# A Level Physics

## Kit Betts-Masters

All the Core Practicals

This is a summary document of all the Core Practicals in Edexcel, but in reality they are useful for all exam boards.  I've tried to give the briefest of descriptions of the method, (you'd need to add detail, like the apparatus and techniques in an exam,) but I've listed the usual evaluative points that go with each practical below each summary.  Where I have a video on the practical I've included it on this page!  I went through this entire document in a recent live feed, so if you prefer to listen to me discuss each one rather than read, go for it!

Every exam board has to satisfy a list of Apparatus and Techniques which are published by the DfE, so that means this list is the same for every exam board, the only difference is the set of practicals that they designed to allow you to use the apparatus and learn the techniques!

• 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

• use appropriate digital instruments, including electrical multimeters, to obtain a range of measurements (to include time, current, voltage, resistance, mass)

• use methods to increase accuracy of measurements, such as timing over multiple oscillations, or use of fiducial marker, set square or plumb line

• use stopwatch or light gates for timing

• use calipers and micrometers for small distances, using digital or vernier scales

• correctly construct circuits from circuit diagrams using DC power supplies, cells, and a range of circuit components, including those where polarity is important

• design, construct and check circuits using DC power supplies, cells, and a range of circuit components

• use signal generator and oscilloscope, including volts/division and time-base

• generate and measure waves, using microphone and loudspeaker, or ripple tank, or vibration transducer, or microwave / radio wave source

• use laser or light source to investigate characteristics of light, including interference and diffraction

• use ICT such as computer modelling, or data logger with a variety of sensors to collect data, or use of software to process data

• use ionising radiation, including detectors

g by free-fall

- Drop a thing

- Measure time and distance

- Use suvat to get g

Compare accurate methods for timing, trap door vs light gates (vs stopwatch)

Discuss improvements using percentage uncertainty,  for example the very small percentage uncertainty using a light gate versus timing with a stopwatch..

Compare a final result to an accepted (true value) as a percentage difference.

Understand the systematic error caused by air resistance Including accounting for the direction that this will change your result, i.e. the measured value for gravitational acceleration will be lower than the true value.

Resolve some algebra into y=mx+c and understand the usefulness of a gradient as it takes less account of anomalies.

Resistivity of a metal

- Measure resistance of various lengths of wire

- Measure diameter of the wire

- Use equation to find resistivity

Comparison of methods using an Ohmmeter directly or of using voltmeter and ammeter.

Usefulness of variable resolution on a multimeter, essentially you are always trying to measure to 3 significant figures..

Take account of the potential Systematic error by heating using a current through the wire

Non-zero resistance in a circuit causing a systematic error which just shifts the graph up but doesn't affect the gradient

Using a micrometer and measuring to 0.01 of a millimetre, taking multiple readings at different positions, discard anomalies and take an average.  This is a usual technique for a random error.

To find resistivity of non-metals you need a much larger area and a shorter length to get an accurately measurable resistance.

EMF and internal resistance

- Model internal resistance with a fixed resistor

- Vary load resistance and measure voltage and current

- Manipulate the algebra to make internal resistance the gradient

We model wires, ammeters and other circuit components as having a negligible resistance which means it is much smaller than other values in the circuit and too small to consider.

An ammeter has a near zero resistance as it needs to be connected in series, a voltmeter has a very large resistance as it needs to be connected in parallel,  neither meter should affect the current in the series loop.

We need to use a fixed resistor on about 10 ohms to model internal resistance because the internal resistance of a normal chemical cell is very low and would be hard to measure accurately.

Falling ball viscometer

- Drop a ball through viscous fluid

- Measure speed through various distances to obtain a terminal velocity

- Consider weight, upthrust and Stoke’s force to calculate viscosity

Can't use light gates as the liquid would block the beam and it would be difficult to get the ball to fall directly through the beam.  Relate your improvements to the method, i.e. It is not always appropriate just to state use a light gate to give you an accurate timing method.

Works best with smaller balls and more viscous fluids as accurate timing is difficult, consider using video analysis.   Increase the values of readings to reduce  percentage uncertainty.

Stoke’s Law is a model using a perfectly spherical object and doesn't consider the extra drag with being near to the edge of a measuring cylinder,  this will be much more accurate in a wider tank of liquid.

Calculate and consider uncertainties in experiment and compare uncertainties of different measurements as a percentage uncertainty.

Young Modulus of Copper

- Measure diameter and original length of a copper wire

- stretch the wire with increasing force measure it's extension

- plot a stress over strain graph to get the young modulus

You need to use a long wire, greater than 2m, so that the extension which, is going to be quite small, is large enough to be measured.  We could also use a vernier scale to measure very small extensions,  or we could have a very small cross sectional area to allow a larger extension for the same force.

Percentage uncertainty in extension is likely to be quite large.

Measure diameter twice at each point at right angles to each other repeat this five times along the length of the wire and take an average of all 10 results.  Again, repetition deals with random errors.

We always give our final results to the same number of significant figures as the least significant figures we measured to.

Speed of Sound

- Compare the electrical signal with the sound wave it produces on an oscilloscope

- Record points where the microphone signal is in phase with the electrical signal,  these points are one wavelength apart

- Measure frequency using oscilloscope and use the wave speed equation to calculate the speed of sound.

It's difficult to find exactly when the two waves are in phase,  especially as the microphone signal can have Interference leading to a thick trace.  Also the microphone signal reduces in amplitude as you get further from the loudspeaker.

We can adjust the time base on the oscilloscope to allow us to get a high resolution time period, hence reduce percentage uncertainty of the frequency we calculate.

Each point where the two waves are in phase have travelled one extra wavelength of path difference.

Use a fixed ruler and a set square to line up a certain point on the microphone to accurately measure the points of one wavelength path difference.

The frequency chosen the sound wave needs to give a wavelength in the region of 10 cm which is a good order of magnitude to measure to a low percentage uncertainty with a metre ruler.

Mass per unit length

- Set up a standing wave on a string either very tension or vary frequency

- find resonant frequencies and measure wavelength of these

- manipulate algebra to the form y=mx+c  with mass per unit length being the gradient

Comparing the sources of uncertainty in the practical,  frequency uncertainty is low if using an oscilloscope, wavelength has high uncertainty as the node tends to be a thick blur.

Deciding on independent variables and dependent variables and which variables to control by inspection of the algebraic rule you are trying to use.

It is difficult to find points of resonance you can move slightly high frequency or slightly low frequency until you are happy that you found the point of sharpest node or highest amplitude.

This technique is very useful to us for example in measurement of the speed of light,  standing waves with lasers are also used to measure very small distances.

Diffraction grating interference patterns

- Shine a laser through a diffraction grating

- the distance between the grating and the wall, measure the distances between the maxima produced

- use trigonometry to work out an angle and hence a wavelength

Lasers are monochromatic meaning of the light is all the same frequency, they are also coherent which means they have a constant phase difference.

They produce bright spot maxima even at longer distances allowing us to reduce the percentage uncertainty in our distances we measure.

If we used white light we would have maxima which would have a Spectrum and it would be difficult to accurately measure the distances between maxima.

Impulse

-Accelerate a trolley using ten gram masses hanging over pulley

- use Newton's second law to decide what to measure and what to fix

- Manipulating algebra to give a graph with a gradient of mass the entire trolley hanging masses and string.

We can use lines of worst fit, i.e. most and least slope which still fit through error bars,  to calculate a percentage uncertainty in gradient.

When working with kinematics we should always consider compensating for friction, we can do that either by using an inclined plane or by using a linear air track.  If you don't do this will introduce a systematic error which will make our accelerations smaller than expected.

There are many ways to conduct this experiment, consider what gives you the lowest percentage uncertainty with the apparatus available.

You could use the same apparatus to conduct a conservation of energy experiment, whereby gravitational potential energy for the Spotted masses is converted into kinetic energy of the accelerated trolley,  with some energy dissipated as heating to surroundings.

Momentum in 2D

- Video collisions of two ball bearings

- use tracker software to analyse the momentum before and after a collision

- resolve these momenta into x dimension and y dimension using trigonometry

You could discuss frame rate as an indication of uncertainty of time and also discuss the parallax error with using a camera.

It will be very difficult to repeat exactly the same collision more than once.

A closed vector triangle shows momentum is conserved.

Capacitor discharge

- charge a capacitor fully

- discharges through a resistor measuring voltage

- analyse the exponential decay using a log graph

The capacitors should be connected in series with a resistor when discharging.   The voltmeter or the oscilloscope has a very high resistance in parallel with the capacitor so does not affect the rate of discharge of the capacitor.

It's difficult to measure  very small periods of time accurately.   The time constant depends upon the resistance and capacitance chosen,  you can estimate the time taken for the decay and hence choose a method appropriate to the length of time expected.

Uncertainty is introduced by having to look at a time and then a voltage simultaneously.

You could use a data logger to give you a continuous readings of voltage, or you could video the voltmeter and take readings as it refreshes using the video to give you a time base.

You can repeat the experiment and measure the time to get to certain potential differences rather than run the experiment continuously.

Or you could use a square wave signal generator and an oscilloscope to repeatedly charge and discharge a capacitor and take readings from the decay curve produced.

Calibrate a thermistor

- vary temperature and measure the resistance of a thermistor

- use a log graph to find a resistance at a  set temperature

- use that resistance and a fixed resistor to design a potential divider circuit such that it will have a set voltage output at the set temperature

It's more accurate to interpolate from straight line graphs than from curves.

Consider the success of this experiment as measured by percentage difference from the estimated values when designing a potential divider and the final values.

It's difficult to ensure that the thermometer is actually reading the temperature of the thermistor rather than water around it, give the thermistor time to reach a thermal equilibrium with the water around it.  Data Loggers continuously reading temperature should limit the effect of this thermal inertia.

As this practical involves hot water it's worth considering a risk assessment potentially considering the specific heat of the hot water.  (Use physics to estimate your level of risk.)

Specific latent heat

- Crush some ice and let it melt in some water

- Measure the masses, starting and final temperatures

- Write an equation which equates the energy to melt the ice with the energy to lower the temperature of the water and raise the temperature of the (now melted) ice

As you're finding a temperature difference by subtracting one temperature from another the uncertainty is added, in this case half a scale division on a thermometer would be 0.5 degrees Celsius so the total uncertainty is 1 degree Celsius.

The ice should be already melting because we know that a ice and water mixture is at 0 degrees Celsius,  but you should allow the water to drip off so that is just ice that goes into the beaker.

The ice should be crushed so that it melts quickly and this limits the time during which it is melting, limiting the energy transferred to the ice and water mixture from the surroundings.

As there will be heating from the surroundings to the room will lead to a smaller than accurate value of the latent heat for fusion of ice.   This is an example of a systematic error.

Boyle's law

- Vary the pressure and measure the volume for a trapped mass of gas.

In our Boyle’s Law apparatus pressure gauge is analogue and so it's an example of when you have to interpolate between scale markings.   This can be less precise but it also doesn't have as much random error by a fluctuating digital reading.

Difficulty in controlling the temperature for example by doing work on the gas causing its internal energy to rise.

Volume of a gas is made off an analogue scale therefore reading should be taken close to the scale and at 90 degrees to the scale to avoid parallax error.

Absorption of gamma ray through lead

- thickness of Lead between a gamma emitter and a Geiger Muller tube

- plot a log graph of counts per second vs fitness

Careful risk assessment should be taken before conducting this practical including consideration of limiting irradiation and insuring contamination is not possible.   The main safety procedures are distance, line of sight and time.  As doubling the distance will quarter the intensity of the radiation, limiting the time limits the dose of radiation.

Radiation is random so the uncertainty is random therefore increasing the length of time measuring counts increases the accuracy as would doing repeats all the same thicknesses.  Measuring the thickness should be taken in at least five places and averaged with a micrometer.

Some Geiger-Muller tubes measure a cumulative count and to divide by time to get a count rate, others will measure a counts per second directly the suitability of this will depend on the activity of the source.   Using large periods of time reduces the percentage uncertainty if using a stopwatch to measure the time.

Mass-spring system

- vary mass and measure the time period

- manipulate algebra to get a straight line graph relating mass to time period

- use your mass spring system to measure an unknown mass

Using multiple oscillations, for example timing 10 full Cycles and dividing by 10, is an accurate way to use a stopwatch to measure a short time period.  You should also use a fiducial marker, (or timing mark), to help you see that you are coming from exactly the same place in the oscillation.

You can also use an ultrasound position sensor underneath the mass-spring system and this would give a sine wave graph and allow you to accurately measure a time period from the graph.

This is an example of a free oscillation, a forced oscillation would not be appropriate as we are looking to you to find a natural frequency based on the equation for mass spring systems.  This is an example of a lightly damped oscillation as well,  and this means that energy leaves the system each swing to heating up the surroundings.

When designing your own practicals make sure that you identify the variables that you are going to measure clearly including the apparatus that you are going to measure them with.

Ensure you have clearly stated the independent variable, dependent variable and control variables.

Give evaluative points based on the evaluative points for the core practicals, in other words the practical is different but the apparatus and techniques is the same as those covered by the 16 core practicals.

Ensure that you indicate what graphs, where possible in the form y = mx + c, and what would be shown by the gradient of it.

Always be thinking about ways to reduce the percentage uncertainty in the practical.

And now!  Some practice questions for all this stuff!