05. Ohm's Law

In 1826, Georg Ohm discovered that the current flowing through a metal wire

is proportional to the potential difference across it (providing the temperature remains constant).

Resistance

The thin wire in a lamp tends to resist the movement of electrons in it. We say that the wire has a certain resistance to the current. The greater the resistance the more voltage is needed to push a current through a wire. The resistance of a wire is calculated by:

Resistance, R = p.d. across the wire (V)

current through the wire (I)

R = V

I

Resistors

Resistors (sometimes made of a length of nichrome wire) can be used to reduce the current in a circuit.

A variable resistor or rheostat is used to vary the current in a circuit.

For more information on Resistance and Ohm's Law, look at the BBC Bitesize link and try to answer the questions.

Four factors affecting resistance

  1. As the length increases, the resistance...

  2. As cross-sectional area increases, the resistance...

  3. As temperature increases, the resistance of a wire...

  4. The material the resister is made from.

When a material exhibits a linear relationship between p.d. (V), and current (I), it is said to be ohmic. Not all materials are ohmic, but metals are under usual conditions. For ohmic conductors the relationship is directly proportional, but the amount of current flowing for each volt of p.d. will depend on the conductive properties of the material. The proportionality is called the resistance (R) of the material, where

V = IR

and R is measured in ohms, which have the Greek symbol omega, Ω. This relationship is called Ohm's law.

This table shows the resistivity of common materials.

Resistivity is the inverse of conductivity, so

resistivity, ⍴ = ______1_______

conductivity, 𝛔

Resistivity is the resistance of a material for a unit area per unit length - in other words, for a fixed size of material.

Resistance causes energy to be lost in a conductor, in the form of heat. This is why resistance results in a potential difference. Resistance can be a problem. Carefully controlling resistance in a circuit enables us to control the p.d. and current too. Circuit components called resistors are precisely engineered to a known resistance, with a certain percentage uncertainty.