AQA P2 2.3 Currents in electrical circuits

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  • Created by: Rchilds
  • Created on: 01-06-17 22:31
What is static electricity?
When INSULATING materials are rubbed against each other they become electrically charged. Negatively charged ELECTRONS are rubbed off one material and onto the other one due to FRICTION
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Why can static electricity only be built up on insulators?
Once transferred, the electrons can't move through an insulating material. If a conductor was used e.g. metal, the transferred electrons would conduct away (i.e. not be static). (Note: static can be built up on conductors IF they are insulated)
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How do insulators become positive?
It loses electrons
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How do insulator become negative?
It gains electrons
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When two insulators are rubbed together, what can you say about how the charges on each compare?
They are opposite charges but equal in side (i.e. the number of electrons lost by one must equal the number of electrons gained by the other)
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What happens when two positive charges or two negative charges are brought together?
Similar charges repel each other (they feel a repulsive electrostatic force)
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What happens when a positive charge and a negative charge are brought together?
Opposite charges attract (they feel an attractive electrostatic force)
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Why can't metal objects build up static electricity? (unless they are insulated from the Earth e.g. by air or rubber (planes and helicopters, clouds etc)
Metals are conductors so the electrons will easily flow through the metal to the ground and therefore CHARGE will not build up
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What is an electric current? What is it measured in?
The rate of flow of electric charge e.g. electrons in metals. It is measured in Amps (A)
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What is a coulomb?
It is the unit of charge. (There is no point in measuring the charge on an individual electron as it's so small so we measure them in groups called a coulomb. 1 coulomb has approx. 6250000000000000000 electrons in it)
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What is the difference between a series and parallel circuit
In a series circuit all the electrons must flow through the same path (and therefore do the same thing), in a parallel circuit there is more than one path avaliable
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What does an ammeter do and how should it be placed in a circuit?
It measures current (DO NOT SAY IT MEASURES 'THE AMPS') and is placed in series with the circuit
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What does current do in a series circuit?
It is the same the whole way round (as current is the flow of charge (electrons) and in series they all do the same thing)
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What does current do in a parallel circuit?
It splits up. When the current reaches a junction in the wires, electrons will take different paths but the total current entering the junction will equal the total current leaving the junction
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What is potential difference (p.d.) and what is it measured in?
the work done (energy transferred) per coulomb of charge that passes between two points in a circuit. It is measured in volts
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What unit is the same as a volt and why? a) A/s b) J/C c) Ohms/C d) A/C
b) J/C because the definition of p.d.is the work done (in joules) per unit charge (in coulombs) so V = W/Q but in units volts = joules/coulombs so they are 2 ways of saying the same thing (equal)
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What does p.d. do in a series circuit?
the p.d. from the power source is shared between the components. (power source p.d. equals the total p.d. over the components added together)
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What decides how the p.d. splits between components?
How much resistance they have. Equal resistance means it will split evenly. Higher resistance requires more pd.
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What does p.d. do in a parallel circuit?
The p.d. over each path is the same as the p.d. from the power source
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What is the total p.d. when you place several cells in series?
total p.d. = sum of individual p.d.
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What is resistance and what is it measured in?
Anything that reduces the flow of charge in a circuit, measured in Ohms (Ω). The greater the resistance, the smaller the current
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Why do electrons in circuits experience resistance?
The electrons COLLIDE with the METAL IONS as they move through the metal
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How would you find the resistance of a component?
Use an ammeter in series to find the current and a voltmeter in parallel to find the p.d. then do R=V/I. If you need several values to then plot a graph, you need to also include a VARIABLE RESISTOR to change the p.d. over the component.
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When will the resistance through a resistor be constant and what does the current - potential difference graph look like for this?
When the temperature is constant. If resistance remains constant then you will get a straight line graph through the origin
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What happens if two resistors are placed in series with each other?
The total resistance = the sum of the individual resistances
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What relationship is shown for a straight line graph through the origin for a fixed resistor at constant temperature?
The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor
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What does the steepness of a current-potential difference graph tell you?
Steep means lower resistance
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What are two ways of increasing the current flowing through a circuit?
1) increase the p.d. of the power source 2) reduce the total resistance of the circuit
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What are the advantages of using a parallel circuit?
Components on separate paths can have their own switch. You can make sure each component has the same p.d. but can also have different currents
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What are the advantages of using a series circuit?
Resistance is higher so less current which is safer as there is less heating effect. You can ensure all components get the same amount of current.
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What does the current-potential difference graph look like for a bulb and what does this tell you?
It is curved, starting steep and gradually flattening out. This is because resistance INCREASES (don't just say 'changes' and if you are given number, take reading from the graph at a low p.d. and a high p.d. and calculate resistance to prove this)
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Why does the resistance of a filament bulb increase with p.d.?
Because it's temperature increases. This causes the IONs in the metal lattice to VIBRATE MORE which means the ELECTRONS COLLIDE with them more often, making it more difficult for charge to flow so resistance has increased
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What is a diode?
A component that only allows current to flow in one direction
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What is an LED?
A diode that emits light when a current flows through it in the forward direction.
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What can a diode be used for?
1) to protect a circuit and prevent current flowing the wrong way e.g. when you put batteries in backwards in a remote control 2) to turn alternating current into direct current
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Why does a diode only allow current to flow in one direction?
Because it has an extremely high resistance in the other direction due to the combination of materials used in it
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What does the current- potential difference graph look like for a diode?
It stays at 0 Amps for all negative voltages (as current can't flow) and for the first little bit of the positive voltages, then the current increases and the graph is NEARLY a straight diagonal line upwards (as resistance drops)
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What is an LDR?
Light dependant resistor. When light intensity increases, resistance decreases so more current can flow
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What can an LDR be used for?
Any electrical circuit that needs to respond to light e.g. turning on automatic street lighting at night, adjusting how long a shutter on a camera should be opened for depending on light levels etc
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What is a thermistor?
A resistor that is effected by temperature. When temperature increases, resistance decreases so more current can flow
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What can a thermistor be used for?
Any electrical circuit that needs to measure or respond to temperature e.g. an electric oven, a kettle (so the kettle knows when to stop boiling), automatic air conditioning system/thermostat, digital thermometer
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Other cards in this set

Card 2

Front

Why can static electricity only be built up on insulators?

Back

Once transferred, the electrons can't move through an insulating material. If a conductor was used e.g. metal, the transferred electrons would conduct away (i.e. not be static). (Note: static can be built up on conductors IF they are insulated)

Card 3

Front

How do insulators become positive?

Back

Preview of the front of card 3

Card 4

Front

How do insulator become negative?

Back

Preview of the front of card 4

Card 5

Front

When two insulators are rubbed together, what can you say about how the charges on each compare?

Back

Preview of the front of card 5
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