IB Physics SL A.3.12 Power

Practise calculating average or instantaneous power from energy transfer, work or force and speed, while applying efficiency and checking time, area and energy-density conversions.

Exam points

calculate average power from transferred energy divided by elapsed time after converting all units to SI
use P = Fv only for the force component parallel to instantaneous velocity
apply efficiency as useful output power divided by input power and keep the energy direction clear

[Maximum number: 2]

This section consists of three questions: B1, B2 and B3.

A 300 W immersion heater is placed in a beaker containing 0.25 kg of water at a temperature of $18^{\circ} \mathrm{C}$ . The heater is switched on for 120 s , after which time the temperature of the water is $45^{\circ} \mathrm{C}$ . The thermal capacity of the beaker is negligible and the specific heat capacity of water is $4.2 \times 10^{3} \mathrm{Jkg}^{-1} \mathrm{~K}^{-1}$ .

Determine the rate at which thermal energy is transferred from the water to the surroundings during the time that the heater is switched on.

energy transfer $=[300 \times 120]-\left[2.835 \times 10^{4}\right]=7.65 \times 10^{3} \mathrm{~J}$ ;
rate of transfer $=\frac{7.650 \times 10^{3}}{120}=64 \mathrm{~W}$ ;

Marking guidance:

Allow ECF from (b)(i).

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A.3.12—Power question 1