Look at the picture below

What else might effect the resistance of a wire?

P6 – Electricity for Gadgets

Lesson 2 – Non-ohmic conductors

Learning aim: Demonstrate an understanding of resistance and non ohmic conductors

Learning Objectives Success Criteria

Understand what current in a metallic conductor is.

Use atomic models to explain electrical resistance in metallic conductors.

Calculate resistance and interpret voltage–current graphs for filament lamps.

Recall what current in wires is (Grade E-D)

Describe what causes resistance. (Grade C)

Use kinetic theory to explain the link between resistance and temperature in a metallic conductor. (Grade C)

Describe and explain the voltage–current graph for a filament lamp. (Grade C-A)

True or False

False

An electric current is a flow of electrons.

True

The unit of electric charge is the ampere (A).

If the resistance of a circuit is increased, the current decreases.

Resistance = voltage ÷ current.

If the voltage is 12 V and the current 0.5 A, the resistance is 6 Ω.

The voltage is the same all around a series circuit.

A variable resistor can be used as a dimmer switch.A variable resistor can be used to control the speed of an electric motor.

The voltage across a 100 Ω resistor is 5 V when the current in it is 0.5 A.

If the length of a piece of resistance wire is doubled, its resistance doubles.

Electron flow in a wire

Electricity is the flow of electrons along a wire.

As the electrons move along the wire they collide with the metal atoms.

These collisions make the atoms vibrate more……which makes the metal hotter.

All wires and devices have some resistance, which is why electrical appliances always waste some energy as heat.

Electron flow and resistance

Material – Different materials have different resistances because some materials are better conductors. Nichrome wire has a higher resistance than a copper wire of the same size.

Length – The longer a wire is the higher its resistance. When electrons travel down a long wire they can collide with more metal ions than in a short wire.

Thickness – A thin wire has a higher resistance than a thick wire.

Temperature – The higher the temperature of a wire the higher its resistance. Metal ions vibrate more at higher temperatures and so collisions with electrons are more likely to happen.

Factors that affect resistanceThe resistance of a wire depends on several factors:

Copy and complete:

The resistance of a wire depends on 4 factors:1. ____________2. ____________3. ____________4. ____________

Higher:For each of the factors, explain how resistance is affected using ideas about electron movement.

Plot the current and voltage readings for a filament bulb.

The higher the temperature of a wire, the higher its resistance.

The graph produced is not a straight line but a curved line.

Increasing the voltage across the filament in the bulb causes this wire to get very hot and give out light.

Current-voltage graphs for a bulb

As the wire gets hotter, its resistance gets higher, which means the current flow is less. So as the temperature rises the current is not proportional to the voltage.

Vo

ltag

e (

V)

Current (A)

1. A wire or resistor

Which of the components obeys Ohm’s Law?

I

V

I

V

I

V

1 2 3

Current-voltage graphs and Ohm’s Law

2. A filament lamp

3. Wires of different materials

Resistance – true or false?Plenary

Learning Objectives Success Criteria

Understand what current in a metallic conductor is.

Use atomic models to explain electrical resistance in metallic conductors.

Calculate resistance and interpret voltage–current graphs for filament lamps.

Recall what current in wires is (Grade E-D)

Describe what causes resistance. (Grade C)

Use kinetic theory to explain the link between resistance and temperature in a metallic conductor. (Grade C)

Describe and explain the voltage–current graph for a filament lamp. (Grade C-A)

Plenary

Complete the sentences – P6a6

Current and Charge revision

An electric current is the flow of charges. Current is measured in amperes with an

ammeter connected in series Charge is measured in coulombsOne coulomb is the total charge supplied by

a current of one ampere in one secondCharge = current x time ( Q = I x t )

Q

tI