CAIE A-Level Physics 5.1.6 Using P = W/t

Practise calculating average power with P = W/t, rearranging for work or time, converting units and interpreting lifting or energy-transfer contexts.

Exam points

calculate power using P = W/t from work done and elapsed time
rearrange P = W/t to determine work done or time and handle unit conversions
interpret power in lifting, propulsion or energy-transfer contexts using the stated work or energy

[Maximum number: 4]

(a)

When falling at a steady speed, the mass in (b) falls through a vertical distance of 1.4 m in a time of 4.0 s . This causes a current of 90 mA in the resistor. The resistance of the resistor is $47 \Omega$ .

Calculate:

[ 4 ]

(i)

the rate of work done by the falling mass
rate of work done = W

[ 2 ]

$P=m g(\Delta) h /(\Delta) t$ or F v

C1

$\begin{aligned} P =(0.60 \times 9.81 \times 1.4) / 4.0 \text { or } 0.60 \times 9.81 \times(1.4 / 4.0) =2.1 \mathrm{~W} \end{aligned}$

A1

(ii)

the power dissipated in the resistor
power = W

[ 2 ]

$P=I^{2} R$ or I V or $V^{2} / R$

C1

$\begin{aligned} =0.09^{2} \times 47 \text { or } 0.09 \times 4.23 \text { or } 4.23^{2} / 47 =0.38 \mathrm{~W} \end{aligned}$

A1

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5.1.6—P = W / t question 1

Question (a)

Question (i)

Question (ii)