Potential Difference and Electromotive Force

Potential difference (work done per unit charge)
- —— energy transformed from electrical to other forms per unit charge.
Electromotive force (work done per unit charge)
- —— energy transformed from other forms to electrical.

Internal Resistance

Internal Resistance: resistance to current flow within the power source; reduces p.d. when delivering current.Voltage across resistor: V = IRV=IR
Voltage lost to internal resistance: V = IrV=Ir
Thus e.m.f.: E = IR + IrE=IR+IrE=I(R+r)

Kirchhoff's 1^st Law

Sum of currents into a junction IS EQUAL TO Sum of currents out of junction.
Kirchhoff’s 1^st law is another statement of the law of conservation of charge.

Kirchhoff's 2^nd Law
Sum of e.m.f.s in a closed circuit IS EQUAL TO Sum of potential differences.
Kirchhoff’s 2^nd law is another statement of the law of conservation of energy.

Applying Kirchhoff’s Laws

Example: Calculate the current in each of the resistors

Deriving Effective Resistance in Series

Deriving Effective Resistance in Parallel

Potential Divider

A potential divider divides the voltage into smaller parts.
Usage of a thermistor at R₁:
- Resistance decreases with increasing temperature.
- Can be used in potential divider circuits to monitor and control temperatures.
Usage of an LDR at R₁:
- Resistance decreases with increasing light intensity.
- Can be used in potential divider circuits to monitor light intensity.

Potentiometers

A potentiometer is a continuously variable potential divider used to compare potential differences.
Potential difference along the wire is proportional to the length of the wire.
Can be used to determine the unknown e.m.f. of a cell.
This can be done by moving the sliding contact along the wire until it finds the null point that the galvanometer shows a zero reading; the potentiometer is balanced.
Example: E₁ is 10 V, distance XY is equal to 1m. The potentiometer is balanced at point T which is 0.4m from X. Calculate E₂