Energy in stores; power in pathways
Teaching Guidance for 14-16
Keeping the lived-in world and imagined world separate but connected
Drawing on the previous sections, we have seen that it's possible to develop an
energy description of electric circuits at a number of different
levels. For example, suppose we have a simple circuit loop consisting of a 12 volt supply and a bulb in which there is a current of 2 ampere (maybe a car headlamp bulb). It's possible to describe the loop in a number of different ways:
- Physical description: the bulb is connected to the supply cell and lights up.
- Shifting energy between stores: energy is shifted from the chemical store of the cell to the thermal store of the surroundings.
- Shifting energy along pathways: energy is shifted from the chemical store of the cell to the thermal store of the surroundings, first along an electrical working pathway, then along the
- Power and pathways: energy is shifted at the rate of 24 joule inverse second. There is a power of 24 watt in the electrical pathway and in the heating-by-particles and heating-by-radiation pathways.
A fundamental feature of any pathway is that it's possible to calculate the rate at which energy is shifted. This is the power in the pathway. In the case of the electrical pathway in this example, it's helpful for students to go back to first principles in thinking through the rate of shifting of energy along the pathway: this is the operating power of the pathway.
So: with the 12 volt cell, the potential difference across the bulb is 12 volt. This means that 12 joule of energy is shifted for each coulomb of charge passing through the resistance of the bulb. With a current of 2 ampere, 2 coulomb pass through the bulb each second.
So 24 joule of energy must be shifted each second by the bulb. 24 joule of energy is shifted each second, from cell to bulb, along the electrical working pathway, and 24 joule of energy is shifted each second from bulb to surroundings, along the heating pathways.