AQA A Level Physics Paper 3 Section B: Engineering physics June 2022 question paper 7408/3BC

Figure 1 shows a fairground ride.

The ride consists of a rotor that rotates in a vertical circle about a horizontal axis. The rotor has two rigid arms. A pod containing passengers is attached to each arm. The rotor is perfectly balanced. The direction of rotation of the rotor is reversed at times during the ride.

Figure 2 shows the variation of the angular velocity ω of the rotor with time t during a 12 s period.

Determine the mean angular velocity of the rotor during the 12 s period.

The moment of inertia of the rotor about its axis of rotation is 2.1 × 10⁴ kg m². A constant frictional torque of 390 N m acts at the bearings of the rotor.

Calculate the power output of the driving mechanism during the first 2 s shown in Figure 2.

Calculate the maximum torque applied by the driving mechanism to the rotor during the 12 s period.

Calculate the magnitude of the angular impulse on the rotor between t = 2.0 s and t = 7.0 s.

Which graph best shows the variation of the torque T applied to the rotor for the 12 s period? Tick () one box.

A copy of Figure 2 is provided to help you.

A moving tram is powered by energy stored in a rapidly spinning flywheel.

When the tram is at a tram stop, the flywheel is ‘charged’ by being accelerated to a high rotational speed.

The mass of the tram, flywheel and passengers is 1.46 × 10⁴ kg.

The distance between tram stops is 500 m.

Figure 3 shows that between stops A and B the track is inclined at a constant 5.0° to the horizontal.

The tram must travel 500 m along this incline on one charge of energy.

The total resistive force on the tram due to its motion is constant at 1.18 kN.

The flywheel is a solid steel disc of diameter 1.00 m. It has a moment of inertia of

Calculate the minimum angular speed of the flywheel when the tram leaves stop A so that the tram reaches stop B using only energy stored in the flywheel.

Between stops C and D the tram travels downhill.

Suggest two advantages of keeping the flywheel connected to the driving wheels when the tram travels downhill.

The same tram is to be used on a track where the stops are further apart, so the flywheel system needs to be modified.

Discuss the design features of the flywheel that will enable it to store more energy without increasing the mass of the tram.

In your answer you should consider:

• the design of the flywheel

• how the choice of materials used to make the flywheel is influenced by its design and maximum angular speed

• other design aspects that allow for high angular speeds of the flywheel.

Explain what is meant by an adiabatic change.

Figure 4 shows the p – V diagram for an ideal diesel engine cycle.

In this cycle, air is compressed adiabatically from a pressure of 1.0 × 10⁵ Pa and volume V₁ to a pressure of 67 × 10⁵ Pa and volume V₂. The adiabatic index γ for air = 1.4 Calculate the compression ratio .

Explain why the compression ratio for a diesel engine must be greater than the compression ratio for a petrol engine.

The dashed lines in Figure 5 show the p – V diagram for the ideal diesel engine cycle.

Draw, on Figure 5, a typical indicator diagram for a real four-stroke diesel engine with

Mark with an X on your diagram the point where the injection of fuel starts.

Explain two differences between the ideal cycle and the indicator diagram for the real engine.

Figure 6 shows a low-voltage solid-state thermoelectric cooling element. The element is a square of side 40 mm and is 4 mm thick.

Figure 7 shows how the element is used as part of a thermoelectric refrigerator to keep small quantities of medicine at a low temperature.

The manufacturer’s data for the element show that when the temperature of the hot side is 35 °C and the temperature of the cold side is 5 °C:

• the rate at which energy is dissipated from the hot side is 65 W

• the electrical power supplied is 28 W.

It is claimed that the coefficient of performance (COP) of a thermoelectric refrigerator is much less than the COP of an ideal refrigerator.

Discuss whether the claim is valid for the thermoelectric refrigerator in this question.

Suggest why a small value of the COP might be acceptable for this particular application of a thermoelectric cooling element.