SAQs Flashcards
Describe what is meant by accuracy and precision (2)
- Accuracy is how close a measured value is to the true value
- Precision is the consistancy of values obtained by repeated measurements
The temperature of the air in a room is measured using a mercury-in-glass thermometer.
Describe how the value for the temperature may be precise but not accurate. (2)
- Repeat readings can give very similar measurements so value precise
- Value is not accurate because of a systematic error
“If the distance from the lens to the screen is doubled, the brightness of the image is halved.”
Assess the validity of this statement. (2)
- I is proportional to 1/x2
- So intensity fall to one quarter so statement is incorrect
Explain why magnetic levitation is an advantage for a high-speed transport system. (2)
- Reduces frictional forces
- Reduces work done against friction
Explain why the astronauts in the ISS have an apparent weight of zero. (2)
- Astronauts are in free fall in the space craft
- So there is no contact force acting on them
State what is meant by simple harmonic motion. (2)
- Acceleration is proportional to displacement from equiolibirum position
- and acting towards the equilibrium position
It is observed that electrons, with energy of 5.5 keV, are diffracted as they pass through the thin gold foil.
Explain a conclusion about the electrons that can be made from this observation. (3)
- Electrons are behaving like waves
- Wavelength = h/p
- Electron wavlength must be similar to the atromic spacing in the foil
In curling:
Explain how sweeping the ice directly in front of the stone makes it travel further. (2)
- Smooths out/melts the surface
- So frictional forces are reduced
Explain an advantage of using a phone camera rather than a stopwatch.
- Won’t be reaction time errors
- so the uncertainty will be reduced
OR - The recording can be replayed
- this may lead to a more accurate value
Describe the procedure to determine an accurate value for the distance between the centres of adjacent heaps of sand. (3)
- Measure over at least 3 heaps
- Divide by the number of gaps between heaps
- Repeat and calculate average
A student measured the diameter of the steel wire and obtained a value of 2.52mm.
Explain which instrument he used to measure the diameter. (2)
- Micrometer Or DIGITAL calipers
- Because the value indicates a resolution of 0.01 mm
Explain one advantage of locating the space station in a geostationary orbit. (2)
- Satellite would always be above the same point on the Earth’s surface
- So that contact would be maintained at all times
“The balance could read to the nearest 0.01g, which makes my values for the magnetic force both accurate and precise.”
Comment on this statement. (3)
three of four
- high precision means a small spread of values
- high accuracy means close to the true vale
- the students is referring to the resolution
- Should have said: There was a low uncertainty in the value of the force OR a high resolution doesn’t garantee accuracy/precision
In an A.C. circuit:
Another student suggested that a voltmeter would be more accurate than using an oscilloscope to determine the magnitude of the p.d.
Comment on this suggestion. (3)
three of five
- Voltmeter must measure alternating p.d.s OR voltmeter would indicate zero for a.c.
- A.C. voltmeter would give an r.m.s. p.d. directly
- Voltmeter may draw current and affect the circuit
- Accuracy would depend on the calibration of the voltmeter
- A voltmeter would give better resolution than measuring trace height on an oscilloscope
- All stars are in the main sequence
- In an older cluster, there would be red giants
When determining the period of oscillation for each mass, the student measured the time for 20 oscillations. He repeated this measurement to obtain a mean time for 20 oscillations.
Explain how the student’s procedure contributed to the accuracy of the measurement (3)
- Timing a large number of oscillations minimised percentage uncertainty
- Repeating each measurement and calculating a mean minimised the effect of random errors
- Taking a repeat measurement allowed a check for gross timing errors
In a diffraction experiment, the light from the laser pointer was not quite perpendicular to the screen.
Explain how this would change the diffraction pattern produced on the screen. (3)
- Maxima on one side move closer to the central maximum
- Maxima on the other side move further away from the central maximum
- Intensity of maxima would be different on each side
Describe how the student should determine the extension of the spring as accurately as possible. (3)
- Calculate mean value of position of mass by taking loading and unloading positions of mass
- Use a pointer on mass Or read at eye level Or Use a set square to read from metre rule Or bring metre rule as close as possible to mass
- Check the metre rule is vertical Or fix in position
Explain how she should have measured the thickness of the slides in order to minimise the percentage uncertainty. (2)
- Measure the thickness of a stack and divide by the number of slides
- Uncertainty would be the same, but measurement would be larger
Assume that no energy is transferred from the chicken to the surroundings.
Explain whether the assumption is realistic. (2)
- There is no way to insulate the chicken Or there will be a temperature difference with the surroundings
- So the assumption is unrealistic
Both microphones were initially positioned the same distance from the loudspeaker. The two signals were in phase on the oscilloscope screen. The student slowly moved microphone 2 towards the loudspeaker, until the two signals on the oscilloscope were in phase again. He then measured the distance d between the microphones to determine the wavelength λ of the sound waves.
(a) Comment on the student’s experimental technique to determine λ.
- Should have moved the microphone over more in-phase positions
- This would reduce the uncertainty
OR - Move microphone between antiphase positions
- Easier to judge when waves are in antiphase
Explain how the principle of conservation of energy applies to the inelastic collision between a projectile and a block.
- Total energy is constant, buy KE decreases
- Projectile does work on block Or internal energy of block increases
Part (ii)
- The effective resistance of the combination is less than the lamp
- the combination has a smaller fraction of the total resistance
- Hence a smaller fraction of the p.d. falls across the lamp
Criticise the method (2)
- First rubber band is too near the surface
- No method for checking the ball is at terminal velocity
- Number of charge carriers in the thermistor decreased
- resistance of the thermistor increased
- A larger fraction of supply p.d. is across the thermistor
A student measured the diameter d of the wire using a micrometer.
Explain one technique the student should use when measuring d. (2)
- Take readings in different positions/orientations along the wire
- As diameter may not be uniform
OR - Check for zero error
- Zero error reduces the accuracy
Diffraction grating experiment:
“The value of x was measured with a metre rule. A metre rule has a precision of 0.1cm, so this value was determined with a high degree of accuracy.”
Comment on this conclusion. (3)
- Has a RESOLUTION of 0.1 cm (not precision)
- Precision is how closely grouped readings are
- Accuracy is how close the measured value is to the actual value
- Systematic error may shift values away from the actual values (e.g. zero error or parallax)
Battery has internal resistance.
Explain how the brightness of bulb 1 changes when the switch is closed. (3)
- Current (from battery) increases
- Therefore terminal p.d. decreases
- so brightness of bulb 1 decreases
OR - The resistance of the (external) circuit decreases
- So there is a smaller proportion of p.d. across the bulbs
- So brightness of bulb 1 decreases
- Ammeter should be in series with the heater
- The range of the ammeter (mA) is too small
Both parts
(i)
- Changes due to the support bending Or temperature are minimised
Or allows use of a vernier scale
(ii)
- Keep the reference wire taut/straight
Measuring extension against force:
Explain why the test wire should be both long and thin. (3)
- Thin wire has a large stress
- A long/thin wire has a large extension
- Hence percentage uncertainty is reduced
