P5 Revision.

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  • Created by: ElishaG
  • Created on: 15-06-16 21:34
What happens when 2 insulating materials are rubbed together?
Electrons are scraped off and dumped on one another.
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What charge do electrons have?
Negative charge.
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What happens when electrons are transferred?
Positive static charge on one object (electron scraped off) Negative static charge on other(gained electrons).
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What does the way electrons are transferred depend on?
The materials involved.
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Give an example.
Polythene and acetate rods being rubbed with cloth duster, with polythene electrons move from duster to rod. With acetate, electrons move from rod to duster.
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True or false- Only electrons are moved.
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What is left over?
Poisitively charged particles left over- called ions.
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What are positive and negative electrostatic charges only produced by?
Movement of electrons. Positive charges don't move.
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What is a positive static charge caused by?
By electrons moving away.
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What do like charges do?
Repel each other.
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What do opposite charges do?
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What is static electricity?
When oppositely charged objects try to repel each other but cannot move apart as positions are fixed.
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What is a build up of static caused by?
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What is static electricity responsible for?
1)Attracting dust.2)Clinging clothes and crackles.3)Hair standing up.
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Describe how static attracts dust.
Dust particles tiny and easily attracted to anything that's charged-glass,wood,plastic etc.
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Describe how static makes clothes cling and crackle.
Synthetic clothes are dragged over each other in drier or your head,electrons scraped off-static on both parts so they are attracted. There are sparks and crackles as the charges rearrange themselves.
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Describe how static effects hair.
Static builds up on hair, giving each strand the same charge, so they repel.
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What is electric current?
A flow of charge around a circuit.
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What are the metal conductors full of in an electrical circuit?
Charges(electrons) that are free to move.
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Why does electric charge flow through metal conductors?
Because the electrons are free to move around.
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What is an insulator?Give an example
Something that current cannot flow through as there are few charges free to move. E.g. plastic.
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What happens in a complete circuit?
The battery pushes the free charges through wires.Charge flows all the way around the circuit and back to the battery,it's not used up.
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What does current depend on?
Voltage and resistance.
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When will current flow through a component?
If there is a voltage across the component.
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What is current measured in?
Ampere (amps or A).
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What is voltage?
The driving force that pushes current round.
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What is voltage measured in?
Volts (V).
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What is resistance caused by?
Things in the circuit(components e.g. lamps) that resist the flow of charge(slows it down).
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What is resistance measured in?
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How do resistance and voltage decide how big the current will be?
The voltage is trying to push the current around the circuit and the resistance is opposing it.Relative sizes decide size of current.
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Finish the sentence: If you increase the voltage..
..then more current will flow. If you increase the resistance-the less current will flow.
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What is power?
The rate at which an electrical power supply transfers energy.
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What is power measured in?
Watts (W) or Kilowatts (Kw).
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What is anything that is supplying electricity also supplying?
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What do power supplies do?Give examples of power supplies.
Transfer energy to the charge, which then transfers it to the components. Power supplies= cells,batteries etc.
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Why is work done?
Because energy is transferred.
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What does an appliance with high power do?
Transfers a lot of energy in a short time. Energy comes from the current. Means appliance has high power and large current.
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What is the formula for power?
Power(W) = Voltage (V) x Current (A).
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The component, ammeter and variable resistor are all in a series circuit, what does this mean?
They can be put in any order in the main circuit.
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Where does the ammeter need to be placed?
In series with the component to measure the flow of current through it.
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Where does the voltmeter need to be placed?
Parallel around the component.
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What does varying the variable resistor do?
Alters the current flowing through the circuit.
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What is potential difference?What does it tell us?
Voltage. Tells us how much energy is transferred to or from each unit of charge as it moves between two points.
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What does a battery do?
Transfers energy to the charge as it passes- the 'push' that moves charge round the circuit.
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What do components do?
Transfer energy away from the charge as it passes e.g. to use as light in a lamp.
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What is potential difference a measure of?
Work done on or by a charge as it passes between 2 points.
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What does the voltage of a battery show?
How much work the battery will do to charge that passes through it.
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What is a voltmeter used for?
To measure the potential difference between 2 points.
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Where must a voltmeter be placed?
In parallel with a component so it can compare energy the charge has before and after passing through the component.
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What do voltage-current graphs show?
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What is a current through a component proportional to?
The voltage across it when resistance is constant.
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Why is resistance in wires usually ignored?
It is very small.
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What does the slope of a voltage-current graph show?
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Complete the sentence: The higher steeper the slope...
..the lower the resistance.
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What is the formula for resistance?
Resistance(Ohms)= Voltage(V) / Current(A).
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What happens to resistors when currents pass through them?
They get hot.
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Why do resistors get hot?
Because when the electrons move through a resistor, they collide with positive ions in the resistor-collisions cause vibrations-makes it hotter.
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What does LDR stand for?What is it?
Light dependent resistor. It changes its resistance depending on the amount of light.Bright light-resistance falls. Darkness-resistance high.
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What is a thermistor?
Like LDR but its resistance depends on temperature. Hot-resistance drops.Cold-resistance goes up.
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What are series circuits?
When the components are connected in a line, to both ends of the power supply.(Except voltmeters).
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Explain how potential difference is shared.
In series circuits PD is shared between the components. The PD round a circuit is always equal to the PD across the battery.
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Describe what current is like in a circuit.
In series circuit, current is the same everywhere.
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How is the sum of resistance in a circuit calculated?
By adding the sum of resistances on all components.
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Describe resistance in a circuit.
Resistance is bigger when there are multiple components as the battery has to push charge through them all.The bigger the resistance, the bigger the PD as more work is done.If the resistance changes, then the PD will as well/
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Describe cell voltages in a circuit.
In series, the more cells there are, the bigger the voltage. In parallel the voltages don't add up.
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What is cell current like?
Adding cells in series increases voltage without increasing resistance so current increases.In parallel, cells increase current of circuit but the current through each cell is less as they join together to make total current.
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What is the layout of parallel circuits like?
Each component is separately connected to the positive and negative ends of the supply.
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What happens if you remove or disconnect a component in parallel?
It often hardly effects the other components.
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What is PD like across components in parallel?
Equal to the PD of the battery.
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What is current like in parallel circuits?
Current in the separate branches is equal to the current flowing from the battery.
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What is resistance like in a parallel circuit?
Resistance of whole circuit is always lower than the lowest resistance of the branches Resistance lower as there are multiple branches-only some charge on each one-circuit has a higher current.
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What does a current going through a component depend on?Explain why.
Resistance.Each component in parallel is separately connected to battery.Resistance controls how much current the voltage is able to push through.Component with least resistance has largest current.
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What is the voltage of UK mains supply?
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What is the process called where UK electricity is produced?
Electromagnetic induction.
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What kind of current is Mains electricity?What does this mean?
Alternating current (AC). Means current is constantly changing direction.
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What current do batteries have?What does this mean?
Direct current (DC). Current always flows in same direction.
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Why is AC used for mains electricity?
It's easier to generate than DC and is easier to distribute over long distances.
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How is a voltage induced in a coil?
By moving a magnet.
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Explain electromagnetic induction.
1)As magnet is moved, magnetic field through coil changes.Change induces voltage across ends of coil.2)If ends of wire connected, current will flow.3)Direction of voltage depends on the way you move magnet.
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What happens when you change the magnet from north to south in a coil?
The voltage is induced in the opposite direction.
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Explain AC generators.
1)Generator rotates in coil of wire , as magnet turns,magnetic field changes.This change induces voltage.2)Every half turn, voltage flows in opposite direction-AC current.
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What are four factors that affect the size of the voltage?
1)Add iron core inside coil.2)Increase strength of magnetic field.3)Increase speed of rotation.4)Increase number of turns on the coil.
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What are transformers used for?How do they do this?
Change the size of voltage- use electromagnetic induction to 'step up' or 'step down' the voltage.
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What do transformers consist of?
Primary coil, secondary coil that are both wound around an ion core.
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Explain transformers.
The AC in the primary coil causes the magnetic field to change-this induces a changing voltage in the secondary coil.
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What are step-up transformers?
Increase voltage.Have more turns on the secondary coil than the primary coil.
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What are step-down transformers?
Decrease voltage.Have less turns on secondary coil than primary coil.
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Explain how transformers work.
1)Primary coil produces magnetic field-stays in iron core.2)As there's AC current in primary coil,magnetic field in core is always changing direction.3)Changing direction induces alternating current across secondary coil.Continued....
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How transformers work continued..
4)Relative number of turns on coils determines whether voltage induced on secondary coil is less or greater than voltage in primary coil.5)Won't work with DC as no voltage would be induced across secondary coil.
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What is a magnetic field?
A region where magnetic materials and wires carrying currents experience a force acting on them.
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What does a current-carrying wire create?
A magnetic field.The field is made up of concentric circles.
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What does a rectangular coil reinforce?
Magnetic field.
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Explain magnetic fields in a circuit.
1)Current carrying wire or coil can exert force on another due to magnetic field.2)When wire is put in different magnetic field-magnetic fields affect one another-put force on wire.3)Wire has to be at Right angle to feel full force.
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Describe Fleming's left hand rule.
Thumb=motion. First finger=Field. Second finger=Current.
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Explain how magnetic fields make coils turn.
If rectangular coil is placed in uniform magnetic field,force will cause it to turn.This is called the motor effect.
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Describe an electric motor.
1)Coil on spindle,forces act one up, one down so it rotates.2)Split ring commutator swaps contacts every half turn,this reverses direction of current to keep coil rotating in same direction.3)Otherwise, motor would change direction every half turn.
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Explain how different devices use electric motors.
1)Link coil to axle,axle spins.2)Fan attached to axle.3)Axle on DVD players.4)Electric cars and trains wheels attached to axles.5)Washing machines,fridges etc use motors.
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What happens when electrons are transferred?


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Give an example.


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