AQA A Level Physics Paper 3 Section B: Engineering physics June 2025 question paper 7408/3BC

The rotating part of an electric motor is called the rotor.

Figure 1 shows an end view of a rotor turning clockwise due to a driving torque from the motor. In this question, the clockwise direction is treated as positive.

Figure 1
Figure 2

The rotor can be brought to rest rapidly by reversing the electrical supply connections to the motor. Figure 2 shows the rotor at time t = 0 when the supply connections are reversed. The rotor then slows down due to a constant anticlockwise retarding torque so that it

The angular velocity of the rotor at t = 0 is 98.0 rad s⁻¹ clockwise. The applied torque on the rotor at t = 0 is anticlockwise. The applied torque produces a constant angular acceleration of −303 rad s⁻². Friction torque is negligible.

Determine t₁.

The electrical supply remains connected and the rotor now accelerates uniformly anticlockwise with an acceleration of magnitude 303 rad s⁻². At a later time t = t₂, the angular velocity of the rotor is −120 rad s⁻¹.

Determine the number of anticlockwise revolutions made by the rotor between

Another way of quickly stopping the motor is to use a disc brake. The motor is brought to rest by switching off the power supply and then forcing two brake pads against the disc.

Figure 3 shows the two brake pads each applying a constant retarding force of 182 N to the disc.

moment of inertia of rotor and disc about the axis of rotation = 0.101 kg m² diameter of disc = 0.160 m diameter of pad = 22 mm

Compare the angular deceleration produced by the disc brake system with the angular deceleration produced by reversing the electrical connections.

The moment of inertia I of any rolling object about its axis of rotation is given by

where A is a constant that depends on the shape of the object m is the mass of the object r is the radius of the object.

Figure 4 shows an object that starts from rest and rolls down a slope without slipping.

The object falls a vertical distance h. It then has a linear velocity v and angular velocity ω. For a rolling object the linear velocity v of the centre of mass is related to the angular velocity ω by v = rω.

Show, by considering the energy transfers that occur, that

Table 1 gives the moments of inertia of a solid sphere P, a hollow sphere Q, a solid cylinder R and a hollow cylinder S about their axes of rotation.

P, Q, R and S have the same mass m and outer radius r.

Explain why the hollow sphere Q has a greater moment of inertia about its axis of rotation than the solid sphere P.

Figure 5 shows all four objects at rest at the top of a slope. They are released at the same time and roll down the slope without slipping.

Deduce the order in which the objects reach the bottom of the slope.

State what is meant by an isothermal change.

Figure 6 shows a p – V diagram in which a fixed mass of an ideal gas is taken through a cycle of three processes.

A → B: adiabatic change B → C: isothermal change C → A: reduction in volume at constant pressure

The maximum temperature of the gas in the cycle is 536 K and the minimum temperature is 291 K.

Calculate, in mol, the amount of gas.

Show that the work done in process C → A is about 100 J.

Complete Table 2 for each process and for the whole cycle.

Two criteria for a heat engine to achieve the maximum theoretical efficiency are:

1. the gas must always be at the same temperature as the hot source when energy is being transferred to the gas by heat transfer

2. the gas must always be at the same temperature as the cold sink when energy is being transferred from the gas by heat transfer.

The temperatures of the source and sink do not change.

Deduce whether the cycle in Figure 6 satisfies either or both of these criteria.

The overall efficiency of a heat engine is defined as

An engineer suggests designing a real engine based on the cycle shown in Figure 6.

Discuss the engineer’s suggestion.

In your answer you should:

• use Table 2 to determine the overall efficiency of a theoretical engine that uses the cycle

• compare this overall efficiency with the efficiency of an ideal engine working between the same temperatures as the cycle

• discuss with reasons the problems that would need to be overcome in making a real engine work on this cycle

• explain whether the engineer’s suggestion would lead to a useful engine.

State what is meant by the coefficient of performance of a heat pump.

A jeweller hopes to reduce workshop heating costs by replacing a gas-fired boiler with an electric heat pump. The heat pump will provide underfloor heating using water at a temperature of 40 °C.

The air outside the building provides the low-temperature reservoir for the heat pump. Over the winter period the outside temperature will vary between 12 °C and −8 °C.

The salesperson for a heat pump company claims that:

• the coefficient of performance will vary greatly, depending on the outside temperature

• the colder the outside temperature, the lower the energy costs will be for running the heat pump.

Comment on the salesperson’s claims.

A heat pump is installed. The average COP_hp over the winter period is 7.5 In this period the heat pump delivers 2880 kW h of underfloor heating.

What is the electrical energy used by the heat pump in this period?

Tick () one box.