Electricity and Magnetism

Episode 115: Energy and power

Lesson for 16-19 IOP TAP

In this episode, students find that the energy transferred by an electrical component depends on the potential difference across it, the current through it and the time for which it operates.

Lesson Summary

  • Demonstration and discussion: Lamp and joulemeter (10 minutes)
  • Student experiment: Power of a lamp (30 minutes)
  • Student questions: Electrical power (20 minutes)

Discussion and demonstrations: Lamp and joulemeter

Start by reviewing the effect of increasing the voltage across a filament lamp. The current through the lamp increases and the lamp gets brighter.

Episode 115-1: Energy transferred by a lamp (Word, 28 KB)

Remind your students that the pd is defined as the energy transferred per coulomb and that the current is the number of coulombs per second. If the lamp is connected to a joulemeter you can show that the number of joules transferred increases with time and that the rate increases when voltage and current increase. This should lead to the word equation:

Energy transferred = number of joules per coulomb × number of coulombs

Energy transferred = pd across lamp  ×  current through lamp  ×  time

ending up with the equation:

energy transferred = V  ×  I  ×  t

Now remind them that power is rate of transfer of energy leading to P = I × V (they may also need to be reminded of the SI units, watts).

This exercise can be rounded off by calculating power and showing that (at the rated voltage) it is equal to the quoted power of the lamp.

Student experiment: Power of a lamp

At this stage, students should investigate the way power depends on pd across the lamp. Ask them to predict the effect of doubling the voltage. Some will expect the power to double. Some students may suggest that it quadruples (arguing that both I and V double). In fact neither answer is correct in practice. At higher currents the filament is hotter so the resistance is higher and the current does not double when the voltage doubles.

The results of the experiment will lead to useful discussion about energy, power, voltage, current and resistance.

Episode 115-2: relationship between power & applied potential difference (Word, 35 KB)

Student questions: Electrical power

Applying the above ideas.

Episode 115-3: Types of light bulb (Word, 50 KB)

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