# 3.01 Resistance.

Objectives.

After completing this section you will be able to:

• Define the term resistivity.
• Calculate the resistance of a material given its resistivity, length and cross sectional area.
• Recall that resistance converts electrical energy into heat.

Introduction.

We have now seen that electrical current is defined as, the rate of flow of charge in an electrical circuit. Voltage sources such as batteries produce an electromotive force (e.m.f.). The e.m.f increases the electrical potential (the energy per unit charge) of the electrons, as they move through the power supply. If a conductive circuit is then connected between the terminals of the battery, the electrons will drift through the circuit due to the attraction of the positive terminal. The electrons carry charge and so this produces an electrical current. As the electrons move through the circuit, their electrical potential is reduced by the resistance in the circuit, which converts the energy supplied by the e.m.f., into heat energy. In this section we will consider how resistance restricts the current flow and how it converts electrical energy into heat energy.

## Drude's model of resistance.

The electrons are initially accelerated due to the attraction of the positive terminal, (and the repulsion from the negative terminal). As the electrons move through the conductor, they collide with the molecules in the material, loosing energy in the process. The electrons kinetic energy, gets transferred to the molecules during the collision. This brings the electrons to a stop and causes the molecules to vibrate. The electrons are then accelerated again due to the force they experience, until a further collision occurs. This process is repeated continuously, as the electrons move through the cct. The net result of this, is that the motion of the electrons is impeded and therefore current flow is restricted. The vibrational energy of the molecules, is heat energy. Therefore the collisions supply heat to the material, raising its temperature. This heat energy is then dissipated into the environment. The net result is that resistance limits current flow, reduces potential and converts electrical energy into heat.