Electromotive force and potential difference
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
Electromotive Force (e.m.f.)
- Electromotive force (e.m.f.) is the electrical work done by a source in moving a unit charge around a complete circuit
- e.m.f. is measured in volts (V)
Calculating e.m.f.
Extended Only
- Formula:
- E = (W/Q)
- Where:
- E = e.m.f. (V)
- W = work done (J)
- Q = charge (C)
Potential Difference (p.d.)
- Potential difference (p.d.) is the work done by a unit charge passing through a component
- p.d. between two points is measured in volts (V)
- the p.d. is 1 volt if 1 joule of energy is transferred when 1 coulomb of charge passes between the points
- 1 V = 1 J/C
Calculating p.d.
Extended Only
- Formula:
- V = (W/Q)
- Where:
- V = p.d. (V)
- W = work done (J)
- Q = charge (C)
Worked Example: Calculating p.d.
Museli finds that 8 J of work is done when 2 C of charge passes through a lamp, and wants to find the p.d. across it.
- Step 1: write down the equation. V = (W/Q)
- Step 2: substitute the values. V = (8/2)
- Step 3: calculate the answer. V = 4 V
Worked Example: Calculating Energy Transferred
Muchindu has a lamp with a p.d. of 12 V across it, and wants to find the energy transferred when a current of 2 A flows for 5 s.
- Step 1: find the charge that flows. Q = I × t = 2 × 5 = 10 C
- Step 2: find the energy transferred. W = Q × V = 10 × 12 = 120 J
Energy Transfers in a Circuit
- The source (battery or cell) supplies electrical energy to the charges passing through it
- Each component (lamp, motor, resistor) transfers energy from the charges to the surroundings, as heat, light, sound or kinetic energy
- The charge itself is not used up - the same amount of charge flows into and out of every component; it is the energy it carries that gets transferred around the circuit
A unit of charge picks up energy at the source and delivers it at each component
Using Voltmeters
- A voltmeter measures potential difference
- It must be connected:
- in parallel (across) the component being tested
- with its + terminal on the side where current enters the component
| Ammeter | Voltmeter |
|---|---|
| Measures current (A) | Measures p.d. (V) |
| Connected in series | Connected in parallel |
| Very low resistance | Very high resistance |
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