4.2.3 Electromotive force and potential difference
electromotive force (e.m.f.) and potential difference (p.d.)
Electromotive Force (e.m.f.)
Definition: The electrical work done by a source in moving a unit charge around a complete circuit.
Measured in: Volts (V).
Explanation: Any device that pushes current around a circuit (e.g., cells, batteries, power supplies) is a source of e.m.f. It describes how much energy is transferred to the charge as it moves through the source.
Potential Difference (p.d.)
Definition: The work done by a unit charge as it passes through a component (e.g., a resistor).
Measured in: Volts (V).
Explanation: The p.d. across a component is the difference in electrical potential between the points where current enters and leaves. It measures how much energy each coulomb loses as it passes through the component.
Voltmeters
Connection: Always connected in parallel with the component they are measuring.
Types:
Analogue: Uses a needle to indicate voltage.
Digital: Displays a numerical voltage value.
Ranges: Voltmeters can measure:
Volts (V): Larger values.
Millivolts (mV) and Microvolts (μV): Smaller values.
Tip: The unit “V” (volts) is upright, while the symbol V (voltage) is italic in books.
Combining e.m.f.s
In Series: The total e.m.f. is the sum of individual e.m.f.s:
E=E1+E2
In Practice: If one cell in a series is reversed, its e.m.f. subtracts from the total.
Equations for e.m.f. and p.d
For electromotive force (e.m.f).:
$$e.m.f.(E)={{work\:done (W)}\over{charge(Q)}}$$
$$E={{W}\over{Q}}$$
For potential difference( p.d).:
$$p.d(V)={{work\:done (W)}\over{charge(Q)}}$$
$$V={{W}\over{Q}}$$
Example:
The p.d. across a lamp is 12 V. The lamp is connected for 10 seconds. Calculate how many joules of energy are transferred when:
a charge of 1 C passes through it
a charge of 5 C passes through it
a current of 2 A flows.
Answer:
use of equation: V=W/Q
W = VxQ
W = 12V x 1C
W = 12J
W = VxQ
W = 12V x 5C
W= 60J
For answer c you need to use the following equation from last topic to find the charge
I = Q/t
Q= It
Q= 2A x 10s
Q = 20C
Now you have found charge use the p.d equation to find the energy
