Tag Archive for: A-level
Plan B
/0 Comments/in News/by CarolAs part of the ESH Building my Skills programme each year, NUSTEM staff take part in a mock interview day with students from North East secondary schools. During the day, students have the opportunity to be interviewed once or twice by representatives from local businesses and other organisations. At NUSTEM we see the interviews as part-practice and part ‘behind the scenes’ to gives students an insight into what they might be asked and why in an interview.
I ask the students to tell me about themselves, and what career they might thinking about. As part of Building my Skills they will have already done some research into possible sectors of interest to them, so they all have something to say. There’s always an wide range of detail in their answers; with some young people knowing very clearly what they want to do and why, and others who have only a vague idea.
Regardless of their answer, my follow-on question is:
‘What is your plan B?’
This often throws the students as I suspect they’re not often asked what happens if they’re not successful.
What is most interesting to me is that, in their answers, students will often change the whole direction of what they would like to do for their plan B. For example, I’ve had students who had been interested in midwifery suggest that their plan B would be ‘something to do with drama’, or who wanted to be a tennis player, but their plan B would be ‘I dunno, maybe history?’. Very few give suggestions that are in a similar sector to the one they are planning for.
At this point in the interview, I’ll talk about other possibilities that the students could do that is not their first choice, but that is linked to it. Often I’ll suggest websites or resources that they might like to investigate. For example, if a student wants to study medicine, but their predicted grades make that look unlikely we’ll talk about what is it about medicine that interests them. They could study radiography, occupational therapy, Information management and so on. (Although I do have to admit that if it’s the salary that they find attractive, then the other options aren’t so well paid!)
I would encourage all students (and teachers and parents/carers) to think about their Plan B. Just in case.
Some useful websites:
Health careers from the NHS – an invaluable website for students that want to work in healthcare, and for their teachers and families to find out the huge range of careers.
This is Engineering from Engineering UK – looking at the opportunities in engineering from a range of different viewpoints e.g. design, space, fashion, sport
National Careers Service website – a government backed website which includes an A-Z description of over 800 different careers.
Connecting with Physics
/0 Comments/in News/by CarolWhen I did my A-levels a couple of decades ago, there were only two or three girls in my physics class. The situation has got a little better since then, but many girls still find they are in a minority in their physics class. Whilst this doesn’t stop the students enjoying physics and doing well, it can sometimes feel a bit isolating.
To help the situation here in the North East, Think Physics is running a second year of our Physics Connect Network. This aims to allow girls from different schools to connect with each other through on-campus meetings and an online support group.
The network kicks off on January 28th with a Saturday morning session. Award-winning physics communicator Dr Jess Wade will be talking about her research at University College, London, on flexible solar cells. We’ll also look at where physics can lead to in terms of careers.
Later in the term there will be sessions on practical work using K’Nex, an Easter revision morning, and a visit to a local physics-related industry (watch this space for details!).
You can find more about the network sessions here, and the timetable for January 28th, including a booking link, here.
Reece Engineering Summer School
As well as Physics Connect, Think Physics organises a three-week summer school for Year 12 female Physics and Engineering students. Funded by the Reece Foundation, the course provides an introduction to engineering in its many forms. It’s an intense and hectic few weeks, with industry visits, challenges, individual and group research, presentations… everything we can cram into the time.
Applications are now open for the 2017 school: for more information and the application form, click here.
See you this week? Timandra Harkness on Big Data and SUN at Life
/0 Comments/in Events, News/by JonathanThis week we’ve not one but two outstanding opportunities to get your dose of fascinating, curious and very-slightly-sideways science:
- On Thursday, broadcaster, writer and comedian Timandra Harkness gives the finale of our Physics Matters! series of public lectures, on Big Data. 6:30pm, Ellison Building, Northumbria campus: full details and tickets here. These talks are aimed at sixth-formers, but all are welcome. Free, but please register.
- On Friday, as part of the Friday Night Life event at the Life Science Centre, poet Katrina Porteous and composer Peter Zinovieff present the premiere of their work ‘SUN’. The Chronicle have a lovely write-up, and full details are on Life’s website, where you can also buy tickets. Over-18s only.
We’re very much looking forward to both events, and we’ll hope to see you at one or both.
The Amazingly Enormous STEM Careers Poster
/0 Comments/in Careers, News/by JonathanHere’s a neat resource from the terrific folks behind the globe-spanning celebration of the achievements of women in STEM which is Ada Lovelace Day: the aptly-named “Amazingly Enormous STEM Careers Poster”.
We’ve used it a couple of times and can recommend it. The only thing we’d say is that – as with all these sorts of resources – it can be slightly tricky to convey the idea that the list of jobs isn’t exhaustive. That’s particularly challenging when there’s little apparent connection between the job and the degree course… which is rather the point of this particular poster.
So: this is a really nicely-prepared resource, which benefits from a little thought and care about how you introduce or use it.
It’s available for download and self-printing, or you can buy physical posters, both via the links.
Calendar updates
/0 Comments/in News/by JonathanIf you’ve not visited our calendar of upcoming events recently, now would be a good time. We’ve added a bunch of stuff for the term ahead, from ourselves and others. Right now, we’re taking bookings for an excellent programme of lectures aimed at sixth form students, Physics Matters!, and we’re shortly kicking off the second year of our networking and support programme for girls studying physics, Physics Connect.
We add to the calendar whenever we come across something we think you might find useful or interesting, so do keep an eye on it!
A-Level Physics teachers: your thoughts welcome
/6 Comments/in News/by JonathanA few months ago, we made a film of an A-level core practical: measuring g via the free-fall method. Many teachers responded to our invitation to comment, and to our shameless request for recommendations for funders. Well… that worked. Thanks for your kind words, and thanks to your kind words we’re making more of these films. We’re not yet revealing the funder, but we can reveal the first three (or four) practicals we’re filming. We’d also like your help again.
We’re filming next weekend, 21st/22nd May, and we’d be delighted if these films could reflect your experience with practicals you’ve completed, your thoughts about ones you’ve yet to teach, and so on. We’ve a crack team of advisors and supporters already involved, but nothing beats the broad experience of teachers across the UK (and internationally).
So: here are the outlines of the films we’re planning to make. Please leave a comment below if you’ve any pertinent thoughts. It’s extremely helpful if you sign your comments with your real name, and note your affiliations (ie. school, that you’re a teacher / head of department / examiner etc) if appropriate. As before, the films are intended primarily to support teachers, but may be of use to students for revision purposes.
Laser diffraction
- Introduction to traditional two-slit diffraction apparatus, with recap of explanation.
- Plotting slit/screen distance vs. slit spacing.
- Discussion of laser safety issues and suppliers.
- Suggestions around practicalities, and the value of the practical for exploring issues of experiment design.
- Alternative arrangement using a wire rather than traditional double slit.
- Second alternative using diffraction gratings and vertical arrangement.
- (possibly – this film’s already getting quite long!) third alternative using diffraction from a CD, as suggested by OCR.
- Discussion of historical context and significance.
Finding the EMF and internal resistance of a battery
- Conceptual basis of internal resistance; review of relationship between EMF, terminal potential difference, current and internal resistance.
- Apparatus, using multimeters, variable resistor, bare wire contacts.
- Variations, including array of known resistors; switched contact; analogue meters.
- Comparison of internal resistance of different battery types.
- Discussion of value of this practical for exploring key lab skills, including careful but quick working.
Discharging a capacitor through a resistor
- Using a data logger to explore capacitor behaviour.
- Initial verification of \(V = V_0 e^{-t/RC}\); demonstrating that voltage decay half-life is constant, and the time taken to decay to \(1/e\) of the original value.
- Manipulation of \(V = V_0 e^{-t/RC}\) to a form comparable with \(y = mx + c\); processing and plotting data accordingly.
- Low-budget version of practical using voltmeter and stopclock, and with hand-processing of data.
- Extend the practical to finding the value of an unknown capacitor.
- Discussion of error.
Force on a current-carrying conductor in a magnetic field
- The standard ammeter and balance arrangement.
- Sequence of
- Determining magnetic field strength.
- Alternative arrangement with U-shaped wire segment.
Thanks in advance for all your comments and suggestions. Inevitably, we won’t be able to incorporate everything everybody suggests, but if you’ve come across a brilliant way of covering one of these practicals which we’ve not mentioned above, or have thoughts on aspects your students find particularly challenging – we’ll do our best to incorporate your ideas.
Final note: this post was written by Jonathan. Hello. I’m the film-maker behind all these videos, and while I am technically a physicist, I last saw most of these practicals in my own A-level studies more than 25 years ago. Any glaring howlers in the above are due to my misunderstanding of the scripts, and you can be reasonably confident that the many teachers involved in the filming will politely roll their eyes before we commit film-based crimes against physics.
Booklet: What is So Exciting About Physics?
/0 Comments/in Careers, News/by CarolQuestion: What do the following people have in common?
- Roma Agrawal – Structural Engineer
- Isabel Jamal – Barrister
- Tatia Engelmore – Data Scientist
Answer: They all studied a physics degree, and are all in a new booklet called What is so Exciting About Physics?
Put together by a group of students at Cambridge University called Cavendish Inspiring Women, the booklet introduces a range of people discussing what they find exciting about Physics, and where it has taken them in their careers so far. The booklet’s a quick, punchy read that introduces a diverse range of role models, several of whom are working outside what you might think of as traditional physics-related jobs. Teachers, it’s well worth passing this one on to your students.
You can download a copy of the booklet from the CiW website, and follow the project via Twitter.
A-level Subject Take-up
/0 Comments/in News/by JonathanOfsted have published an analysis of the numbers and proportions of girls and boys studying A-level subjects in England:
Until now there has been no single source of data for schools or inspectors to consult that sets out the numbers and proportions of girls and boys that progress from Year 11 to AS levels and then from AS to A level. This report provides that data, so that schools can compare their own performance against the national picture. Several subjects have significantly unequal numbers of girls and boys, for example physics.
Do read the report for the figures in detail, but Dom McDonald, Programme Manager, Outreach at the Royal Society of Chemistry has the STEM subjects summary:
key "Science" numbers (Girls:Boys)
Chemistry 1:1
Geography 1:1
Physics 1:4
Maths: 1:1.5
Further Maths 1:3
Biology 1:0.66
Psychology 1:0.4— Dom McDonald (@TheOxfordDom) May 13, 2015
Also on Twitter, he went on to note that the most extreme subject appears to be Computing, with a girls:boys ratio of 0.09:1.
Yikes.
Future Opportunities: Atom Bank creating new jobs
/0 Comments/in Careers/by EmmaAtom Bank is a new company which hopes to open as a bank in October 2015, and aims to employ 450 employees over the next five years. Located in Durham, Atom Bank describes itself as “designed for digital” and wants to offer the customer a new, innovative experience in banking, for those who engage with new ideas and new technologies.
Teachers: this is a great example to share with pupils to highlight career possibilities within the financial sector, which combine banking with digital and business roles. In Atom bank, and companies like it, there will be careers in:
- People and customer experience
- Technology
- Marketing and propositions
- Finance and risk
- Operations
- Business
Job titles include: marketing, business analysts, solutions architects, technical architects, credit risk manager and financial crime (though we think that means preventing crime, not carrying it out).
Post 16 subject choices: Combinations of Maths, Computer Science, Physics, Further Maths, Business Studies, and ICT will be useful for students aiming for careers in this sector.
Website: www.atombank.co.uk
Twitter: @atom_bank
Tag Archive for: A-level
Simple harmonic motion
/1 Comment/in Required Practical/by JonathanA-Level Physics Required Practicals:
Simple Harmonic Motion
(and a bit of error analysis)
Supporting notes for this film will follow shortly.
Assessment
— to follow
Student Worksheet
— to follow
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Inverse Square Law
/0 Comments/in Required Practical/by JonathanA-Level Physics Required Practicals:
Inverse Square Law
Supporting notes for this film will follow shortly.
Assessment
— to follow
Student Worksheet
— to follow
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Force on a Current-Carrying Wire
/3 Comments/in Required Practical/by JonathanA-Level Physics Required Practicals:
Force on a Current-Carrying Wire
Supporting notes for this film will follow shortly.
Assessment
— to follow
Student Worksheet
— to follow
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Diffraction
/4 Comments/in Required Practical/by JonathanA-Level Physics Required Practicals:
Investigating Diffraction Using a Laser
All the new specifications include a required practical that asks students to measure the wavelength of light by diffraction. Some awarding bodies insist on the use of a laser, while others allow alternative light sources. Some expect the use of Young’s double slits, whilst others suggest a diffraction grating.
In both cases, the light diffracts as it passes through the slits, leading to a broader spread of light on a screen. The different beams diffracted by each slit interfere with each other, either constructively or destructively, depending on their relative path length between slit and screen.
This is a great opportunity to carry out careful measurements, to revise geometry and trigonometry, and to discuss the nature of light itself.
In this film, we show first the double slit method set up conventionally, and then a method using a diffraction grating arranged vertically. Both approaches use laser pointers. The vertical arrangement has advantages in terms of both space in the lab and experiment safety, which are covered in the film.
Using lasers
Many teachers are – quite rightly – concerned about the safety implications of using multiple lasers in a class, even with reasonably sensible students. Particular concerns include:
- With multiple sets of apparatus it can be hard to avoid having laser beams (or their reflections or diffraction paths) criss-crossing the lab.
- In many labs it’s hard to achieve a good blackout, and that carries its own classroom management issues: do you have appropriate places to store coats and bags, for example? What about trailing cables and other trip hazards, in a darkened environment?
- Lasers can be sourced for very little money, but they’re often more powerful than the claimed “<1 mW”. Christina started what turned out to be an interesting discussion about how one might check this, on Talk Physics (registration required).
As ever, one should check with CLEAPSS. Here’s their guidance on lasers (PDF link). Their recommendation is to purchase from an established office or IT supplier, and to ensure the specific unit you receive has a CE marking and a classification BS EN 60825 Class 2. But that’s a summary: read the CLEAPSS advice in full.
It is possible to avoid lasers by using a white light source with a coloured filter. However, to get good results the source needs to be very bright, ideally narrow (collimated), and as close to monochromatic as possible. In practice the filter often costs more than a suitable laser, but if you’d like to explore the option, here’s a write-up at Practical Physics.
Young’s Double Slits
As shown in the film, different exam boards and textbooks use different notation for the formula governing the diffraction of light using a double slit aperture:
\(\lambda = ax / D\)
where:
- \(\lambda\) = wavelength of light (to be found)
- \(a\) = slit spacing, between centres – this information is probably printed on a double-slit slide.
- \(x\) = fringe separation, between adjacent maxima or minima. Measure with a ruler or Vernier callipers, or use mm squares on graph paper as the screen.
- \(D\) = distance from slits to screen, which should be as large as possible (ideally 2 m or more).
Alternatively:
\(\lambda = ws/D\)
where:
- \(s\) = slit spacing
- \(w\) = fringe separation
A straightforward calculation can be done, or you can use the graphical method Alom describes in the film at 1’04”, plotting the fringe separation obtained for a range of different \(D\) values and using the gradient to calculate \(\lambda\). You can check the value you obtain against the value stated on the laser and see if it agrees, within your experimental uncertainty.
Questions you might ask your students:
- Why is a laser a particularly suitable light source for this experiment?
- What would happen to the fringe spacing if we used a green or blue laser? Why?
- Why do we want \(a\) (or \(s\)) to be as small as possible, and \(D\) as large as possible?
- What are the uncertainties in the measured quantities, and how do we combine those to arrive at an uncertainty in \(\lambda\)?
Diagram
We have a PDF version of the above diagram available for download.
Diffraction Grating
This is an alternative or additional method. The pattern obtained is easier to see, since the bright fringes (maxima) are well-defined ‘spots’. however, the mathematics involved is a little more involved, and students must use trigonometry to find the diffraction angle \(\theta\).
In the film, Christina sets up the apparatus vertically, which both requires less space in the lab and reduces concerns over laser safety. However, you will still need to be cautious for stray reflections.
The relationship here is
\(n\lambda = d \sin\theta\)
where:
- \(n\) = the order of the maximum, with \(n=0\) as the central maximum, \(n=1\) for the ‘first order’ to either side, and so on.
- \(\lambda\) = wavelength of light
- \(d\) = slit spacing. Usually a diffraction grating slide states the number of lines per mm. For example: 300 lines/mm implies 300,000 slits/m, so \(d = 1/300,000~\mathrm{m}\).
- \(\theta\) = angle between the straight-through direction (helpfully marked by the zero-order maximum) and the maximum being investigated. This must be found using trigonometry:
As stated in the film (at 3’15”), we cannot use the small angle approximation here, since the angles are too large. Hence:
Using the measurements in the film, Alom’s miraculous ‘off-the-cuff’ calculation went like this:
- Total distance = 191 mm, so \(x\) = 191/2.
- \(D\) = 225 mm
- So \(\theta = \tan^{-1}((191/2)/225) = 22.9985\) ≈ 23.0°
Now, to calculate the wavelength:
\(n\lambda = d\sin\theta\) or \(\lambda = \frac{d\sin\theta}{n}\)
Since we measured two fringes either side of the central maximum (for accuracy):
- \(n = 1\)
- \(d = \frac{1}{300,000}~\mathrm{m}\)
- \(\lambda = (\frac{1}{300,000}\sin 22.9985)/2 = 6.5118 \times 10^{-7} = 651~\mathrm{nm}\) (to 3 sf).
Again, students can compare this with the manufacturer’s stated value; the students should be able to assess their uncertainty to check that the manufacturer’s value falls within their uncertainty range. The same discussions as above can be had about how to minimise uncertainties, and what would happen if a different colour laser were to be used.
Further work
Christina suggests in the film that for an extension activity, students could be given an unmarked diffraction grating with a different (and unknown) slit spacing, and asked to use the laser wavelength they’ve calculated to measure the slit spacing. As set out earlier, white light could be used with a colour filter, and the comparisons of uncertainties would be interesting.
Materials & Costs
Double slit slides: can be bought for around £10 each, for example from Philip Harris.
Gratings: eg. 300 lines/mm for around £15, again from Philip Harris.
Assessment
Common Practical Assessment Criteria
At the time of writing, the exam boards appear to agree that this practical might be used to address CPAC 3 and 4, or subsets of them. For example, AQA suggests:
- using appropriate analogue apparatus to record a range of measurements (to include length/distance, angle)
- using a laser or light source to investigate characteristics of light, including interference and diffraction.
Student Worksheet
We’ve drafted a student worksheet for this practical, which you may find useful:
- Diffraction Practical Worksheet (PDF, 1.7Mb)
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Discharging a Capacitor
/1 Comment/in Required Practical/by JonathanA-Level Physics Required Practicals:
Investigating the Discharge of a Capacitor
All the new specifications include a required practical that asks students to investigate capacitors charging or discharging through a resistor. The awarding bodies agree that this practical is an ideal opportunity for students to develop their skills in using ICT.
What’s in the Film
It’s possible to measure the changing potential difference across the capacitor using: In their experiments, both Alom and Carol do without a two-way switch and instead simply disconnect the capacitor from the power supply to make it discharge through the resistor. As Alom mentions in the introduction, the uses of capacitors are quite interesting for giving the students some context here. He refers to a previous film:
The circuit will be similar whichever approach you use. A meter connected in parallel across the resistor R measures V. With the switch connected at A, the capacitor C charges (almost instantly, since there is negligible resistance in the circuit). When the switch is flicked to position B the capacitor discharges through the resistor, with both the current and voltage decreasing exponentially.
The Datalogger Method
In core practical film, Alom uses Edu-lab data loggers (see below for approximate costings), but there are many alternatives. You could also use a digital oscilloscope which stores data, or a computer-based ‘Picoscope’ or similar which functions equivalently.
From 1’32” in the film Alom suggests that students can check the discharge curve is exponential by seeing if there is a constant half-life (the time taken for the potential difference to fall by a half). Or, better, to measure the time constant and if it that is… er… constant (the time constant is the time taken for the measured potential difference to drop to 1/e of its original value – see the graph above).
Earlier in the film, Alom chooses values for C and R which multiply to give a time constant which is measurable: neither too fast nor too slow. In this case:
\(C = 4700~\mathrm{\mu F}\)
\(R = 10~\mathrm{k\Omega}\)
So time constant \(\tau = 4700\times10^{-6} \cdot 10 \cdot 10^3 = 47~\mathrm{s}\)
The exponential equation:
\(V = V_{0}e^{-t/RC}\)
is one which students will need to be able to handle. Alom shows how to manipulate this into the form of a straight-line graph. First, take natural logs of both sides:
\(ln(V) = ln(V_0) – \frac{t}{RC}\)
Compare with:
\(y = mx + c\)
If \(ln(V)\) is plotted on the y-axis against \(t\) on the x-axis, then \(ln(V_0)\) will be the y-intercept and \(-\frac{1}{RC}\) the gradient, hence \(C\) if \(R\) is known.
Resistor colour code
This might be a useful opportunity to show students the resistor colour code:
- Wikipedia page.
- The best little web app we’ve found for calculating and interpreting resistor bands. —Seriously, this page is awesome. We have it as a shortcut on our browser toolbars.
Questions
You might like to ask your students:
- Why is a datalogging method appropriate for this practical?
- Does it matter which way round we connect the capacitor? (see Carol’s comment at 3’21”).
- For how long should we time the discharge?
- What would happen if we instead measured V while the capacitor charges?
- Why process the data so we get a straight line graph? Why is this better than a curve?
- What should the units of \(ln(V)\) be?
The Direct Measurement Method
Carol uses a multimeter as a voltmeter and a stopclock to do the timing manually. She suggests measuring V every ten seconds: it’s hard to record it any more frequently, but take less frequent measurements and the graph will be hard to draw, with too few data points. Any voltmeter will do: digital, analogue, multimeter, or even an oscilloscope.
Students could be challenged to find an unknown capacitance by measuring the time constant obtained with a particular resistor. In the film, Carol explains that some preliminary runs will be needed to test the circuit and to ensure they have a suitably measurable time constant.
This method lends itself to the use of ICT, in that students can put their data into a spreadsheet and manipulate them into a straight line for (as shown above). Alternatively, data can be processed by hand.
You might as your students:
- Why do we not need to worry about starting the timing immediately the discharge begins? (This will test their understanding of the exponential decay function.)
- What are the causes of uncertainty in the experiment – not only our measurements, but the manufacturers’ tolerances for their components?
- How can we reduce the uncertainty in our readings?
Safety
Since \(Q = CV\), a large capacitor charged to a huge voltage stores a lot of charge, which will lead to a high initial charging or discharging current, with consequent heating effect. The values used in the film (and noted above) offer a good guide to what works well – and safely.
Capacitors should not be charged to a voltage higher than that stated on their labelling or product information sheets. Electrolytic capacitors should be connected with the correct polarity.
It’s also important to ensure the capacitor is discharged before touching it: disconnect the power, then operate the circuit to discharge the capacitor through the resistance before dismantling the apparatus.
Costs
The Edu-logger system used in the film sells for around:
- £45 for the voltage sensor
- £43 for the USB module to connect to a PC (standalone display/tablet/smartphone alternatives are available).
- £43 for a battery module.
Those figures are without VAT. Other systems are available for similar prices, or you can spend considerably more on systems with increasing levels of flexibility. Most suppliers will offer discounts on bulk orders.
Further work
The IOP’s Teaching Advanced Physics website has a series of articles about teaching topics around capacitance:
Assessment
Common Practical Assessment Criteria
At the time of writing, the exam boards appear to agree that this practical might be used to address, in whole or in part:
- CPAC 3: Safely use a range of practical equipment and materials.
- CPAC 4: Makes and records observations.
Student Worksheet
We’ve drafted a student worksheet for this practical, which you may find useful as a starting point:
- Capacitor discharge lab manual (Word .docx)
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Measuring the EMF and Internal Resistance of a Cell
/3 Comments/in Required Practical/by JonathanA-Level Physics Required Practicals:
Measuring the EMF and Internal Resistance of a Cell
All the new specifications include “measure the internal resistance of a cell” as one of the practicals. This is probably a new bit of physics for your students, and although the practical is straightforward to set up, collecting and processing the data is more of a challenge. Comparing two different types of cell, as shown in this film, can make the practical more interesting, with potential for differentiation by ability.
What’s in the Film
The film starts (to 1:24) with the theory which you’ll probably introduce to students before carrying out the practical. From 1:30 onwards, the film illustrates how you might go about conducting the practical with conventional cells, and also with a button cell (watch battery). Christina and Alom do several things in the film to limit the current so the cell doesn’t overheat: they use a limiting resistor, start with low currents, and connect the circuit only momentarily. This represents safe working practice, but heating the cell would also affect the resistance we’re trying to measure.Safety
AA Cell
We used a 10 Ω resistor to limit the current in the circuit. A simple fixed resistor would do, but make sure it can handle the maximum power you expect in the circuit – a few Watts. We didn’t have such a resistor to hand for filming, hence the huge switchable resistance box.
To vary the current to get multiple readings we used an old rheostat, rated at about 16 Ω. In practice anything with a range up to 50 Ω or so should work. It’s also possible to use a range of different fixed resistors, or a switchable resistance box.
Digital or analogue voltmeters or ammeters could be used instead of multimeters, but as Christina points out in the film, the use of multimeters is a skill your students will need to develop anyway. Students will need to select the most appropriate range, which is likely to be 20 V DC for the voltmeter and 200 mA DC for the ammeter (taking care to convert back to amps when processing the data).
Alom’s multimeter films may be dull, but they’ve had a third of a million views so… they may have some redeeming merit. Click through to YouTube or to maximise the film from these tiny windows:
Measuring Voltage with a Multimeter
Measuring Current with a Multimeter
Collecting & Processing Data
Working in pairs, this experiment can be be done very quickly. Systematic data are nice, but as long as there’s a good spread of data points across the whole range of currents, students should get a good result.
From our data, we arrived at:
Gradient = -2.10
y-intercept = 1.415
so:
EMF = 1.415 V
Internal resistance = 2.10 Ω
We would normally expect an AA cell to have an EMF of about 1.5 V and an internal resistance of about 1 Ω. Ours was old and cheap, which probably explains our results: it’s worth noting that poorer-quality cells can make for a more interesting experiment!
You might ask your students:
- Is their result what they would expect from the cell packaging or label?
- How could they assess the uncertainty in their data?
Christina mentions tolerance at 4’11”, which is a concept with which students may not be familiar. All components have a stated manufacturer’s tolerance, which notes the ±% range which might be expected when the component is in normal use.
Coin Cell
We used a standard CR2032 watch battery. The readings for this sort of cell vary much more wildly than for an AA cell. We’d assumed this is due to internal heating of the cell, but in writing these notes we’ve started to wonder if it’s more about the chemistry that’s going on inside – if there’s a limit to the reaction rate, that would explain why the voltage drops away rapidly (particularly with high current drain cases), before the cell recovers after ‘resting.’ Comments welcome, and for now we’ll move on…
Taking photos of the meters is one way of dealing with rapidly-changing readings. Another approach would be to use analogue meters, which can be easier to read by eye.
At 6’09” you’ll see Christina using a ‘best fit’ ruler – a clear ruler with a slot through the middle. We recommend these! Our results:
With the increased uncertainty in the readings, Alom suggests repeating the whole experiment twice. Each repeat could be plotted onto the same axes and the gradients and y-intercepts compared. Students could then find the mean EMF and internal resistance, together with their associated uncertainties.
We would normally expect a 3 V cell to have an EMF of about 3 V, and an internal resistance which is much higher than the AA cell – which indeed is what we found, measuring an internal resistance of 15 Ω.
You might ask your students:
- Is there a better way to record the fluctuating readings?
- Is a simple mean a legitimate way to combine repeat readings?
- What’s the best way to deal with data which looked bunched-up on a graph because you need to include the y-intercept? (You could investigate mathematical extrapolation methods here.)
- Why do cells have different EMFs and internal resistance? What chemicals do they contain and how are they structured inside? (a useful resource here is Battery University, though it gets a little… detailed, shall we say?)
Other Notes
Costs
- 50 AA cells should cost about £12.
- 40 3 V coin cells should cost about £5.
Further Work
Some teachers like to challenge their students further by investigating the EMF and internal resistance of a cell made with copper and zinc electrodes and an item of fruit or a vegetable, for example: a ‘potato battery.’ Further guidance on this can be found at the Practical Physics website. Cutting the potato into different shapes can make for an interesting comparison.
Assessment
Common Practical Assessment Criteria
At the time of writing, the exam boards appear to agree that this practical might be used to address, in whole or in part:
- CPAC 1: Follows written procedures
- Correctly follows instructions to carry out the experimental techniques or procedures
- CPAC 4: Makes and records observations.
- Makes accurate observations relevant to the experimental or investigative procedure.
- Obtains accurate, precise and sufficient data for experimental and investigative procedures and records this methodically using appropriate units and conventions.
You can likely prioritise other CPACs should you so choose. There are some more notes on this in the draft student worksheet, below.
Student Worksheet
We’ve drafted a student worksheet for this practical, which you may find useful as a starting point:
- EMF and internal resistance worksheet (Word .docx).
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
Measuring g via Free Fall
/4 Comments/in Required Practical/by JonathanA-Level Physics Required Practicals:
Measuring g using a free fall method
A-level specifications from all the exam boards include “measure the acceleration due to gravity of a freely falling body” as one of the practicals. Students might be a bit uninterested in measuring the value of a constant with which they are already familiar. However, this practical is likely to be undertaken close to the beginning of an A level course. As such, it can be used to make a number of valuable points, each of which is worth introducing our students to at this stage:
- By comparing more than one method, students can practice thinking critically about experimental methods.
- Learn to assess and reduce uncertainty.
- Consider how to present and process data.
- Discuss what is meant by ‘constant.’
What’s in the Film
The film shows four different methods of measuring g using a falling object: Which of these methods you use might depend on your students’ skill, your own preference, apparatus availability, ease of data collection or processing, and class size. It’s good to compare at least two methods (even if one is shown as a demonstration only), to prompt and inform discussion about precision, accuracy and uncertainty. Since we anticipate this practical being used early in the A level course, we’ve not included comments about how to do a full error analysis. There’s more error handling in some of the other films in the series, and we’re planning to complete a film specifically about error as the series continues. The ASE book The Language of Measurement (£13.50/£8.50 members) refers to precision as: A measurement is precise if values cluster closely. In the film, at about 2:22, the word ‘precision’ is used to mean the timer’s smallest scale division. This is us showing our age! A better term might be ‘resolution’. Other ASE definitions: Accuracy: a result is accurate if it is close to the true value. Uncertainty: the interval within which the true value can be expected to lie.
What’s not in the film
An aside about language
Calculating g from h and t
Methods 1, 2 and 4 give values of the time \(t\) for a ball to fall from rest at a height \(h\). From the equation:
\(s = ut + \frac{1}{2}at^2\)
we have:
\(g = \frac{2h}{t^2}\)
Measuring \(t\) for different values of \(h\) allows a graph to be drawn of \(h\) against \(t^2\). The gradient of that graph is \(g/2\).
In method 3 the ball falls through two light gates separated by a distance \(h\). Each gate gives a value for the average speed of the ball as it passes through, so we can use \(v^2 = u^2 + 2as\) to find its acceleration between the gates.
Uncertainties
Uncertainties arise in four ways:
- Starting the timer.
- During the ball’s descent, due to air resistance or other factors.
- Stopping the timer.
- Determining the height of the fall.
Students can think about these as they compare the different methods. They can try to assess whether each uncertainty will make the values of \(t\) and \(h\) too big or too small, or just more uncertain. They can also discuss how each factor will affect the value of \(g\).
For example: if the measured value of \(t\) is too big, the calculated value of \(g\) will be too small, because t is on the bottom line of \(g = 2h/t^2\).
Error in g
Eventually, students must be able to assess the overall uncertainty in their measurements. For example, they might state their measured value as \(g = 8.7 \pm 1~\mathrm{ms^{-2}}\). For now, however, it is a good start to be able to calculate the percentage error, e.g.:
\(\frac{9.8 – 8.7}{9.8} \times 100\% = 11\%\)
Method 1: Dropping a ball
Drop a ball from a measured height \(h\); start and stop a timer to find \(t\).
Students should be able to comment on the problems with starting and stopping the timer at the correct instants. Reaction time here is not the same as the human response to a random stimulus, since we can watch the ball falling and anticipate the correct moment to stop the timer. It’s still, however, a poor way to measure \(g\)!
Repeated measurements for a fixed value of \(h\) will give a range of values for \(t\). Some students may be better at using a consistent technique, which will give a smaller spread in the values of \(t\). This reduces the random error, but there may still be systematic error, which itself may be revealed by repeating the experiment for different values of \(h\).
Questions to ask your students:
- What’s a better way to release the ball?
- What’s a better way to measure the time?
- Why don’t we try to reduce the uncertainty in measuring \(h\)?
Method 2: The g-ball
These cost about £20 + VAT from education suppliers, one of which is Timstar (as of 2020-08-03 Timstar appear to have stopped selling the G Ball. We’ve found other UK suppliers: Philip Harris, Better Equipped, Breckland Scientific. Search for ‘G Ball’ to find others. There appear to be at least two manufacturers, Unilab and Mollic).
The g-ball starts timing when released and stops timing as soon as it hits the floor. Like most stopclocks, it measures to a resolution of 0.01 seconds. The switch release can limit accuracy, but overall the g-ball is a quick way to collect a large number of data points. In the film, Alom and Christina use an L-shaped bracket clipped to a metre rule to press the release switch, to aid a clean release.
Questions to ask your students:
- How is this better than using a stopwatch?
- How can they ‘average’ their data?
Your students could just repeatedly drop the g-ball from the same height and average all their data, then substitute their average \(t\) into \(s = \frac{1}{2}gt^2\) to find \(g\). Much better would be to exclude anomalous values first, and better still would be to plot a graph of \(h\) against \(t^2\) to spot those anomalies.
If you’re going to plot a graph, however, you might as well have two meaningful variables. In the film, Christina and Alom drop the ball from different heights and collect a lot of (not very good!) data quickly.
This approach:
- Helps to spot anomalies visually – an indication of random error.
- Shows that \(g\) is (more-or-less!) constant: the graph is a straight line.
- Allows \(g\) to be determined from a gradient, an important skill.
- Might lead on to a discussion about systematic errors (should the graph pass through the origin?).
Systematic errors can often be eliminated by plotting a graph. For example, if the height is measured 1 cm too short each time, the line on the graph will be shifted downwards. The gradient will be unchanged, and the systematic error should be easily detected.
Another approach, incidentally, would be to plot \(2s\) against \(t^2\), giving a simpler gradient of \(g\). This is a matter of personal preference.
Language:
Random error: a measurement error due to results varying in an unpredictable way.
Systematic error: A measurement error where results differ from the true value by a consistent amount each time.
Method 3: Light gates & data logger
Using light gates is a great way to get students familiar with data loggers. Watch out for the common misconception that using a computer will automatically give a better result! In this case, the timing typically does have a higher resolution, and it’s possible to collect lots of data quickly – both good reasons for using the apparatus.
With two light gates, there are three possibilities:
- Display values of \(u\) and \(v\), and calculate g using \(v^2 = u^2 + 2as\).
- Display values of \(u\), \(v\) and \(t\), and calculate g using \(v = u – at\).
- Position the top light gate just below where the ball is released so the initial velocity is close to zero, then proceed as above.
Note that the measured speeds are always average values, because the ball is accelerating during the time it takes to pass through the light gate.
Questions to ask your students:
- Can we be sure that the data logger determines accurate times?
- If we vary \(h\), what graph should we plot to determine \(g\)?
- How can a graph help to reveal problems with the experimental technique?
Apparatus
If you don’t have multiple sets of data loggers, you could have one set up and have students use it in turn. However, the experience of setting up the apparatus is itself valuable, as it prompts the student to think through the role of each piece of equipment rather than to approach the configured apparatus as a ‘black box.’
Method 4: Electronic Timer
The commercially available apparatus that Alom and Ronan discuss in the film is available from Philip Harris, for about £100+VAT. Instructions for its use are available from the Nuffield Foundation, which also includes circuit diagrams for a DIY version. (update 2020-08-03: it looks like the old practicalphysics.org site has been taken down. The content has moved to spark.iop.org/practical-physics, and the resources on this particular experiment are: Acceleration due to gravity, and Measurement of g using an electronic timer).
Ronan’s version addresses several subtle issues, including that of releasing the ball cleanly, and using a plumb-line to confirm that \(h\) is being measured vertically. We’ll update this article with more details of Ronan’s apparatus when we have them.
Questions to ask your students
- Between which two points should we measure the height of fall \(h\)?
- Can we be sure that the timer starts and stops at the exact moments we want it to?
- If we vary \(h\), what graph should we plot to determine \(g\)?
Assessment
Common Practical Assessment Criteria
At the time of writing, the exam boards appear to agree that this practical might be used to address, in whole or in part:
- CPAC 2: Applies investigate approaches and methods when using instruments and equipment.
- CPAC 4: Makes and records observations.
Apparatus & Techniques
Each exam board has published a list of apparatus and techniques with which students much be familiar, along with suggestions as to which elements might be addressed by each practical. For example, Edexcel’s guidance for this practical suggests:
- 1. Use appropriate analogue apparatus to record a range of measurements (to include length/distance, temperature, pressure, force, angles, volume) and to interpolate between scale markings
- 2. Use appropriate digital instruments, including electrical multimeters, to obtain a range of measurements (to include time, current, voltage, resistance, mass).
- 4. Use stopwatch or light gates for timing.
- 11. Use ICT such as computer modelling, or datalogger with a variety of sensors to collect data, or use of software to process data.
Check your exam board’s resources: there should be a mapping document to help you decide which criteria to assess on each practical.
Student Worksheet
We’ve drafted a student worksheet for this practical, which you may find useful:
- Acceleration due to Gravity Worksheet (Word .doc file)
Comments & Feedback
As ever, no single film can encompass everything one might wish to say about a practical. Please, leave comments with your thoughts about the approach we’ve taken, and your suggestions for alternatives or improvements.
A-Level Physics Required Practicals
/0 Comments/in Advanced, CPD/by JonathanA-Level Physics Required Practicals
About this project
As of September 2015, practical work in A-Level Physics is no longer assessed through examination – but it’s still a core part of the curriculum. We’ve been keeping a close eye on the support and resources available to teachers, and we think there’s a gap. To start filling it, we’ve teamed up with physics teacher Alom Shaha and educational charity Physics Partners to produce a series of films, building on the approach of Alom’s previous films about physics demonstrations.
We’ve been joined in the project with financial and editorial input from The Ogden Trust.
As of December 2016, four films are complete, with three more in post-production.
g via free fall
Diffraction
Capacitor Discharge
EMF and Internal Resistance
Supporting Resources
Written support resources will follow!
See also…
If you think the style of these films is somehow familiar, that’ll be because Alom’s previously made a whole raft of videos about practical demonstrations with film-maker Jonathan Sanderson, who’s currently working for… you guessed it, Think Physics. Most of the films are collected at the National STEM Centre library, but some have appeared on YouTube.
The films are used by teachers and teacher trainers world-wide, and we love to hear your stories about how you’re putting them to use, tips and techniques you have to do the demonstrations better, and comments and criticisms for how the films could be more useful.
Alom and Jonathan also made a longer video essay about the use of demonstrations in the science classroom. Demo: The Movie has been particularly popular with PGCE course leaders for prompting discussion.

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