# Physics Topic 2 - Electricity

What is electric current?
The flow of electrical charge, which only flows round a complete circuit if there is a potential difference.
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What is potential difference and what is its unit?
The driving force that pushes the charge round, measured in volts (V).
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What is resistance and what is its unit?
Anything that slows the flow down, measured in ohms.
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What is the formula for charge flow?
Charge flow (coulombs, C) = Current (I) x time (s)
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What is the formula for potential difference?
Potential difference (V) = Current (A) x Resistance (ohms)
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What is the relationship between the resistance of ohmic conductors (eg. a wire) and current?
The resistance doesn't change with current. At a constant temperature, the current flowing is directly proportional to the potential difference.
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How is this different with a filament lamp?
When a charge flows through a filament lamp, some energy is used to heat the filament up. Resistance increases with temperature so as current increases, resistance increases.
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What about for diodes?
The resistance depends on the direction of the current. They let current flow in one direction, but have a very high resistance if it is reversed.
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What does the term 'I-V characteristic' refer to?
A graph which shows how the current (I) flowing through a component changes as the potential difference (V) across it is increased.
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What is a light dependent resistor (LDR) and how does it work?
A resistor that is dependent on the intensity of light. In bright light, the resistance falls. In darkness, the resistance is highest.
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What is a thermistor and how does it work?
A temperature dependent resistor. In hot conditions, the resistance drops. In cool conditions, the resistance goes up.
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Give two ways of using LDRs and Thermistors in sensing circuits?
Set up a circuit with the thermistor followed by the fixed resistor and the fan in parallel. Connect a bulb in parallel to an LDR with a fixed resistor before.
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What is a series circuit?
This is where the different components are connected in a line, end to end, between the positive and negative of the power supply.
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What would happen if you removed or disconnected one component?
The circuit is broken and they all stop, meaning series circuits are not very practical and in practice very few things are connected in series.
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What happens to the potential difference in a series circuit?
The total potential difference of the power supply is shared between the various components, so the potential differences round the circuits always add up to equal the source pd.
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What happens to the current in a series circuit?
The same current flows through all the components. The size of the current is determined by the total pd of the cells and the total resistance of the circuit.
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What happens to the resistance in a series circuit?
The total resistance of two components is just the sum of their resistances. By adding a resistor, the two resistors have to share the total pd. The pd across each resistor is lower, so the current is also lower.
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What is a parallel circuit?
This where each component is separately connected to the positive and negative of the supply. If you remove or disconnect one of them, it will hardly affect the others.
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What happens to the potential difference in a parallel circuit?
All components get the full source potential difference, so it is the same across all components. Identical bulbs connected in parallel will all be the same brightness.
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What happens to the current in a parallel circuit?
The total current is equal to the total of all the currents through the separate compounds. The current splits/rejoins at junctions. Two identical components in parallel will have the same current.
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What happens when you add a resistor in a parallel circuit?
If you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors.
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What are the two types of electricity supplies?
Alternating current and direct current.
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What is an alternating current?
The current is constantly changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating.
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Give the voltage and frequency of the UK mains supply (alternating current)
230V and 50Hz
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What is direct current?
A current that is always flowing in the same direction, created by a direct voltage.
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Give an example of a direct current.
Cells and batteries.
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What is the brown wire in a three core cable?
This is the live wire, providing the alternating potential difference (230V) from the mains supply.
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What is the blue wire in a three core cable?
This is the neutral wire, which completes the circuit and carries away current- electricity normally flows in through the live wire and out through the neutral wire. It is around 0V.
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What is the green and yellow wire in a three core cable?
This is the earth wire, which protects the wiring and stops the appliance casing from becoming live. It only carries current if there is a fault, so is usually at 0V.
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What is the power of an appliance?
The energy that it transfers per second.
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How do you calculate energy transferred by electrical work?
Energy transferred (J) = Power (W) x Time (s)
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Give the formula for linking energy transferred, charge flow and potential difference.
Energy transferred (J) = Charge flow (C) x Potential difference (V)
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What is the equation for power?
Power (W) = Potential difference (V) x Current (A)
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What is the equation for power when you don't know potential difference?
Power (W) = Current (A) x Resistance (ohms)
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What is the national grid?
A giant system of cables and transformers that covers the UK and connects power stations to consumers.
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Why does the national grid use a high potential difference and low current?
The high potential difference is needed to transmit huge amounts of power, but a high current would cause lots of energy to be lost as the wires heat up. It is cheaper to boost the pd up really high (400,000V) and keep the current low.
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How does the national grid get the potential difference up to 400000V?
They use transformers as well as big pylons with huge insulators. The transformers step the potential difference up at one end, for efficient transmission, and then bring it back down safe, usable levels at the other end.
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What causes static electricity?
When certain insulating materials are rubbed together, electrons are scraped off one and dumped on the other. This leaves the materials electrically charged (one positive, one negative)/
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What are the classic examples of the two insulating materials?
Polythene and acetate rods being rubbed with a cloth duster.
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How can too much static cause sparks?
Electric charge builds on the object, so the potential difference between the object and the earth increases. If the pd gets large enough, electrons jump the gap between the charged object and the earth, causing a spark.
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Where is an electric field created and where is it strongest?
An electric field is created around any electrically charged object. The closer to the object you get, the stronger the field is.
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How do you show an electric field line around an object?
You use field lines. On an isolated, charged sphere: electric field lines go from positive to negative, they are always at a right angle to the surface and the closer together the lines are, the stronger the field is.
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What happens to a charged object in an electric field?
The electric fields of each charged object interact with each other, and the charged object feels a force, which then causes attraction or repulsion.
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How do electric fields cause sparks?
A high pd causes a strong electric field between the charged object and the earthed object. The strong electric field causes electrons in the air particles to be removed (ionisation). This makes the air much more conductive so current can flow.
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## Other cards in this set

### Card 2

#### Front

What is potential difference and what is its unit?

#### Back

The driving force that pushes the charge round, measured in volts (V).

### Card 3

#### Front

What is resistance and what is its unit?

### Card 4

#### Front

What is the formula for charge flow?

### Card 5

#### Front

What is the formula for potential difference?