Core Practical Flahcards

What is meant by free-fall?

An object is said to be in free-fall if the only force acting on it is its own gravity. This means that negligible resistive forces are acting (small enough that they can be considered to be effectively zero).

What is 'g'?

Gravitational Field Strength (in our case, on the surface of Earth)

Why can the SUVAT equations be used in this experiment?

The SUVAT equations can be used since the object will fall with uniform acceleration. This is because the force of gravity is approximately constant at the Earth's surface.

When plotting a graph of t² against h, how is 'g' determined?

The gradient of the graph will be t²/h. Consequently, the acceleration ('g') will be equal to 2/gradient. This comes from the equation s=ut + ½at², where s=h, a=g and u=0.

When plotting a graph of v² against h, how is 'g' determined?

The gradient of the graph will be v²/h. Consequently, the acceleration ('g') will be equal to half the gradient. This comes from the equation v²=u²-2as, where s=h, a=g and u=0.

Describe how an electromagnet system can be used to determine 'g'.

A magnetic ball bearing can be released by an electromagnet clamped at a known height. The timing system starts when the electromagnet is switched off, and the timer is stopped when the ball lands on the finish pad.

When using a clamp stand in this experiment, what safety precaution should be taken?

The clamp stand should have a counterweight or G-clamp attached to its base to provide a moment to prevent it from toppling over.

What safety precaution should be taken when using an electromagnet?

Electromagnets heat up over time. To reduce this heating effect, you should switch it off when not in use.

Suggest how light-gates could be positioned to ensure that the ball or dowel falls directly through them.

A plumb line could be used to demonstrate the expected path of the object. This allows the light-gates to be positioned in appropriate places, so that the ball will fall through them.

Why is it advantageous to use a small ball-bearing over a larger ball?

The effect of air-resistance is lesser on a smaller ball bearing. Therefore, our assumption that the effects of air resistance are negligible is more valid if a smaller ball-bearing is used.

Why should there be a gap between the release position and the first light-gate?

There should be a gap to ensure that the time over which the ball is passing through the light gate is negligible (the ball is moving sufficiently quickly at the light gate).

Explain why this experiment would not be valid if the air resistance acting on the ball wasn't negligible.

The ball wouldn't be in free-fall since the acceleration would not be purely due to the force of gravity. The acceleration would also be variable since air resistance increases with speed, and so the uniform acceleration equations couldn't be used.

Suggest why your obtained value of 'g' may not be the same as the accepted value.

- Delays in the timing equipment (if using a stop clock, this will be human reaction time)

- Resistive forces are acting

- Errors in height measurements, such as measuring from different positions on the ball each time.

What is the advantage of using light-gates over a stop-clock in this experiment?

Using light-gates should result in a lower uncertainty in your time measurements. Using a stop-clock would involve human reaction times and would thus create additional uncertainty in timing accuracy.

How could your results be improved?

You should take repeat readings at each height and then calculate the mean time taken, from all non-anomalous results. You should also ensure that height measurements are taken from the same point on the ball every time.

How should you calculate the uncertainty in your time readings?

The uncertainty in time can be considered to be equal to half the range of your time readings, measured for each height. This can then be converted into a percentage uncertainty.

How do you determine the percentage uncertainty in t²?

To calculate the percentage uncertainty for a variable that is squared, you should double the percentage uncertainty of the variable itself. In this case the percentage uncertainty in t² is double the percentage uncertainty in t.

When plotting a graph how should you determine the scales for the axes?

The scales should be chosen so that the graph fills at least half the available space. Using numbers that split easily into the squares on page (such as multiples of 5) will also make plotting simpler.

What is the minimum number of repeat readings you should take in this experiment?

You should take at least 3 repeat readings at each height. This allows for anomalous results to be more easily identified.

What is the equation used to convert an uncertainty into a percentage uncertainty?

Percentage Uncertainty = (Uncertainty/Mean Value) x 100%

How can the percentage difference between your value of 'g' and the accepted value be calculated?

[(Your Value - 9.81)/9.81] x 100%

Would you expect your value of 'g' to be greater or lower than the accepted value?

You will most likely obtain a value that is lower than the accepted value, due to air resistance reducing the downwards force acting on the object.

State the equation used to calculate the resistivity of a wire.

ρ = RA/l

How does the resistance of a wire change when the cross-sectional area decreases?

The resistance of a wire is inversely proportional to the cross sectional and so as the area decreases, the resistance will increase.

How does the resistance of a wire change when the length is decreased?

The resistance of a wire directly proportional to the length of the wire, so as the length decreases the resistance decreases proportionally.

How does the resistance of a wire change if the resistivity is increased?

The resistance of a wire is directly proportional to the resistivity, so as the resistivity increases, the resistance also increases.

What is the unit of resistivity?

Ωm

Ohm - Metres

How do you measure the cross-sectional area of a thin wire?

Using a micrometer, measure the wire's diameter in at least three different places along the wire. Use the average diameter in the circular area equation.

Suggest how the length of conducting wire can be varied when carrying out this experiment.

One end of the wire can be fixed and the other end can be connected to the circuit using a crocodile clip. The length of conducting wire can be changed by varying the position of the crocodile clip.

What device is used to measure the potential difference across the wire, and how is it connected?

A voltmeter, which should be connected in parallel across the wire.

What device is used to measure the current across the wire, and how is it connected?

An ammeter, which should be connected in series with the wire.

Why should the power supply be switched off between readings?

The temperature of the wire should remain constant throughout the experiment. Switching the power supply off between readings will mitigate heating of the wire, during the experiment.

Why should the temperature of the wire remain constant throughout this experiment?

Temperature changes can affect the resistance of the wire. In this experiment, temperature is a control variable.

Why does the resistance of a wire increase when its temperature increases?

As temperature increases, the metal ions gain more kinetic energy and so vibrate more. These vibrating ions make it harder for charges to pass through the wire and so the wire's resistance increases.

How can the resistivity of a wire be determined from a graph of resistance against length?

The gradient of the graph will be R/L and so by multiplying the gradient by the wire's cross-sectional area, you will obtain the wire's resistivity.

Why should the current used in this experiment be kept low?

As current increases the temperature of the wire will increase. By keeping the current low, the heating effect on the wire is kept to a minimum.

Suggest how you could ensure that your length measurements are taken from the same position each time..

A metre ruler could be taped in place below the fixed wire.

What factors lead to uncertainties in this experiment?

There will be resistance between the crocodile clips and wire as well as at the contact of the leads and the power supply. There may also be a zero error due to the positioning of the ruler and crocodile clip at the zero end.

What device could replace the voltmeter and ammeter in this experiment?

Instead of a voltmeter and ammeter, a multimeter could be used to measure the current, potential difference and resistance. Note this may lower the resolution of your data depending on the number of significant figures provided by the devices you have available

What is a source's internal resistance?

A source's internal resistance is the resistance of the materials inside the source. It is equal to the lost volts per unit current in the source.

What is the EMF of a power source?

A power supply's EMF is the work done by the source per unit charge. It is equal to the potential difference across the source when no current flows.

State the equation used to calculate a battery's EMF from its current, load resistance and internal resistance.

ε = I ( R+ r )

R: Load Resistance

r: Internal Resistance

What is meant by the phrase 'lost volts'?

The lost volts of a battery is the difference between the battery's EMF and its terminal potential difference.

How do you calculate a battery's EMF from its terminal potential difference, current and internal resistance?

ε = V + Ir

Demonstrate how the equation of a V-I graph, for a source with internal resistance 'r' can be obtained.

ε = I (R + r) V= I R

ε = V + Ir

V = ε - Ir

V = -rI + ε

Which is in the form y=mx+c

How can you find the internal resistance from a graph of V against I?

V = -rI + ε

y = mx + c

The internal resistance is the positive equivalent of the gradient of the graph.

How can you find the EMF of battery from a graph of V against I?

V = -rI + ε

y = mx + c

The EMF is the y-intercept of the graph

Why should the battery be disconnected between readings?

The temperature of the circuit should remain constant throughout this experiment, so as not to affect the resistance. Disconnecting the battery when not needed will reduce unwanted heating.

Why should you avoid using a rechargeable battery/cell when carrying out this experiment?

Rechargeable power sources have very low internal resistance which would be hard to measure in this experiment.

Why should a new cell/battery be used when carrying out this experiment?

Run-down cells and batteries have internal resistances that may fluctuate throughout the experiment. Using a new source will result in a more constant value.

What safety precautions should be taken when carrying out this experiment?

If connected for long periods of time, the battery and circuitry can become hot. Avoid touching bare metal contacts and disconnect the battery when readings are not being taken.

What device could be used to check your value for the cell's internal resistance?

An Ohmmeter

Suggest why the value for the cell's EMF determined in this experiment may be slightly different to the true value.

Ideal voltmeters are assumed to have an infinite resistance. In reality, a small current may still flow through the voltmeter, resulting in there being an additional PD in parallel with the supply.

With reference to the supply's internal resistance, suggest why high voltage supplies are safe for use in a lab.

High voltage supplies have a very high internal resistance. This is much higher than the external resistance produced if a human were to handle the supply and so the terminal potential is very low. So very little current flows in the circuit.

What safety precautions should be taken when carrying out this experiment?

If any spillages occur, they must be cleaned up immediately. Washing-up liquid is very slippery and so spillages lead to a risk of slipping and inury.

Why is this experiment invalid if the diameter of the tube is too small?

Stokes' law is only valid for laminar flow. If the tube is too narrow, the flow may not be laminar throughout, rendering Stokes' law invalid.

Suggest why light-gates shouldn't be used when carrying out this experiment.

Light-gates rely on a clear breakage of the beam. This will not occur in this experiment since it is unlikely that the ball will cut the beam. The line of the beam may also be disturbed by the coloured liquid.

Explain why the flow may become non-laminar if the ball approaches the tube's wall as it sinks.

If the ball approaches the wall, the surrounding liquid will accelerate upwards around it due to Bernoulli's principle. Faster fluid flow means a lower pressure, which would further guide the ball towards the wall. As a result the streamlines would be non-symmetrical, and so would be non-laminar.

What forces act on a ball as it sinks down a tube of liquid?

The downwards force of weight, and two upwards forces: drag and upthrust.

How can the uncertainty of r² be determined from the uncertainty in r?

The uncertainty in a squared variable is equal to twice the uncertainty in the variable itself. This means that the uncertainty in r² is double the uncertainty in r.

How can the radius of a small ball be measured?

A screw gauge micrometer can be used to measure the diameter of the ball. This can then be halved to give the radius.

Describe the forces on the ball when it reaches terminal velocity.

-At terminal velocity, the downwards forces on the ball equal the upwards forces.

-Weight = Upthrust + Drag

Describe how you should take time readings in this experiment.

Time readings should be taken at eye level to rubber bands, wrapped around the measuring cylinder at set values. The lap function should be used to record the time at each band.

How can you ensure that the time readings are accurate?

The same person should record the time readings throughout. They should measure at eye level to the rubber bands and must ensure they stop the timer at the relative position between the ball and the band each time.

Suggest why it may be advantageous to use a steel ball bearing in this experiment.

Steel ball bearings are magnetic. This means that a magnet can be used to easily lift the ball from the bottom of the liquid after each run.

What must be the case when positioning the highest rubber band on the tube?

The ball should be travelling at terminal velocity by the time it reaches this first band. This means it needs to be positioned sufficiently low enough for this to be the case.

If the ball has reached terminal velocity by the time it reaches the first band, what should be true about the two speeds calculated?

The two speeds should be the same since the ball will not accelerate beyond its terminal velocity.

How should the lower two bands be positioned on the tube?

The lower two bands should be positioned sufficiently far apart so that the time intervals between each are easily observable and measurable

What equation is used to calculate the liquid's viscosity?

η = (2r²g(p-σ))/9v

How can the density of each ball be determined?

The diameter can be measured using a micrometer, and from this the volume of the sphere can be calculated. The mass of the ball can be measured using a mass balance. Dividing the mass by the volume give's the ball's density.

What is the unit of viscosity?

Pascal-Seconds

Pa s

Define Young's Modulus

The Young's Modulus of a material is its ratio of tensile stress to strain. It is a measure of material's stiffness.

How is stress calculated?

Stress = Force/Cross-sectional area

How is strain calculated?

Strain = Change in length / Original length

What is the unit of stress?

Pascal (Pa) or N/m^-2

What is the unit of strain?

Strain is unitless since it is a ratio of two lengths

What is the unit of Young's Modulus?

Pascals (Pa) or Nm^-2

How can the cross-sectional area of a thin wire be measured?

The wire's diameter should be measured in several different orientations and at several different places along the wired, using a micrometer. The average diameter can then be used to calculate the circular cross-sectional area.

What safety precaution should be taken when stretching thin wires?

Safety goggles should be worn since the wire may snap when under a tensile load and this has the potential to cause eye injury.

Why should the temperature of the surroundings be kept constant when carrying out this experiment?

Metals undergo thermal expansion when there is a temperature increase, and contraction when there is a decrease. Temperature changes therefore cause unwanted changes in wire dimensions.

Why should a pre-stress be applied to the wire when setting up this experiment?

A pre-stress should be applied so that all kinks in the wire are removed and the wire is taught, before any length measurements are taken.

How can the Young Modulus be determined from a graph of extension against load?

The gradient of the graph is e/F

E = L/(A x Gradient)

Suggest what has happened if the length of the wire doesn't return to its original length when unloaded.

If the wire doesn't return to its original length when unloaded, the load may have exceeded the wire's elastic limit and consequently the wire has undergone plastic deformation.

How can the load applied on a wire be calculated from the mass added to the end of the wire?

Load = Mass x Gravitational Field Strength

F=mg

What safety precaution should be taken when using hanging masses?

Never stand with your feet below the hanging masses in case the wire snaps and the masses fall. It is good practice to place a padded bucket below them, in case the wire does snap.

Suggest how the extension of the wire may be measured.

A marker, such as a small piece of tape, could be added to the wire. A ruler could then be placed underneath the wire, allowing the movement of the marker to be measured.

Why is the choice of wire diameter important in obtaining successful results?

If the wire is too thick, the extension will be too small to measure. If the wire is too thin, it may begin to deform plastically before a good range of results have been obtained.

Suggest why a comparison test wire is used when conducting this experiment using Searle's apparatus.

A comparison test wire is used when conducting this experiment so that any changes in the environmental conditions, such as a change in temperature, are accounted for and won't skew the results obtained.

Why is your choice of wire length important in this experiment?

The extension of the wire depends on the wire's length since: x=FL/AE.

This means the wire needs to be long enough that extension is easily measurable.

Suggest how the wire may be fixed in place when carrying out this experiment

The wire can be clamped tightly between two blocks of wood at one end. These clocks can then be clamped to the bench.

Alongside a meter ruler, what other tool will help you measure the extension accurately?

A set-square can be used to help read the extension accurately.

When measuring the diameter of the wire in multiple places, why should you rotate the wire between measurements?

You should measure the wire in different orientations to ensure that the wire is circular across the full-length of the wire

What equation links wave speed, frequency and wavelength?

v = fλ

speed (ms⁻¹) = Frequency (Hz) x Wavelength (m)

How can the frequency of a signal be measured from the oscilloscope?

The time between two identical point on adjacent waves can be measured from the time-axis. This is the time period of the signal. The frequency is given by the inverse of the time period.

What two traces should be displayed on the oscilloscope screen?

Both the signal sent to the loudspeaker and the signal taken in by the microphone should be displayed on the oscilloscope screen.

Describe what happens to the two traces as the distance between the speaker and the microphone is increased.

The two traces will move past each other and the phase between the traces will change.

What piece of apparatus should be connected to the loudspeaker to produce a tone?

A signal generator should be connected to the loudspeaker. This generator should also be connected to the oscilloscope so that its signal trace is displayed

Describe how the speaker and microphone traces should be positioned on the oscilloscope display.

The spacing of the two signals inputted into the oscilloscope should be changed so that one is directly above the other. A peak of the lower trace should be inline with a trough of the upper trace.

How should the initial separation of the speaker and microphone be set?

The separation should be adjusted so that a trough on the upper trace touches the peak of the lower trace. This distance should then be measured using a metre ruler.

How can the wavelength be determined by moving the speaker away from the microphone?

The speaker should be moved away from the microphone until the trace has moved to a point where the peak of the lower trace once again touches a trough of the upper trace. The distance moved to achieve this is the wavelength.