Unit 4 - Electricity Topic 3

Resistance A 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.

Calculating Resistance: The unit of resistance is the ohm The symbol is omega ( Ω ) Resistance is measured using an ohmmeter. The relationship between resistance, voltage, and current is called OHM’s LAW V = I x R voltage = current x resistance

Problems: V = I x R I = V R R = V I V = I x R I = V R R = V I

Try this example: I = ? I = ? V I = V / R V I = V / R I = 9 [V] / 18 [Ω] I = 9 [V] / 18 [Ω] I = 0.5 [A] I = 0.5 [A]

Variable Resistors Some resistors can vary the amount of resistance they offer resulting in different amounts of energy being released. Examples: Dimmer switches Temperature controls on stoves Volume controls Variable speed motors like toy trains

There are two types of electrical circuits: SERIES CIRCUITSPARALLEL CIRCUITS

The 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.

PARALLEL 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.

Measuring current A A This is how we draw an ammeter in a circuit. SERIES CIRCUIT PARALLEL CIRCUIT

Measuring current SERIES CIRCUIT PARALLEL CIRCUIT current is the same at all points in the circuit. 2A current is shared between the components 2A 1A

Copy the following circuits and fill in the missing ammeter readings: ? ? 4A 3A? ? 1A ? 3A 1A