Factors Affecting Resistance of a

Factors Affecting Resistance of a

Conductor The amount of resistance offered by a conductor is determined by a number of factors.

Conductor resistance

Length - the greater the length the greater the resistance.

Cross sectional area - the larger the area the smaller the resistance.

The material - the resistance offered by a conductor will vary according to the material from which it is made.

Temperature - most metals increase in resistance as temperature increases.

Series Circuits When resistors are connected so that there is only one path

for the same current to flow through

each resistor they are connected in series. In a series circuit:

Current is the same in all parts of the circuit

 A simple series circuit

Applied voltage equals the sum of the

volt drops around the circuit Total resistance of the circuit, equals

the sum of the individual resistance values.

Parallel Circuits When resistors are connected such that they provide more

than one path for the current to flow in, and have the same voltage across each component, they are connected in parallel. In a parallel circuit:

Voltage across all components of a parallel circuit is the same.

Total current from the source is the sum of the current flowing in each

 A simple parallel circuit

branch. The current splits up depending on each component resistance.

Total resistance of the circuit is the sum of the reciprocal (one divided by the resistance) values.

Magnetism and Electromagnetism Magnetism can be created by a permanent magnet or by an electromagnet. The space around a magnet in which the magnetic effect can be detected Electromagnet is called the magnetic field. Flux lines or lines of force represent the shape of magnetic fields in diagrams. Electromagnets are used in motors, relays and fuel injectors to name just

a few. Force on a current carrying conductor in

a magnetic field is caused because of two magnetic fields interacting. This is the basic principle of how a motor works.

Electromagnetic Induction When a conductor cuts or is cut by magnetism a voltage is induced in the conductor. The direction of this voltage depends upon the direction of the

Induction

magnetic field and the direction in which the field moves relative to the conductor. The size is proportional to the rate at which the conductor cuts or is cut by the magnetism. This effect of induction, meaning that voltage is made in the wire, is the basic principle of how generators such as the alternator on a car work. A generator is a machine that converts mechanical energy into electrical energy.

Mutual Induction If two coils, primary and secondary, are wound on to the same iron core, any change in

Transformer Ignition coil

magnetism of one coil will induce a voltage in the other. This happens when the primary current is switched on and off. If the number of turns of wire on the secondary coil is more than the primary, a higher voltage can be produced. This is called transformer action and is the principle of the ignition coil.

 State the factors that affect the resistance of a conductor.

 Look back over the previous section and write out a list of the key bullet points here:

VEHICLE ELECTRICAL COMPONENTS AND CIRCUITS

Switches ฀฀ A switch is a simple device used to break a circuit, that is, it prevents the flow of current. A wide range of

switches is used. Some switches are simple on/off devices such as an interior light switch on the door pillar. Other types of switch are more complex. They can contain several sets of contacts to control, for example, the indicators, headlights and horn. These are described as multifunction switches.

Resistors Good conductors are used to carry the

current with minimum

Resistors in use

voltage loss due to conductor resistance. Resistors are used to control the current flow in a circuit or to set