Students investigate the efficiency of an electric motor - Edexcel - GCSE Physics - Question 4 - 2018 - Paper 1

The graph of an alternating current at 0.5 Hz would typically show a sine wave oscillating above and below the x-axis, with the frequency corresponding to a period of 2 seconds (since frequency is the reciprocal of the period). The graph should have time on the x-axis (spanning at least 4 seconds) and current on the y-axis.

An efficiency of 40% means that 40% of the input power is converted into useful output power, while the remaining 60% is lost to waste, typically as heat. This implies that for every unit of energy consumed, 0.4 units are effectively used for work, and 0.6 units are not utilized.

To calculate the energy supplied:

1. Convert time from minutes to hours:

   ext{Time (hours)} = rac{3 ext{ minutes}}{60} = 0.05 ext{ hours}

2. Use the formula for energy:

   ext{Energy} = ext{Power} imes ext{Time} = 15 ext{ W} imes 0.05 ext{ hours} = 0.75 ext{ Wh}

   Unit: Wh (watt-hours)

To calculate the cost:

1. Use the energy calculated in the previous step:

   ext{Energy supplied} = 0.75 ext{ Wh} = 0.00075 ext{ kWh}

2. Cost of electricity = 20 pence per kWh.

3. Calculate the cost:

   ext{Cost} = ext{Energy} imes ext{Cost per kWh} = 0.00075 ext{ kWh} imes 20 ext{ pence/kWh} = 0.015 ext{ pence}