AQA A Level Physics Paper 3 Section B: Engineering physics June 2019 question paper 7408/3BC

The fly-press shown in Figure 1 is used by a jeweller to punch shapes out of a thin metal sheet.

The frame holds a screw and punch. Two arms are attached to the screw, each loaded with a heavy steel ball. The screw is driven downwards when the arms are rotated. Kinetic energy is stored in the rotating parts: the balls, arms, screw and punch. This energy is used to punch the shape out of the metal sheet.

When the punch reaches the metal sheet, the rotational speed of the arms is 2.9 rev s⁻¹. At this speed the rotational kinetic energy of the rotating parts is 10.3 J.

Calculate the moment of inertia of the rotating parts about the axis of rotation.

The total mass of the screw, punch and arms is the same as the total mass of the two balls.

Explain why the moment of inertia of the screw, punch and arms about the axis of rotation is much smaller than the moment of inertia of the steel balls about the same axis.

During the punching of the metal sheet, the rotating parts of the fly-press are brought uniformly to rest from an initial rotational speed of 2.9 rev s⁻¹ in a time of 89 ms.

Determine

• the angular deceleration

• the angle turned through by the rotating parts.

Which of the following is the SI unit for angular impulse? Tick (✓) one box.

A turning moment diagram is a graph showing the variation of torque produced by an engine or motor with the angle of rotation of the output shaft.

Figure 2 shows the turning moment diagrams for a single-cylinder diesel engine and an electric motor that have the same output power.

State what is represented by the area between the curve and the angle axis for a turning moment diagram.

The diesel engine or the electric motor may be used to drive a machine that has a low moment of inertia and that requires an almost constant torque.

Discuss why, to drive this machine, the diesel engine would need to be fitted with a flywheel.

In your answer you should explain

• why the electric motor does not require a flywheel

• why the torque of the diesel engine varies over one cycle, including why there are points where the torque is zero

• how the moment of inertia of the flywheel influences the motion of the output shaft of the diesel engine.

In an ideal heat-engine cycle a fixed mass of air is taken through the following four processes.

A → B isothermal compression from an initial pressure of 1.0 × 10⁵ Pa and a volume of 9.0 × 10^–2 m³ to a pressure of 2.2 × 10⁵ Pa. The work done on the air is 7100 J.

B → C increase in volume at constant pressure to a volume of 5.9 × 10⁻² m³.

C → D isothermal expansion to a pressure of 1.0 × 10⁵ Pa and a volume of 13 × 10⁻² m³. The work done by the air is 10 300 J.

D → A reduction in volume at constant pressure to the original volume.

Figure 3 shows the cycle.

Show, by calculation, that the volume at B is 4.1 × 10⁻² m³.

The temperature of the air between A and B is 295 K.

Show that the temperature of the air at C is about 420 K.

An ideal engine based on this cycle uses a device called an economiser. The economiser stores all the energy transferred in the cooling process D → A and gives up all this energy to the air in process B → C. This means that an external source supplies energy to the air by heating only in process C → D.

Complete Table 1 to show the values of work done W and energy transfer Q in each of the four processes. Use the space below Table 1 for any calculations. Use a consistent sign convention.

Explain why W is equal to Q in process A → B and in process C → D.

It is claimed that the efficiency of this engine cycle is the same as the maximum theoretical efficiency of a heat engine operating between the same temperatures.

Deduce whether this claim is true.

Discuss one problem that would be faced by an engineer designing a real engine based on this cycle.

Tumble-dryers blow hot air over wet clothes that are moving in a rotating drum. Conventional tumble-dryers heat the air in the drum electrically; other dryers use a heat pump to heat the air.

A typical conventional tumble-dryer uses about 0.6 kW h per kg of clothes. A heat pump tumble-dryer uses about 0.25 kW h per kg.

Explain why the heat pump tumble-dryer uses less electrical energy than the conventional tumble-dryer to dry the same load.

The cold space of the heat pump is the room in which the tumble-dryer is placed. The hot space is the air in the tumble-dryer and is at a temperature of 160 °C.

A heat pump tumble-dryer can be placed in a kitchen at a temperature of 20 °C, or in a garage at around 5 °C.

Deduce which place would result in lower running costs for the tumble-dryer. Support your answer with calculations.