Topic 5 - Electricity and Magnetism

Coulomb's Force Law (unit: N)

The force between two point charges is proportional to the product of the charges and inversely proportional to their separation squared.

F = | q1q2 | / r²
k = 1/4πε

Electric field strength (unit: N/C \= V/m)

The force per unit charge on a positive point charge.

E = F/q

Electric potential (unit: J/C \= V)

The work done by the electric field per unit charge

V = W/q

Resistance (unit: Ω)

The ratio of the applied voltage to the resulting current.

R = V/I

Electromotive Force (emf) (unit: V)

The energy, per unit charge, available to the entire circuit.

Ohm's 1st law

When voltage is proportional to current (i.e. resistance is constant) we say Ohm's 1st law is satisfied (or we say the device is ohmic). E.g. a metal wire at constant temperature is ohmic.

When satisfied, V = RI

Internal resistance (unit: Ω)

A constant resistance inside a cell, which leads to energy dissipation in the cell. Modeled as a resistance in series with an ideal emf , hence the terminal voltage of a cell is V = E-rI.

Note: Sometimes the above equation is also referred to as Ohm's 2nd law.

Current (unit: A)

The amount of charge per unit time passing through a cross

The copper wires and insulator are both exposed to an electric field. Discuss, with reference to charge carriers, why there is a significant electric current only in copper wires.

• when an electric field is applied to any material «using a cell etc» it acts to accelerate any free electrons

• electrons are the charge carriers «in copper»

• metals/copper have many free electrons whereas insulators have few/no free electrons/charge carriers

Explain, in terms of electrons, what happens to the resistance of the cable as the temperature of the cable increases

• «as temperature increases» there is greater vibration of the metal atoms/lattice/lattice ions OR increased collisions of electrons

• drift velocity decreases «so current decreases»

• «as V constant so» R increases