Efficiency: a motor lifting a load sim
Set the supply's voltage and the load, then time a small geared motor winding a slotted-mass load up beside a half-metre rule: read the energy in from a joulemeter (or V × I × t), work out the useful energy m g Δh and the efficiency, watch a Sankey diagram of where the energy went draw itself, and plot how the efficiency changes with the load.
Energy, work and power · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Energy in = the joulemeter's reading after − before, or V × I × t. Useful energy out = the load's gain in gravitational potential energy, m g Δh, with g = 10 N/kg and Δh read on the half-metre rule (1 mm divisions, zero at the bench). Efficiency = useful energy out ÷ total energy in: never more than 1 (100 %). The rest is wasted: it heats the motor's coils (the current's heating effect), and friction in the gearbox, bearings and brushes turns it into heat and sound. Model (a chosen motor, beyond the syllabus): a small DC motor with a gearbox, coil resistance 24 Ω, motor constant 0.32 N m per A at the drum, friction 0.02 N m, winding radius 6 mm; for example at 6.0 V it lifts 300 g at 59 mm/s drawing 0.119 A, 24.9 % efficient. Each run's supply voltage varies by up to ±1.5 % and its friction by up to ±4 % (seeded, so it repeats on every load). The joulemeter counts in whole 0.1 J steps and drops the rest, so a run can read up to 0.1 J low, and a longer run makes that 0.1 J smaller in proportion: a 5 s lift of 300 g at 6.0 V reads 3.5 J for about 3.56 J, so its efficiency comes out near 25.4 % against the model's 24.9 %. Meters: voltmeter 0.01 V, ammeter 0.001 A, joulemeter 0.1 J (it counts on from the last class until reset), stopwatch 0.01 s. Idealised: the motor reaches its steady speed at once and stops at once; you switch on and start the stopwatch together; the worm gearbox holds the load when the motor is off; the string is light and thin; the supply holds its voltage; the leads and meters take no energy. If the hanger reaches the drum (430 mm up) the motor jams: it stops turning and all the energy heats its coils.
- The load (slotted masses on a hanger): the part to watch
- Geared motor in a clamp on a stand, half-metre rule, DC supply, joulemeter (or voltmeter and ammeter), stopwatch
- Red leads: the positive side of the circuit
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