Paper 3 Flashcards
1
Q
What safety precautions can be taken with experiments involving clamp stands?
A
- Clamp down the clamp stand or add a counterweight to make it more stable.
2
Q
What safety precautions can be taken when using electrical circuits?
A
- Use a low current to reduce heating.
- Use a low p.d. to reduce chance of a shock.
- Don’t leave running for long periods of time if using a component that can heat up.
- Avoid contact with water if using water.
- Avoid contact with circuit connectors.
- Turn off between readings.
3
Q
When dropping a ball bearing through light gates to measure g, what can you do to increase validity?
A
- Use a plumb line to see the path of the ball as it falls, so that the light gates can be placed directly below the ball.
- Use a larger ball bearing as it has less air resistance relative to its weight.
- Leave a notable gap between the starting position and first light gate so that the change in velocity while passing the first beam is negligible.
4
Q
What’s good about a high resolution?
A
- (Not guaranteed, but can be) higher accuracy
- Lower uncertainty
5
Q
How would you determine uncertainty of resistivity readings?
A
- length of wire: resolution (%U)
area of wire: range/2 of diameter - readings (%U) TIMES TWO because squaring
- Resistance as (%U) of ohmmeter OR (%U) of ammeter (resolution) + (%U) of voltmeter (resolution).
Total: length%U + 2diameter%U + resistance%U
6
Q
When determining the resistivity of the wire, what variables must you control?
A
- Wire thickness: do not stretch, measure all along the wire and take a mean value
- Wire temperature: if not cooling the wire, turn off the power supply between taking readings, and keep the current very low.
- Length measure origin: measure from the same point each time by clamping the start of the wire in place, and taping a metre rule to the work surface so that the start of the ruler is the start of the wire.
7
Q
What factors lead to inaccuracy when determining the resistivity of wire?
A
- Resistance in circuit connectors, such as crocodile clips.
- Zero error of the start of the wire.
- Subtle changes in temperature of the wire.
8
Q
What is the disadvantage of using a multimeter to experiment for resistivity?
A
Resolution will probably be lower.
9
Q
What is the equation used to calculate e.m.f. in a circuit?
A
ε = I(R + r)
so ε = V + Ir, where V is terminal potential difference
10
Q
What should be controlled during the e.m.f. experiment?
A
- The temperature of the source: turn off circuit when not using, so that the source doesn’t heat up.
11
Q
What kind of battery should you use in the e.m.f. experiment?
A
- A non-rechargeable battery, as rechargeable ones have very low internal resistance.
- A new battery, as used batteries tend to have fluctuating internal resistance.
12
Q
What device could you use to check your value for internal resistance?
A
An Ohmmeter.
13
Q
What might lead to inaccuracies in a calculated e.m.f.?
A
The voltmeter is unlikely to be ideal and will probably have a small current flow through it.
14
Q
Why are high voltage supplies safe for lab use?
A
Their high internal resistance means the terminal potential difference is lower, and it is safer to be handled.
15
Q
What’s the second method for determining e.m.f.?
A
- Take data as usual: potential difference for varying currents.
- Consider the circuit load and internal resistance as a potential divider.
- Plot power dissipated by load (P=IV) against load resistance.
- When power dissipated is at its maximum, the load resistance equals the internal resistance.
16
Q
What safety precautions can be taken when using liquids?
A
- Wear goggles to avoid splashes in eye.
- Mop up any spills to avoid slipping.
- Wear gloves if allergic to liquid.
17
Q
How can you make sure the viscosity experiment is valid?
A
- Ensure the ball has enough time to speed up to terminal velocity.
- Ensure that the tube radius is significantly more than the ball radius, otherwise the flow may not be laminar.
18
Q
Why can’t light gates be used in the viscosity experiment?
A
Light gates require a clean breakage of the beam. The ball is unlikely to pass directly along its diameter, and the fluid used may disrupt the beam.
19
Q
Why would the flow become non-laminar if the ball approached the walls of the tube in the viscosity experiment?
A
If the ball approaches the wall, the fluid between them will accelerate, decreasing its pressure due to Bernoulli’s principle. This means the lines of fluid flow would not be parallel and therefore would not be laminar.
20
Q
Why are steel balls favoured in the viscosity experiment?
A
They are magnetic and can be removed with a magnet.
21
Q
Why is it important to have two timed regions for the viscosity experiment?
A
- It allows you to make sure the ball is going at terminal velocity.
- It allows you to take multiple readings at once.
22
Q
What is the equation used to calculate viscosity in the experiment?
A
n = (2r²g( pb - pf))/(9v)
23
Q
What’s the unit of viscosity?
A
Pascal-seconds (Pas)
24
Q
How can accuracy be improved in the Young’s modulus experiment?
A
- Use a longer wire and have a larger distance between the paper markers to reduce percentage uncertainty.
- Use reference markers and a set square to avoid parallax error.
- Ensure that all necking is finished before taking a reading.
- Apply a pre-stress to the experiment to straighten out the wire before beginning.
- Choose an appropriate wire diameter. Too thin a wire will stretch too quickly, passing the elastic limit; too thick a wire will result in too small extensions and therefore a higher uncertainty.
25
What are the safety precautions used in the Young's modulus experiment?
- Wear safety goggles.
- Use paper tents at several points along the wire in case it breaks.
- Don't stand with your feet below the masses in case the wire snaps. Put a padded bucket below the masses.
26
What variables should be controlled in the Young's modulus experiment?
- Temperature, as metals undergo thermal expansion.
27
What is Searle's method for measuring Young's modulus, and why is a comparison wire used?
- In Searle's method, the wires are hanging fixed points, side by side.
- Each wire has a scale attached: the comparison wire has the main scale, and the test wire has a vernier scale.
- Weights are added to the test wire, and its extension is measured against the main scale.
- This means that any environmental factors that cause the wires to change in length will be accounted for, as the test wire is measured against the comparison wire, which would have also changed.
28
How do you set up the the speed of sound experiment?
- Set timebase on oscilloscope to 100 ms/cm, and y-gain 0.1 volts/cm
- Connect microphone to input on oscilloscope, activate second beam mode
- Place microphone in front of the speaker and set signal generator to 1000Hz
29
How do you perform the speed of sound experiment?
- Set up the experiment with the oscilloscope reading from the speaker + signal generator and microphone, and the speaker an microphone next to each other at the start of a meter rule.
- Move the microphone away from the speaker until the peaks and troughs line up on the oscilloscope, and record the wavelength.
- Repeat this as many times as you can within the length of the meter rule, dividing by how many waves are between the speaker and microphone to find the wavelength of one wave.
- Repeat this for different frequencies: use 1000Hz, 2000Hz and 3000Hz
- Plot wavelength on the y and 1/frequency as the x, then the gradient is the speed of sound.
30
What are the sources of uncertainty in the speed of sound experiment?
- The resolution of the oscilloscope.
- The resolution of the meter rule.
- The thickness of the lines on the oscilloscope.
31
Why should the frequency be measured with the oscilloscope instead of from the signal generator?
The oscilloscope has a greater resolution.
32
What equation is used for the vibrating string experiment?
v = fλ = √(T/µ)
33
How can a standing wave be made in a piece of string?
The string should be held under tension, with one end hanging over a pulley, and the other end attached to a vibration generator connected to a signal generator.
34
In the vibrating string experiment, what piece of equipment could be used to change the length of the string?
A bridge (triangular-prism-shaped object).
35
Why should a signal generator be run for a few minutes before use?
The frequency needs time to stabilise.
36
What's the relationship between the length of the string used and the frequency of the wave in the vibrating string experiment?
Inversely proportional.
37
What's the equation used to calculate the frequency of the first harmonic of string?
f = √(T / µ) / 2l
38
What safety precautions should be taken in the vibrating string experiment?
- Wear safety goggles in case the string snaps.
- Don't stand under the hanging masses, and ideally place a padded bucket below them.
39
What uncertainties are present in the vibrating string experiment?
- The blur of the nodes.
- The calibration of the signal generator.
40
What is important for the setup of a double slit experiment to determine wavelength?
- The same source should be used so waves are coherent.
- The slits should be small so that more diffraction occurs.
- The slits should be close to each other so that the waves interfere.
41
What adjustment should you make if using white light for the wavelength experiment?
Use a colour filter to isolate one colour, as different wavelengths in white light will diffract different amounts.
42
If using a double slit experiment to determine wavelength, what formula should be used?
λ = ax / D
- a is the distance between two adjacent wave sources
- x is the distance between two adjacent antinodes
- D is the perpendicular distance between the screen and the sources
43
How can we improve the accuracy of the double slit wavelength experiment?
- Perform in a darkened room.
- Increase distance between screen and sources, so node distance increases and percentage uncertainty decreases.
- Measure the distance between all antinodes and find the mean.
- Use a colour filter if using white light.
44
What equation is used for diffraction grating?
nλ = dsinθ
where d is the slit width
45
What safety precautions should be taken when using lasers?
- Don't look directly at the beam.
- Ensure there are no reflective surfaces in the laser's vicinity.
- Display a warning notice so others know the laser is in use.
- Use a matt screen.
46
What does it mean for sources to be coherent?
- They have the same wavelength.
- They have a constant phase difference.
47
Suggest a use for diffraction grating.
- Can be used instead of prisms to analyse spectra.
- Can be used in X-ray crystallography to analyse crystal structure by looking at the diffraction patterns of x-rays passing through crystals.
48
Describe the light coming from a laser.
- Coherent
- Monochromatic
- Collimated (consists of parallel layers)
49
Describe the experiment used to investigate change in momentum.
- Trolley on a ramp with two sets of light gates: one near the start, and one near the end.
- Attach weights on a string over a pulley to the end of the trolly.
- While keeping the mass of the system M constant, change the force being applied to the system by altering weight distribution.
- Measure the velocities at the two light gates and the time difference between them.
- Calculate each momentum as Mv1 and Mv2, then find change in momentum.
- Plot ∆P/∆t against F, and this should produce a straight line relationship.
50
How can friction be accounted for in the trolley experiment?
Use a slope or an air track.
51
How can you do the trolley momentum experiment with just one light gate?
- Measure the time it takes to reach the light gate with a stopwatch.
- Release the trolley from the same position each time, from where the top of the mass hanger just touches the pulley.
- Measure this distance.
- Take starting velocity as zero.
- If time, not velocity, is being recorded on the light gate, take light gate time as t, velocity as L/t and time to travel distance as T.
52
For the tracking software, what should you consider when choosing what balls to use, and where to roll them?
- Balls should be opaque
- Ball colour should contrast the colour of the surface you are rolling them on.
- Surface should be smooth.
53
How should you set up your circuit for the capacitor discharge experiment?
- Have a switch that either completes the capacitor charging setup or the capacitor discharging setup.
- Each side of the circuit should have an ammeter and resistor in series with the capacitor, and there should be a voltmeter over the capacitor.
- One side should be connected to a D.C. power supply.
54
What safety precautions should be taken for the capacitor discharge experiment?
- Ensure that the capacitor is connected in the right direction in the circuit, as some capacitors are polarised. If you don't do this, it may overheat and explode.
- Ensure that the potential difference of the supply is safely below that of the maximum potential difference the capacitor can take, to reduce the risk of the capacitor exploding.
- Ensure that the potential difference is low in general, to reduce the risk of an electric shock.
55
How can you increase accuracy in the capacitor discharge experiment?
- Use a higher resistance resistor, as this will slow the discharge and allow for more accurate readings.
- Repeat the discharge 3 times and find the mean of the readings (assuming you use the same time intervals).
56
How can you verify if your capacitor discharge experiment has worked?
Use your graph to graphically calculate the time constant (the time at which the voltage is 1/e of the original voltage). Then, calculate the time constant by doing τ = RC, and compare these values.
57
What graph can you plot to obtain the time constant of a capacitor discharge?
lnV = lnV0 - (1/RC) t
Then, the time constant RC is the negative reciprocal of the gradient.
58
What is the benefit of doing a trial run of the capacitor discharge experiment before you collect data?
- So that you can determine an appropriate resistor to use so that the capacitor discharges slowly.
- So that you can choose a suitable time interval to take measurements at.
59
How can accuracy be ensured for the thermistor calibration experiment?
- Use a small current and switch off the circuit between readings to avoid additional heating.
- Read the thermometer at eye level to avoid parallax.
- Choose a suitable resistor so that the voltage variance is sensible.
- Make sure temperature change is slow.
60
What safety precautions should be taken for the thermistor calibration experiment?
- Do not handle the beaker when hot.
- Use a safely low voltage to avoid shocks.
- Do not exceed voltage limit of thermistor.
- Keep leads away from the beaker to avoid any melting.
- Support the thermistor to prevent it from tipping the beaker over.
61
What formula is used to calculate the resistance of a negative temperature coefficient resistor?
R = R0 e^(b/T)
62
Why should temperature changes be gradual?
The thermistor and thermometer will have thermal inertia, so if the water is heated or cooled too quickly, the temperature of the thermometer and thermistor won't be the same.
63
What safety precautions should be taken when using a bunsen burner?
- Hair and loose clothing tied back.
- Heatproof mat to protect work bench.
- Safety flame when not heating water.
- Avoid touching non-insulated regions.
64
What graph should be plotted for the thermistor calibration curve?
lnR = b(1/T) + R0
so plot lnR against 1/T
65
How can you ensure accuracy when measuring the changing temperature of a beaker of water.
- Stir occasionally.
- Use gradual temperature changes.
- Take readings from eye level to avoid parallax error.
66
How do you measure the specific latent heat of fusion of water? (Immersion heater method)
- Two setups: a funnel of ice suspended over a beaker on scales; both with an immersion heater, but only one is turned on.
- Turn on the immersion heater, measure the voltage and current through it, and leave it running for five minutes.
- Calculate work done as W = IVt.
- Measure the change in mass of water melted through the funnel in the control beaker with no power input, and subtract this from the mass from the beaker with the powered immersion heater.
- Use Q = mL to calculate L, where Q is total work done, and m is change in mass, excluding the water melted due to heating from environment.
67
What is the uncertainty of the temperature readings in the specific latent heat experiment?
U = ±1ºC (as the uncertainty for each thermometer in the powered experiment and control experiment is ±0.5ºC, and they are summed together).
68
What safety precaution should be taken with experiments involving liquids?
- Clean up immediately after any spills.
- If the liquid can be harmful, wash hands immediately if you come into contact with it.
69
If cooling from room temperature is not taken into account, what can you expect from your value for specific latent heat?
Calculated heat entering the system will be too low, so the value for L will be smaller (Q = mL).
70
What can you do to improve the accuracy of the latent heat of ice experiment?
- Insulate the beaker
- Use crushed ice (so it melts quicker and less heat is absorbed from surroundings
- Make sure the ice is already melting when you start the experiment, so you know that it is at 0ºc and not below
- Don't add too much ice, as it will take longer to melt
71
What are the necessary components of the compressed air experiment?
- Tube of fixed dimension, with volume measurements on it
- Fixed mass of air in tube
- Pump that increases pressure by pumping oil into the tube.
- Pressure gage
72
What do you plot in the compressed air experiment?
- Plot pressure against 1/volume and look for a linear relationship
- P = nRT(1/V)
73
How can you ensure accuracy in the compressed air experiment?
- Note that the pressure gage measures pressure difference, so add on atmospheric pressure
- Use the pump slowly so that you aren't increasing the temperature too much
- Take measurements from eye level to avoid parallax error
- Measure the oil height from the bottom of the meniscus (curve on the oil's surface)
74
What safety precautions should be taken for the compressed air experiment?
- Clamp apparatus to desk
- Press vertically downwards on pump
- Wear safety goggles, as you are working with high-pressure tubing that might be unstable
75
What is Boyle's law?
When kept at a constant temperature, a fixed mass of an ideal gas will have a volume and pressure that are inversely proportional; pV is constant.
76
What is the standard value for atmospheric pressure?
101kPa
77
Why wouldn't it be suitable to take volume measurements for very low pressures?
Some oil may cling to the sides of the tube.
78
Describe the procedure for the gamma ray absorption experiment.
- Measure background radiation before the source is in place
- Geige counter facing a gamma source, all clamped in place, with an aluminium sheet in front of the source to absorb any alpha or beta radiation
- Using a micrometer, find the average thickness of each lead sheet by measuring each one at multiple places
- Measure the count rate, adding a sheet of lead clamped in place between the Geige counter and source each time
79
How do you interpret the results of the gamma ray absorption experiment?
- Plot count rate against lead thickness
- Plot a curve of best fit
- Find multiple values for 'half thickness' and calculate an average
80
What are the safety precautions for the gamma absorption experiment?
- Use a warning sign so that others know a radioactive source is in use
- Keep the gamma source in the lead box when not in use
- Handle the gamma source with tongs
- Keep distance from gamma source, always at an arms length from your torso (inverse square relationship makes it much safer with distance)
- Do not have it point at anyone
- Make sure there is an aluminium sheet in front of the source throughout the experiment
- Minimise contact with lead and wash hands after use
81
How can you ensure accuracy in the gamma absorption experiment?
- Remember to take the background radiation measurement
- The aluminium sheet means that you don't record beta radiation for you zero sheets of lead measurement
- Record count rate for a significantly long time, as decay is random
- To make results clearer, you can plot a recession line by taking natural logarithms.
82
Why is Cobalt-60 a suitable source for the gamma absorption experiment?
- It has a half life of 5 years, so it can be used for a good few years before it needs to be replaced.
- The activity is low enough to be safe, but high enough to produce good results
83
What preliminary experiment could be taken for the gamma absorption experiment?
You could investigate the maximum thickness of lead for which you still get a reasonable count rate.
84
Describe the nature of radioactive decay.
Radioactive decay is random (meaning you can't predict when it will decay) and spontaneous (meaning you can't influence when it will decay).
85
If measuring thickness, how can you reduce %U?
Measure a stack and divide by n.
86
What is a fiducial marker?
A reference point in an experiment (e.g. a fixed pin used for measuring oscillations against)
87
What are the benefits of using a data logger?
- Synchronous readings
- More readings can be taken faster/ higher sampling rate
88
What is an important thing to check when recording data?
- Make sure that the data being recorded is what's actually being observed in the experiment.
- E.g. Don't just record the extension if the equilibrium position isn't at 0 on the metre rule.
89
What is meant by error and uncertainty?
- Error is the difference between the measured result and the true value.
- An uncertainty is the interval/range in which the true value can be considered to lie.
90
Information suggests that the readings taken may be too high or too low. How might this affect your results?
- Refer to the actual MEASUREMENTS, e.g. voltage could be recorded as too high or too low.
- This could cancel out, or it could result in a systematic error, which wouldn't affect the gradient.
91
When measuring resistivity, why do you keep the same portion of wire in the circuit at all times, instead of physically cutting the wire so it's shorter each time?
By keeping the same amount of wire in the circuit the same, the total resistance is the same, so current is constant (which is important if not using an ohmmeter).
92
How can you criticise results?
- Incorrect units
- Incorrect number of decimal places
- Range too small
- Too few values within range
- No evidence of repeats
