Fig. 1 shows a delivery cyclist using brakes while travelling down a hill. The bicycle and rider have a mass of 90 kg. At the bottom, the rider applies the ...
Question 1 Report
Fig. 1 shows a delivery cyclist using brakes while travelling down a hill. The bicycle and rider have a mass of 90 kg. At the bottom, the rider applies the brakes and comes to rest. The brake discs become much hotter and a small amount of sound is heard. The cyclist wants to reduce the heating of the brakes on a long descent.
(a) What energy store decreases as the bicycle slows down? [1]
(b) Describe the useful and wasted energy transfers when the brakes are applied. [2]
(c) Which action would reduce the rate at which energy is transferred to the brake discs? [1]
(d) Give one reason why brake discs need to conduct thermal energy well. [1]
A thermostat in an electric incubator uses a bimetallic strip. Fig. 1 shows two bonded metal layers, P and Q, next to an electrical contact. Metal P expands more than metal Q for the same temperature increase. When the strip bends, the circuit to the heater opens and the heater stops transferring energy to the air.
(a) Which metal layer becomes longer by the greater amount when heated? [1]
(b) Describe how the strip bends when the temperature increases. [2]
(c) What happens to the circuit when the strip touches the contact? [1]
(d) Give one reason why a thermostat saves energy. [1]
Answer Details
Bicycle brakes
- (a) The kinetic energy store decreases as the bicycle slows down. [1]
- (b) Friction at the brakes transfers kinetic energy mainly to the thermal energy stores of the brake discs and surrounding air. [1] Some energy is also transferred as sound, which is wasted. [1]
- (c) The rider could brake more gently, apply the brakes for a shorter time, or use a lower gear earlier. [1] Each reduces the rate at which energy is transferred to the discs.
- (d) Brake discs need to conduct thermal energy well so that energy is transferred away quickly and the brakes do not overheat. [1]
Bimetallic-strip thermostat
- (a) P becomes longer by the greater amount when heated, because the question states that P expands more than Q. [1]
- (b) P expands more than Q. [1] Therefore the strip bends towards Q, with P on the outside of the curve. [1] The outside of a bend must be the longer layer.
- (c) When the strip touches the contact, the circuit is completed and current flows to the heater. [1]
- (d) A thermostat saves energy because it switches the heater off at the required temperature, preventing unnecessary energy transfer. [1]
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