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 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 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 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 resistance Factors that affect resistance The 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 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. Voltage (V) Current (A)

1. A wire or resistor Which of the components obeys Ohm’s Law? I V I V I V 123 Current-voltage graphs and Ohm’s Law Current-voltage graphs and Ohm’s Law 2. A filament lamp 3. Wires of different materials 

Resistance – true or false? 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 coulombs One coulomb is the total charge supplied by a current of one ampere in one second Charge = current x time ( Q = I x t ) An electric current is the flow of charges. Current is measured in amperes with an ammeter connected in series Charge is measured in coulombs One coulomb is the total charge supplied by a current of one ampere in one second Charge = current x time ( Q = I x t ) Q t I

Current questions 1. a) What electrical quantity is measured in coulombs? b)What electrical quantity is measured in amperes? c)What is the connection between “coulombs” and “amperes”? 2. The current through an electric heater is 12 A. a)What does this tell us about the charge flowing through the heater? b)How much charge flows through the heater in 10 seconds? c)How much flows in 2 minutes? 3. A current of 12 A flows for 20 minutes into an electric cooker. How much charge has the cooker used? 4. A current of 3 A flows into a television set. How long would it take for 1500 C of charge to flow through it? coulombs of electricity flow through a car bulb in one minute. What is the current through the bulb? 1. a) What electrical quantity is measured in coulombs? b)What electrical quantity is measured in amperes? c)What is the connection between “coulombs” and “amperes”? 2. The current through an electric heater is 12 A. a)What does this tell us about the charge flowing through the heater? b)How much charge flows through the heater in 10 seconds? c)How much flows in 2 minutes? 3. A current of 12 A flows for 20 minutes into an electric cooker. How much charge has the cooker used? 4. A current of 3 A flows into a television set. How long would it take for 1500 C of charge to flow through it? coulombs of electricity flow through a car bulb in one minute. What is the current through the bulb?