Topic 3

Unit 4 - ElectricityTopic 3

Topic 3:Resisting the Movement of Charge

2ResistanceResistanceA property of a resistor or load that slows charges.It converts electrical energy into other forms of energy.Good conductors offer little resistance like metal wires.A toaster has nichrome wires which offers a lot of resistance creating a lot of heat.

3Calculating Resistance:The unit of resistance is the ohmThe symbol is omega ()Resistance is measured using an ohmmeter.The relationship between resistance, voltage, and current is called OHMs LAW V = I x R voltage = current x resistance

4Problems:V = I x R

I = V R

R = V I

5

Try this example:I = ?V I = V / R I = 9 [V] / 18 [] I = 0.5 [A] 6Variable ResistorsSome resistors can vary the amount of resistance they offer resulting in different amounts of energy being released.Examples:Dimmer switchesTemperature controls on stovesVolume controlsVariable speed motors like toy trains

7Types of circuitThere are two types of electrical circuits:

SERIES CIRCUITSPARALLEL CIRCUITS8The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round.SERIES CIRCUITS

If one bulb blows it breaks the whole circuit and all the bulbs go out.9PARALLEL CIRCUITS

The current has a choice of routes.The components are connected side by side. If one bulb blows there is still be a complete circuit to the other bulb so it stays alight.10Measuring currentAAThis is how we draw an ammeter in a circuit.SERIES CIRCUITPARALLEL CIRCUIT11Measuring currentSERIES CIRCUITPARALLEL CIRCUIT current is the same at all points in the circuit.2A2A2A current is shared between the components2A2A1A1A12Copy the following circuits and fill in the missing ammeter readings:??4A4A4A3A??1A?3A1A1A13