Electromotive force — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 10Definition of e.m.f.

Definition of e.m.f.

Electromotive force (e.m.f.)

• Electromotive force (e.m.f.) the electrical work done by a source in moving a unit charge around a complete circuit.

Not a force

• Not a force despite its name, e.m.f. is energy per unit charge, not a force.

Unit

• Unit the volt (V), where 1 V = 1 J C⁻¹.
• A 1.5 V cell gives 1.5 J of energy to each coulomb that passes through it.

Section 2 of 10The equation E = W / Q (Extended)

• E = W / Q
  • E is e.m.f.
  • (V), W is energy transferred to the charge (J) and Q is charge (C).
  • Rearranged, W = E Q and Q = W / E.

Section 3 of 10Energy transformations in a source

Chemical cell

• Chemical cell transforms chemical energy into electrical energy.

Solar cell

• Solar cell transforms light energy.

Dynamo (generator)

• Dynamo (generator) transforms kinetic energy.

Thermocouple

• Thermocouple transforms thermal energy.

After the source

• After the source the energy is given to the charge carriers, which transfer it to the components.

Section 4 of 10Measuring e.m.f.

Voltmeter

• Voltmeter connected across the terminals of the source.

High resistance

• High resistance a good voltmeter draws almost no current from the cell.

Open-circuit voltage

• Open-circuit voltage the e.m.f. is the reading when no, or a very small, current flows.

Terminal voltage

• Terminal voltage when the cell supplies current, the reading drops slightly because some energy is transferred inside the cell against its internal resistance.

Section 5 of 10Combining cells

Cells in series

• Cells in series the total e.m.f. is the sum, so three 1.5 V cells give 4.5 V.
• A cell connected the wrong way round subtracts its e.m.f.

Cells in parallel

• Cells in parallel with equal e.m.f.s the total e.m.f. is that of one cell, but each cell supplies a share of the charge so they last longer.

Section 6 of 10E.m.f. compared with p.d.

Quantity	Applies to	Meaning
e.m.f.	The source	Energy given to each coulomb of charge
p.d.	A component	Energy taken from each coulomb of charge and converted to other forms

Section 7 of 10Energy in a circuit loop

Series loop

• Series loop in a loop with a 6.0 V battery and two lamps each coulomb receives 6.0 J, which the lamps use up between them.

Sum of p.d.s

• Sum of p.d.s the e.m.f. equals the sum of the p.d.s in the external circuit when internal resistance is negligible.

Old battery

• Old battery a battery with run-down chemicals has a lower e.m.f., so less energy is given to each coulomb and a lamp is dimmer.

Capacity

• Capacity e.m.f. is the energy per coulomb, while capacity in ampere-hours (A h) shows how much charge a battery can supply in total.

Section 8 of 10More from the specification

More from the specification

Definition of electromotive force

• Electromotive force (emf) is the total electrical energy converted from chemical or mechanical forms per unit charge passing through a power source: E = W / Q.

Terminal p.d. vs source emf

• Internal resistance within cells causes terminal potential difference under load to be lower than emf: V = E - Ir.
  What the symbols mean: E or ε = e.m.f. (V) · I = current (A) · R = external resistance (Ω) · r = internal resistance (Ω) · V = terminal p.d. (V)

Voltmeter connection in circuits

• Voltmeters possess very high internal resistance and are connected in parallel across components to measure potential difference without drawing significant current.

Potential difference across circuit components

• Potential difference across circuit components measured potential difference indicates the electrical energy transferred per unit charge between positive and negative terminals.

Gold-leaf electroscope testing

• Gold-leaf electroscope testing bringing a charged polythene rod near the metal plate repels like charges into the leaf, causing detectable deflection.

Electrical power delivered by supplies

• Electrical power delivered by supplies electrical power represents the rate of energy delivery, calculated from current multiplied by source potential difference.

Gold-leaf electroscope

• A device in which a charged gold leaf falls when photoelectrons discharge the plate.

Mains supply

• Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

Electrical power

• Rate of energy transfer, P = I V.
  What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Electroscope

• Instrument with a metal cap, stem and gold leaf used to detect charge.

Conductor

• Material that allows current to flow.

Insulator

• Material that does not allow current to flow.

Potential difference

• Work done by a unit charge passing through a component.

Electric field

• Region of space in which a charge experiences a force.

Alternating current

• Current that continually reverses direction.

Field lines

• Lines showing the direction and strength of a magnetic field.

Direct current

• Current that flows in one direction only.

Section 9 of 10Key terms

Term	Meaning
E.m.f.	Electrical work done by a source in moving a unit charge around a complete circuit.
Volt	Unit of e.m.f. and p.d.; 1 V = 1 J/C.
Open-circuit voltage	Voltage across a source when no current flows.
Terminal voltage	Voltage across a source while it supplies current; less than the e.m.f. because of internal resistance.
Battery	Two or more cells joined together.

Section 10 of 10Exam technique

1

Describing e.m.f. as a force — it is energy per coulomb measured in volts.

2

Saying a higher e.m.f. means the battery lasts longer — capacity decides that, not e.m.f.

3

Confusing e.m.f. with p.d. — e.m.f. is for the source, p.d. is for a component.

4

Adding the e.m.f. of cells in parallel — the e.m.f. stays that of one cell.