AQA GCSE Physics: Unit 2

Electricity Section

Useful Information

• AQA Specification can be found by clicking this link. http://store.aqa.org.uk/sciencelab/AQA-PHYS-W-SP.PDF

• GCSE Bitesize relevant pages (Electricity section)http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/

• S-cool physics current electricity section. http://www.s-cool.co.uk/gcse/physics/static-and-current-electricity

• When certain insulating materials are rubbed against each other they become electrically charged.

• Negatively charged electrons are rubbed off one material onto the other.

• The material that gains electrons becomes negatively charged.

• The material that loses electrons is left with an equal positive charge.

• Have a look at these excellent simulations from the University of Colorado.

• Balloons and Electricity http://phet.colorado.edu/en/simulation/balloons

• John Travoltage http://phet.colorado.edu/en/simulation/travoltage

Attraction and Repulsion

Dangers of Static

• Can you think of any dangers of static electricity?

Uses

• Can you think of any uses of static electricity?• Some ideas here:

http://en.wikibooks.org/wiki/GCSE_Science/Uses_of_static_electricity

Charging by Induction

Can you explain how the balloon sticks to the wall?

Why does this girls hair stand on end?

Conductors

• In conductors such as metals electrical charges can move easily. This is because they have a sea of free electrons.

The Coulomb

• The charge on an electrons is very very small. • It is useful to think of electrons moving around

in groups called Coulombs. • We talk about a Coulomb of charge.

Voltage –see later for a fuller explanation

• Fancy name – Potential difference• When a coulomb of charge moves around a

circuit energy is transferred.• The amount of energy transferred per

Coulomb is called the voltage.

• Voltage = Energy/Charge

Electrical Symbols

• Do you know the symbols for the following electrical components?

Open switch Resistor Lamp

closed switch Variable resistor LED

Cell Lamp LDR

Battery Fuse Thermistor

diode voltmeter Ammeter

Resistance

• It is easier to pass current through some components than others.

• We say that the opposition provided by a component to the current flowing through it is called its resistance.

• The greater the resistance then the smaller the current for a given potential difference.

Georg Ohm

Define Ohm’s Law

• Ohms Law is to do with the relationship between voltage and current. It also is to do with temperature.

• You need to be able to write down the law.

Calculating Resistance

Electrical Current

• Electrical Current is a flow of electric charge. • In metals these charges are electrons. • The size of the electric current is the rate of

flow of electric charge.

Can you do these questions?

• A current of 2 A flows through an electric light bulb. How much charge flows through the bulb in two minutes?

• A charge of 100 Coulombs flow through a component for 20 seconds. What current flows through the component?

Ampere and Volta

Standard Test Circuit

Work out the resistance of each of the colour coded resistors

Colour Current through ( )

Voltage across ( )

Resistance ( )

Current – Potential Difference Graphs – metal wire at constant temperature

• You can learn about circuits easily by simulating them.

• Download this simulator from the University of Colorado and have a go. http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Table of Results Current (A) Voltage (V) Resistance (Ω)

Resisters in Series

What is the total resistance of this combination?

• If the potential difference of the cell is 60 V then what is the current flowing through all three resistors?

Resistors in Parallel

What is the total resistance?

Calculate the current in the two resistors and the total current

Cells in series and parallel

Cells in series

It can be more complicated if the cells are not all connected in the same direction

Current – Voltage graph for a filament bulb

Explaining changes in resistance

Can you explain what a Diode does?

Anyone for LED Eyelashes

How many uses for LED’s can you find?

Make sure you learn these facts about LED’s

• An LED emits light when a current flows through it in the forward direction.

• There is an increasing use of LEDs for lighting, as they use a much smaller current than other forms of lighting.

LDR’s

• Light Dependent Resistors • What are they used for?

How does the resistance change?

Thermistors – Can you think of some uses?

What is the relationship between resistance and temperature?

Learn this about a thermistor

• The resistance of a thermistor decreases as the temperature increases.

• A thermistor that does is said to have a _________ _________ __________.

Mains Electricity

AC and DC

• What does AC and DC stand for and what do they mean?

• Have a listed to AC/DC from starlight express. Click for the Youtube video: http://www.youtube.com/watch?v=c8OX7H1mJpw

Oscilloscope traces

Period and Frequency

• What is meant by the period of an oscilloscope trace?

• What is meant by the frequency of an oscilloscope trace?

• If the period of the mains is 0.02 seconds then what is the frequency?

• Remember f=1/T• What do f and T stand for?

Mains Electricity

• Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz) and is about 230 V.

Mains Cable Structure – Three Core

• Which colour represents the Live, Neutral and Earth?

Mains Cable Structure – Two Core

• What is the difference between two core cable and thee core cable? In what circumstances is 2 core cable used?

Three pin plug

What are the common faults when wiring a plug?

• How to wire a plug. Have a look at this DIY guide. http://www.pat-testing.info/wireaplug.htm

• Watch a video on how to do it. http://www.youtube.com/watch?v=ksuFrRU8BQk

Three Pin Plug Socket – Label each pin hole

What is a fuse?

Fuse Blow Videos – two of many – just search youtube

• http://www.youtube.com/watch?v=pJ9e4eaOiEk - several uses blown in slow motion.

• http://www.youtube.com/watch?v=sngiPk6sTBs - an electric cable in an ice storm.

• If an electrical fault causes too great a current, the circuit is disconnected by a fuse or a circuit breaker in the live wire.

• When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.

Key Points • The fuse is connected in the live wire.• Fuses don’t blow they melt!!

Do you know about fuse wire and cable thickness?

Double insulation

• What is meant by double insulation?

Circuit Breakers • RCCB – What do these letters stand for? • What is the main advantage of a circuit breaker compare to a fuse?

Have you seen something similar to this in your home?

Electrical Power

• Electrical appliances are labeled with an electrical power.

• Just have a look at the Argos catalog electric drill selection to see this. http://www.argos.co.uk/static/Product/partNumber/7114676.htm#pdpFullProductInformation

• A drill transfers electrical energy into kinetic, sound and heat. The useful energy is of course kinetic.

This drill has a power of 850 W. This means that in every second 850 Joules of electrical energy are transferred into kinetic, sound and heat energy.

Light bulb Powers

Which would you buy?

• Compact Fluorescent or filament lamps?

Two ways of Calculating Power

• Way 1 Multiply the current flowing by the voltage across.

• Way 2 Divide the energy transferred by the time taken to transfer it.

Power Equations

•P=IV •P=E/t

Power Equations in Triangles

• It may help you to put the equations into mathematical equations so you can rearrange them easily.

Can you do this question?

• An MP4 player transfers 4J of electrical energy (from the battery) to sound energy in 10 seconds.

• What is the power of the MP4 player in Watts?

What about this one?

• A 100 W lamp is switched on for 2 hours. How much energy is transferred to the room?

and this one………

• The information plate on a vacuum cleaner is labeled with the following.

Model 5349230 V ~ 50 Hz 500 WBEAB approved

1. Calculate the current in the vacuum cleaner.

2. What size of fuse should be fitted into the vacuum cleaner?

Calculating Fuse Values

• A fuse should be chosen that is just above the current value in the current.

• This means that if there is a fault not too much current will be allowed to flow before the fuse melts.

• This could be potentially dangerous and give an electric shock!

• If an appliance uses 4.4 A then a 5 A fuse should be fitted not 15 A etc……….

Voltage and Energy

• If is important to understand the difference between current and voltage.

• Current is a flow of charge whereas voltage is all about energy.

• As charges flow through components energy is transferred.

• The amount of energy per Coulomb of charge is called the voltage.

An equation linking voltage and energy

• Voltage = Energy/Charge or in symbols………• V=E/Q•Do you know how to put this

equation into a triangle?

Can you answer this question?In this very simple circuit the battery has a voltage of 6 V. In 10 seconds 30 Coulombs of charge flow. How much energy is transferred during this time?

Further Explanation

• If the voltage across a light bulb reads 12 V then what does this mean in terms of energy?

• If the voltage across a battery reads 24 V then what does this mean in terms of energy?

Homework questions and past paper style questions links

• Static Electricity • Circuits • Mains Electricity • Power in electrical appliances • Answers – No cheating!