Physics paper 3 (practical) A-level (AQA)

name the SI base units

metre - m

kilogram - kg

second - s

ampere - A

kelvin - K

mole - mol

candela - cd

what are random errors, how do they arise and how can they be reduced?

they cause readings to randomly fluctuate about the mean affecting precision

due to observational or reading errors

or due to the environment e.g varying temperature or supply voltage

you cannot reduce random error but you can reduce their effect by taking repeat readings and taking an average

what are systematic errors, how do they arise and how can they be reduced?

systematic errors cause each reading to be consistently wrong, cause every result to differ from the true value by the same amount affecting accuracy

due to:

-instrument error: the instrument has not been calibrated properly

-reading error: for example parallax error that will affect all readings

-poor experimental design: for example ignoring the affect of an external factor

it is hard to spot if they are there in the first place

but you can:

-redo the experiment with a different method

-you can calibrate your apparatus which will reduce zero errors

what does it mean if a result is repeatable and reproducible?

repeatable- if the experiment is repeated by the experimenter an gives consistent results

reproducible- if other experimenters can get similar results with different methods

what do precision and accuracy tell you?

precision- tells you whether the results are numerically close together

accuracy- how close a result is to the true value

how can you increase the percentage uncertainty and thus the precision of repeatable events?

measure the time over multiple events

thus sharing the absolute uncertainty across a greater overall reading so the percentage uncertainty will decrease.

how do you obtain the overall uncertainty if you are adding or subtracting quantities?

you need to add their absolute uncertainties

how do you obtain the overall uncertainty if you are multiplying or dividing quantities?

you need to add their percentage uncertainties

how can you overcome the uncertainties for each measurement on a graph and how do you decide what the gradient is

uncertainty in each measurement- use 'error bars' and draw a best fit line that goes through all the error bars

uncertainty in best fit line- draw two lines of best fit one with the maximum gradient going through all error bars and the other with the minimum gradient going through all error bars.

-find the range of the two gradients and divide by two for the absolute error in the gradient

what are valid results?

a valid result arises from a suitable procedure to answer the original question, keeping all the other variables controlled bar the one that you are investigating

what areas must you look at when evaluating a method?

-whether all the variables are controlled, if not how could they be controlled

-whether any anomalous results have crept into the data and could have been prevented

-whether the uncertainty could have been reduced (repeated readings, using appropriate equipment with higher resolution, calibrating apparatus)

RP01

investigating the variation of the frequency of stationary waves:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-check that the string is horizontal, this can be done with a spirit level or a set square

-ensure the independent variable is varied over as wide a range as possible and that all other variables are controlled

-repeat readings as there is an element of judgment as to when the standing wave is produced, also fine adjustments of the signal generator are needed to get the maximum ensure you've got very close to the maximum amplitude

RP01

investigating the variation of the frequency of stationary waves:

what are the hazards and how can they be reduced?

-the stand may fall over and cause damage and injury

so a counterweight can be used to increase stability

RP01

investigating the interference effects:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-Make sure that the screen and double slit are aligned perfectly normal to the laser to avoid parallax error in the measurement of the fringe width. A set square can be used in conjunction with the ruler.

-use a vernier calliper to measure the width of fringes this will decrease the absolute error

-measure across multiple fringes to decrease the percentage error when determining the fringe width

-measure the slit separation using a travelling microscope

RP01

investigating the interference effects:

what are the hazards and how can they be reduced?

laser beams diverge very little so intensity does not decrease much with distance so:

-do not shine at people especially near their eyes as it could cause blind spots on the retina

-only have it on for as long as necessary

-have a hazard sign up

-user laser protection goggles

RP03

determining the acceleration due to gravity:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-The ball bearing should be dense to help mitigate the effects of air resistance.

-To reduce parallax error in measuring the height, the ruler can be clamped directly

next to the light gates.

-readings repeated for each value of s and ensure a wide range of values for the distance the ball drops

RP03

determining the acceleration due to gravity:

what are the hazards and how can they be reduced?

-the stand may fall over and cause damage and injury

so a counterweight can be used to increase stability

RP03

determining the acceleration due to gravity:

how do you determine g from your results?

the equation is

s=ut + 0.5gt^2

so plotting s on the y axis and t^2 on the other axis

will give a gradient of 0.5g

RP04

measuring the Young's modulus of a wire:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

measuring the stress:

-when determining the cross-sectional area of the wire, measure the diameter using a micrometer in several places and take an average

-when finding the initial length add the smallest weight necessary to straighten the wire to determine the initial length

-use a long wire as the extension will be greater to decrease percentage error

-use Searle's apparatus as it includes a control wire so that all the effects on extension due to external factors such as heat do not affect the experiment

RP04

measuring the Young's modulus of a wire:

what are the hazards and how can they be reduced?

If the wire breaks, the masses could fall and cause injuries, so a sand tray should be

placed beneath them to catch them.

RP05

determining the resistivity of a wire:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-measure the diameter in several places along the wire with a micrometer and take an average

-ensure there are no kinks in the wire to ensure the length measured is as accurate as possible

-repeat the process several times as there will be a random error in reading the measurement for the length of the wire each time you move the flying lead

-The wire heating up might additionally cause the resistance of the wire to change,

affecting measurements. To reduce this, disconnect it in between measurements or

reduce the voltage of the supply so the current is lower.

RP05

determining the resistivity of a wire:

what are the hazards and how can they be reduced?

-Disconnect the crocodile clips in between measurements to avoid the wire heating

up and causing burns if touched. If the current rises too high, reduce the voltage

using the variable power supply.

RP05

determining the resistivity of a wire:

how do you determine the resistivity of the wire from the results?

plot the graph of length on the x axis and residence on the y axis

gradient will be equal to R/L

this equal p/A

so multiply the gradient by the cross-sectional are of the wire

RP06

investigating the emf and internal resistance of a cell:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-the circuit should be switched off between readings because:

*the current will tend to heat up the internal resistor and change its resistance

*the battery may begin to run down , which would increase its internal resistance

-Use fairly new batteries/cells because the emf and internal resistance of run down batteries can vary during the experiment.

RP06

investigating the emf and internal resistance of a cell:

how do you determine the emf and the internal resistance of the cell?

alter the resistance of the circuit with a variable resistor

plot current of the circuit on the x axis and voltage across the cell on the y axis

the y-intercept will equal the emf of the cell

the gradient will equal the NEGATIVE of the internal resistance

RP07 part 1

investigation into simple harmonic motion using a mass-spring system:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-f the spring starts to move horizontally during its oscillation, stop the oscillation and start it again making sure it is pulled vertically downwards.

-The fiducial marker should be at the equilibrium position so the mass is moving past it at the fastest speed and there is the least uncertainty in starting and stopping the stopwatch.

-A motion tracker and data logger can be used to find a more accurate value for the time period and eliminating random error of the stopwatch.

-measure the time for a large number of oscillations to then determine time period this will reduce parentage error

RP07 part 1

investigation into simple harmonic motion using a mass-spring system:

how do you determine the value for k from the results?

plot mass on the x axis and T^2 on the y axis

the gradient will be (4pie^2) / k

RP07 part 2

investigation into simple harmonic motion using a simple pendulum:

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-The fiducial marker should be at the equilibrium position so the mass is moving past it at the fastest speed and there is the least uncertainty in starting and stopping the stopwatch.

-A motion tracker and data logger can be used to find a more accurate value for the time period and eliminating random error of the stopwatch.

-measure the time for a large number of oscillations to then determine time period this will reduce parentage error

-The angle should not be more than 15° at maximum amplitude

RP07 part 2

investigation into simple harmonic motion using a simple pendulum:

how do you determine the value of g from the results?

plot length on the x axis and T^2 on the y axis

the gradient will be (4pie^2) / g

RP08 part 1

investigating pressure and volume relationship (Boyle's law):

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-when measuring the height of the oil in the tube ensure that the line of sight of the eye is at the same level as the meniscus in order the reduce parallax error

RP08 part 1

investigating pressure and volume relationship (Boyle's law):

what are the hazards and how can they be reduced?

-if the method includes pressurising a glass container ensure you wear goggles and don't use too high ma pressure

RP08 part 2

investigating volume and temperature relationship (Charles' law):

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?

-The tube needs to be perfectly clean with no traces of other chemicals to prevent the thread of sulfuric acid from splitting.

-ensure you stir the water before you take the measurement to ensure the temperature recorded is more accurate

RP09 part 1

investigating the variation of capacitor charging current with time:

how do you determine the relationship between current and time and what will be the time constant?

you can use a log-linear graph to confirm the exponential relationship between the two

plot time on the x axis and ln(current) on the y axis

you will get a straight line showing that it is exponential

the time constant = -1/gradient

RP09 part 2

investigating the variation of capacitor discharging current and pd with time:

how do you determine the relationship between current or pd and time and what will be the time constant?

use a log-linear graph to confirm that it is an exponential relationship between any of the variables and time

plot time on the x axis and ln(current or pd) on the y axis

you will get a straight line showing that it is exponential

the time constant = -1/gradient

RP10 part 1

investigation of how he force on a wire varies with current using a top pan balance

why does the reading on the top ban scale increase when current is flowing through the wire?

the wire is securely clamped and cannot move

the field exerts an upward force on the wire so the wire exerts an equal and opposite force downwards (N3L)

this downward force makes the reading on the scales increase

RP10 part 1

investigation of how he force on a wire varies with current using a top pan balance

what is the relationship with current and force and why is this?

as current increases force increases

because F=BIL and as length and magnetic flux are constants F is proportional to the current

RP10 part 2

investigation of how the force on a wire varies with length using a top pan balance

how do you vary the length of the wire in the magnetic field?

-use a number U-shaped wires with different distances between the two parallel sections

RP10 part 2

investigation of how the force on a wire varies with length using a top pan balance:

what are the hazards and how can they be reduced?

large currents are required so the bare copper wires will get hot and could cause burns so do not touch the wire.

RP11

investigating flux linkage and search coil

what are some good practical techniques for this experiment to ensure the experiment produces reliable results?