Energy in Circuits
In circuits there are two fundamental types of component: energy givers and energy takers.
Electromotive Force (emf), ε
Energy givers provide an electromotive force, they force electrons around the circuit which transfer energy.
The size of the emf can be calculate using:
This is similar to the equation we use to find voltage/potential difference and means the energy given to each unit of charge. We can think of this as the energy given to each electron.
The emf of a supply is the p.d. across its terminals when no current flows
EMF is measured in Joules per Coulomb, JC-1 or Volts, V
Energy takers have a potential difference across them, transferring energy from the circuit to the component.
emf = energy giver p.d. = energy taker
Energy is conserved in a circuit so energy in = energy out, or:
The total of the emfs = The total of the potential differences around the whole circuit
Internal Resistance, r
The chemicals inside a cell offer a resistance to the flow of current, this is the internal resistance on the cell.
Internal Resistance is measured in Ohms, Ω