aqa gcse physics: unit 2
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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 - PowerPoint PPT PresentationTRANSCRIPT
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AQA GCSE Physics: Unit 2
Electricity Section
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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
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• When certain insulating materials are rubbed against each other they become electrically charged.
• Negatively charged electrons are rubbed off one material onto the other.
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• 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
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Attraction and Repulsion
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Dangers of Static
• Can you think of any dangers of static electricity?
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Uses
• Can you think of any uses of static electricity?• Some ideas here:
http://en.wikibooks.org/wiki/GCSE_Science/Uses_of_static_electricity
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Charging by Induction
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Can you explain how the balloon sticks to the wall?
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Why does this girls hair stand on end?
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Conductors
• In conductors such as metals electrical charges can move easily. This is because they have a sea of free electrons.
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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.
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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
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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
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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.
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Georg Ohm
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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.
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Calculating Resistance
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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.
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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?
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Ampere and Volta
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Standard Test Circuit
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Work out the resistance of each of the colour coded resistors
Colour Current through ( )
Voltage across ( )
Resistance ( )
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Current – Potential Difference Graphs – metal wire at constant temperature
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• 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
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Table of Results Current (A) Voltage (V) Resistance (Ω)
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Resisters in Series
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What is the total resistance of this combination?
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• If the potential difference of the cell is 60 V then what is the current flowing through all three resistors?
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Resistors in Parallel
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What is the total resistance?
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Calculate the current in the two resistors and the total current
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Cells in series and parallel
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Cells in series
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It can be more complicated if the cells are not all connected in the same direction
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Current – Voltage graph for a filament bulb
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Explaining changes in resistance
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Can you explain what a Diode does?
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Anyone for LED Eyelashes
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How many uses for LED’s can you find?
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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.
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LDR’s
• Light Dependent Resistors • What are they used for?
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How does the resistance change?
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Thermistors – Can you think of some uses?
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What is the relationship between resistance and temperature?
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Learn this about a thermistor
• The resistance of a thermistor decreases as the temperature increases.
• A thermistor that does is said to have a _________ _________ __________.
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Mains Electricity
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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
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Oscilloscope traces
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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?
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Mains Electricity
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• 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.
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Mains Cable Structure – Three Core
• Which colour represents the Live, Neutral and Earth?
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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?
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Three pin plug
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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
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Three Pin Plug Socket – Label each pin hole
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What is a fuse?
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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.
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• 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!!
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Do you know about fuse wire and cable thickness?
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Double insulation
• What is meant by double insulation?
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Circuit Breakers • RCCB – What do these letters stand for? • What is the main advantage of a circuit breaker compare to a fuse?
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Have you seen something similar to this in your home?
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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
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• 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.
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Light bulb Powers
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Which would you buy?
• Compact Fluorescent or filament lamps?
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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.
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Power Equations
•P=IV •P=E/t
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Power Equations in Triangles
• It may help you to put the equations into mathematical equations so you can rearrange them easily.
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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?
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What about this one?
• A 100 W lamp is switched on for 2 hours. How much energy is transferred to the room?
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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?
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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……….
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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.
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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?
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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?
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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?
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Homework questions and past paper style questions links
• Static Electricity • Circuits • Mains Electricity • Power in electrical appliances • Answers – No cheating!