Resistivity

What does resistance of a conductor depend on?

• Length

• Cross sectional area

• Material

• Temperature

Resistivity

Electrical resistance of a conductor of unit cross sectional area and unit length

Formula for resistivity

rho = RA/l

Where R is resistance, A is cross sectional area(m²) and l is length(m)

Micrometer

A device used to measure small distances - i.e the diameter of a wire.

A resistor

A device that converts electrical potential energy into some other form.

Multimeter

Electrical instrument which can be used to measure voltage, current or resistance.

Purpose of a Wheatstone bridge

To find resistance of an unknown resistor.

Formula and setup of a Wheatstone bridge:

R1/R2 = R3/R4

Galvanometer

Very sensitive analogue ammeter which registers whether or not there is any potential difference between two points. If a Galvanometer is balanced there will be no deflection of the pointer.

Uses of a Wheatstone bridge

• Temperature control (the resistance increases when it heats up)

• A stress gauge (The resistance of a conductor in a strain gauge increases when a force is applied to it.)

The Metre Bridge

• Same as Wheatstone bridge except two of the resistors are replaced by a single strip of uniform resistance wire.

• The balance point can be reached by simply sliding the contact wire along this lower uniform resistance wire

• One of the resistors is known and the two lengths can be measured.

• Resistance is proportional to length so instead of using R3/R4 we can use l1/l2.