AQA A Level Physics Paper 3 Section A: Practical skills and data analysis June 2024 question paper 7408/3A

This question is based on a method to determine the resistivity of a wire (required practical activity 5).

Figure 1 shows a micrometer screw gauge.

Figure 2 shows an enlarged view of the scales.

State, in mm, the resolution of the main scale.

What is the reading on the micrometer?

Tick () one box.

A wire X is placed in the gap between the anvil and the spindle.

State and explain how this gap is closed just before taking a reading of the diameter of X.

Figure 3 shows a circuit used to determine the resistance per metre of wire X.

Two terminals are used to mount X on a ruler. Clips are used to connect a voltmeter across the 1.2 Ω resistor. When the switch is closed, the voltmeter reading is 931 mV.

The switch is then opened and the voltmeter is connected to X as shown in Figure 4.

When the switch is closed, the voltmeter reading is 397 mV.

Show that, for the arrangement in Figure 4, the resistance R of the wire between the clips is about 0.5 Ω.

The length of wire between the clips is L. Values of R are determined for different values of L. Figure 5 shows these data.

Determine the resistance per metre of X.

Table 1 shows the resistance per metre of various metal wires. The diameter of X is one of the values of d shown in Table 1.

Identify the metal used for X. Go on to determine the resistivity of the metal. State an appropriate SI unit for your answer.

A student adds error bars for R and L to each point on Figure 5.

She estimates that

• each value of R has a percentage uncertainty of 6%

• each value of L has an absolute uncertainty of 5 mm.

Compare her error bars for the point at L = 209 mm with her error bars for the point at L = 388 mm.

Outline how error bars are used to determine the uncertainty in the gradient of a linear graph.

Figure 6 shows apparatus used to investigate how the resistance R of a light-dependent resistor (LDR) varies with illumination.

The ohm-meter

• always displays a four-digit reading of R

• can be set to the different ranges A to E shown in Table 2.

Explain why the reading displayed in Figure 6 shows that the ohm-meter is set to range C.

The quantity E_V is a measure of the intensity of the light incident on the LDR.

The resistance R of the LDR is given by

Show that E_V for the arrangement shown in Figure 6 is about 130 lx.

R is recorded for different values of the vertical distance x between the lamp and the LDR. E_V is calculated for each value of R.

Figure 7 shows how E_V varies with x.

It can be shown that E_{V 2}

Describe a method to show that Figure 7 confirms this relationship. You do not need to show any calculations.

Deduce the value of x when E_V = 130 lx.

R is measured when x = 450 mm. Figure 8 shows how the ohm-meter displays the values of R when set to range B and when set to range C.

The uncertainty of the reading on the ohm-meter is ±2% of the displayed reading plus ±2 in the least significant digit.

This means that:

• using range B the maximum value of R is 1.02 × 1681 + 2 = 1717 Ω

• using range C the minimum value of R is 0.98 × 1.68 – 0.02 = 1.63 kΩ.

Complete Table 3. Go on to explain whether range B or range C should be used to measure R.

Figure 9 shows the LDR being used to investigate the transmission of light through glass slides.

The lamp and ohm-meter are switched on. R is recorded with different numbers of slides placed on the LDR. E_V is calculated for each value of R.

The positions of the lamp and the LDR are not changed during the experiment.

Identify two other control variables.

For the arrangement in Figure 9 it can be shown that

where N is the number of slides μ is a constant.

Explain how μ can be determined from a linear graph.

In an experiment μ = 9.0 × 10⁻²

Deduce the minimum number of slides needed to reduce E_V by 50%.

This question is about a method to investigate how the force on a conductor varies with flux density and current (required practical activity 10).

Figure 10 shows a copper rod clamped above a horizontal bench.

Figure 10

Describe a method to show that the copper rod is horizontal. Your method must include the use of a metre ruler. You may annotate Figure 10.

Figure 11 shows the copper rod positioned above a digital balance.

Two identical magnets are mounted on a steel yoke with their opposite poles facing each other. The balance is zeroed. The yoke is then placed on the balance so that a horizontal uniform magnetic field is applied perpendicular to the copper rod. The ends of the rod are connected as shown.

When the switch is open, the reading on the balance shows the mass of the yoke and the two magnets. When the switch is closed, the reading on the balance decreases.

Explain, with reference to Figure 11, the direction of the horizontal magnetic field.

The current I in the rod is varied. The balance reading M₁ is recorded for different values of I.

The switch is now opened. Two additional magnets, identical to those used before, are attached to the yoke.

Figure 12 shows how this new arrangement compares with the arrangement in Figure 11.

The balance reading with four magnets attached to the yoke is M₂. With the switch open, M₂ is the mass of the yoke and the four magnets.

The switch is now closed. M₂ is recorded for different values of I.

Figure 13 shows data from both experiments. Values of M₁ and M₂ are plotted using different vertical axes.

It can be shown that

M = kBI + nZ + Y

where

M = balance reading when the current is I B = magnetic flux density of the horizontal uniform magnetic field n = number of magnets attached to the yoke Z = mass, in g, of each magnet Y = mass, in g, of the yoke k is a constant.

Deduce the fundamental base units for k.

Determine Y.

A student sets up the apparatus with the copper rod positioned incorrectly.

Figure 14 shows how the student’s arrangement compares with the correct arrangement.

The student produces a graph of M₁ against I.

Compare the student’s graph with the graph of M₁ against I (the solid line) in Figure 13. Explain your answer.