ELECTRICITY

ELECTRICITY
Key notes:
1. Current is the rate of flow of charges
2. Current is measured by an ammeter in amperes (A).
3. Current only flows between two points in a circuit if there is a potential
difference between the two points.
4. A voltmeter is use to measure the potential difference.
5. Unit for potential difference is the volt (V).
6. The electromotive force (e.m.f.) of a cell is the energy dissipated by the cell in
driving each coulomb of charge around the complete circuit.
7. The unit for e.m.f. is also the volt (V).
8. Electrical resistance is the resistance to current flow. It is measured in ohms,Ω.
9. A rheostat has a variable resistance.
10. For most metallic conductors, resistance increase with temperature.
11. For thermistors, resistor decreases with temperature.
12. At a junction in a circuit, the sum of the currents entering the junction is equal
to the sum of the currents leaving it. I1 + I2 = I3 + I4
13. In a series circuit,
a) the current flowing through at every point is the same.
b) the combined resistance is the sum of the individual resistances.
c) the potential difference across the whole circuit is the sum of the potential
difference across each component.
14. In a parallel circuit,
a) the current from the voltage source is the sum of the currents in the separate
branches.
b) the potential difference across the effective resistance is the same as the
potential difference across each component.
Formulas:
 Direction of Convectional Current & Electron Flow
O LEVEL PHYSICS REVISION NOTES
I = Q / t
ampere = coulomb / second
= coulomb per second
 Electromotive Force, Volts
Volt = J/C
joule / coulomb
 Resistance of a Wire
Resistance, R ∝ 1 / A
R = ρl / A
l - length
A - cross sectional area
ρ - constant which depends on the material of the wire.
 OHM’S LAW
I ∝ V
V/I = R
or V = IR
 Series Circuits
Total resistance, R = R1 + R2 + R3
I = V/(R1 + R2 + R3)
V = V1 + V2 + V3
 Parallel Circuits
I = I1 + I2 + I3
1/R = (1/R1) + (1/R2) + (1/R3)
V = V1 = V2 = V3