AQA A Level Physics Paper 3 Section A: Practical skills and data analysis June 2018 question paper 7408/3A

This question is about using a digital balance to investigate the force on a wire placed in a magnetic field when there is an electric current in the wire.

A student carries out the procedure shown in Figure 1 and Figure 2. A metre ruler is pivoted at the 1.0 cm mark and a prism is placed on a digital balance. The free end of the ruler is raised and the balance is turned on and then set to zero, as shown in Figure 1.

The ruler is then supported by the prism with the apex of the prism at the 30.0 cm mark as shown in Figure 2. The height of the pivot is adjusted so that the ruler is horizontal.

Deduce the mass of the ruler. State one assumption you make.

The student attaches a uniform wire to the upper edge of the ruler, as shown in Figure 3.

The ends of the wire are connected to terminal blocks P and Q which are fixed firmly to the bench. A power supply and an ammeter are connected between P and Q.

These modifications cause the balance reading to increase slightly.

A horizontal uniform magnetic field is applied, perpendicular to the wire, between the 85 cm and 90 cm marks, as shown in the close-up diagram in Figure 3.

The balance reading M is recorded for increasing values of current I. A graph of these data is shown in Figure 4.

State and explain the direction of the horizontal uniform magnetic field.

It can be shown that B, the magnitude of the magnetic flux density of the horizontal uniform magnetic field, is given by

where change in force acting on the prism per unit current in the wire L = length of the region where the magnetic field cuts through the wire.

Determine B.

The experiment is repeated with the ruler pivoted at the 99.0 cm mark. Nothing else is changed from Figure 3.

This arrangement is shown in Figure 5.

Tick () one box in row 1 and one box in row 2 of Table 1 to identify the effect, if any, on the magnitude of the forces acting on the apparatus as a certain current is passed through the wire.

Tick () one box in row 3 and one box in row 4 of Table 1 to identify the effect, if any, on the graph produced for this modified experiment compared with the graph in Figure 4.

Figure 6 shows the balance being used to measure the forces between two wires. The connections joining these wires to the power supply are not shown.

The pan of the balance moves a negligible amount during use and it supports a straight conducting wire X of horizontal length L. Terminal blocks are used to connect X into the circuit. The weight of these does not affect the balance reading. A second conducting wire Y is firmly supported a distance d above X.

Show, by adding detail to Figure 6, the wire connections that complete the circuit. The currents in X and Y must have the same magnitude and be in the directions indicated.

The vertical force F on wire X due to the magnetic field produced by wire Y is given by

where k is a constant d is the perpendicular distance between X and Y I is the current in the wires and L is the horizontal length of wire X.

A student wants to measure k using the arrangement in Figure 6.

The student is told that the following restrictions must apply:

• L is fixed

• I must not exceed 5.0 A

• the result for k must be obtained using a graphical method

• the experimental procedure must involve only one independent variable.

Explain what the student could do to find k.

This question is about an experiment with a retractable steel tape measure.

The tape measure is placed at the edge of the bench and about 1 m of the steel tape is extended so that it overhangs the bench.

The tape is then locked in this position to stop it from retracting.

A student measures the dimensions x and y, the horizontal and vertical displacements of the free end of the tape, as shown in Figure 7.

Describe a suitable procedure the student could use to measure y. You may add detail to Figure 7 to illustrate your answer.

By changing the extension of the tape, the student obtains further values of x and y.

These data are shown in Table 2.

Suggest why the student chose to make all measurements of x greater than 70 cm

The data from the experiment suggest that y = Axⁿ where n is an integer and A is a constant.

These data are used to plot the graph in Figure 8.

Determine n using Figure 8.

Figure 8

Explain how the numerical value of A can be obtained from Figure 8.

Estimate the order of magnitude of A. You should use data for x and y from any one row in Table 2 on page 11. Give your answer with an appropriate unit.

This question is about an experiment to estimate absolute zero.

Figures 9a to 9d show the stages in the procedure carried out by a student.

An empty flask fitted with a tube and an open valve is placed in water bath H containing hot water. The air inside the flask is allowed to come into thermal equilibrium with the water. The valve is then closed, trapping a certain volume of air, as shown in Figure 9a.

The flask is inverted and placed in water bath C in which the water is at room temperature. The air inside the flask is again allowed to come into thermal equilibrium with the water, as shown in Figure 9b.

The valve is opened and some water enters the flask, as shown in Figure 9c.

The depth of the inverted flask is adjusted until the level of water inside the flask is the same as the level in the water bath. The valve is then closed, trapping the air and the water inside the flask, as shown in Figure 9d.

Explain why the volume of the air in the flask in Figure 9c is less than the volume of the air in the flask in Figure 9d.

Explain why Charles’s Law can be applied to compare the air in the flask in Figure 9a with the air in the flask in Figure 9d.

The flask is removed from water bath C and the valve and stopper are removed. The volume of the water in the flask is V₁ The flask is then completely refilled with water and the valve and stopper replaced. The volume of the water now in the flask is V₂ The volumes V₁ and V₂ are shown by the shaded parts in Figure 10.

Explain how V₁ and V₂ can be determined.

In your answer you should

• identify a suitable measuring instrument

• explain a suitable procedure to eliminate possible systematic error.

Plot on Figure 11 points to show the volume V and the temperature θ of the air in the flask when

• the flask is as shown in Figure 9a

• the flask is as shown in Figure 9d.

The temperature of the hot water bath is 86 C

Add a best fit line to your graph in Figure 11 to show how V should vary with θ according to Charles’s Law.

Determine the value of absolute zero in C using your graph in Figure 11.

value of absolute zero =