6.04 Energy in d.c. circuits.

In the previous text we have described how an e.m.f supplies energy to electrons in d.c. circuits and how resistors convert this energy to heat as the electrons move around the circuit.
We defined voltage (E or V) as the ratio of energy to charge.

• E = W/Q (or V=W/Q) .
• Therefore W = EQ .
• Also I = Q/t .
• Therefore Q = It .
• Substituting for Q gives W = EIt .

Therefore the energy supplied by an e.m.f, or the energy dissipated by a resistor, can be calculated using the voltage, current and the time period over which the energy was supplied or dissipated.

W = EIt.

• W = Energy (work done) in Joules (J).
• E = e.m.f. voltage in Volts (V).
• I = current in amps (A).
• t = time in seconds (s).

The energy dissipated by a resistor is given by:

W = VIt.

• W = Energy (work done) in Joules (J).
• V = Voltage drop across the resistor (V).
• I = current in amps (A).
• t = time in seconds (s).

In either case it is clear that: Energy = voltage x current x time.