# Physics chapter 5

• Created by: abiiwelsh
• Created on: 23-06-18 12:00
What is direct current?
Current that only goes in one direction
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What is alternating current?
Current that repeatedly reverses direction
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What is the frequency of UK mains supply?
50 Hz- 50 cycles per second
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How much PD does a power station generate?
25000 V
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How is electricity transferred from station to national grid?
Set- up transformers- they increase potential difference from 25,000 V to 132,000 V
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How is electricity transferred from national grid to consumers?
Step- down transformers- decrease potential difference from 132,000 V to 230
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Why is grid PD so large?
Less current needed to transfer same amount of electricity- less resistance- less energy lost- efficient
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When does the earth wire carry a current?
If there is a fault- it is usually at 0 V
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Why is the earth wire connected to the longest pin?
If you plug appliance with metal case into wall socket and live wire breaks, you could become electrocuted- earth wire means it is earthed and appliance is safe
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Which metal is used for pins?why?
Brass- strong, conductive, doesn’t oxidise or rust
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Where is the fuse in a plug?
Between live wire and pin- melt if too much current
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What is the brown wire?
Live
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What is the yellow and green wire?
Earth
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What is the blue wire?
Neutral
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What are the wires made of?
2-3 copper wires surrounded by rubber
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Why is copper used for wires?
Flexible and good conductor
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Why are cables joining wall sockets thicker than cables joining light fittings?
Bigger current flowing through cables for wall socket- need bigger cable so less resistance- doesn’t heat up as much
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What is a short circuit?
When a live wire and neutral wire touch- big current
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What are the formulas for working out power transferred to appliance?
Power (W)= energy transferred (J) / time (s) OR power = current^2 x resistance
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What is the formula for working out power supplied to appliance?
Power supplied = current x potential difference
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What is the formula for charge flow?
Charge flow (C) = current x time
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How is the work done by the battery related to energy transfer to resistor?
The work done by battery forceing electrons round is equal to the enegy transferred to resistors kinetic and thermal energy store
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What is the formula for energy transfered to a resistor in given time?
Energy = charge flow x potential difference
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What is the formula for energy transfered from mains?
Energy transfered = power x time
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What is the formula for efficiency of device?
Efficiency= ( output power / input power ) x100%
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## Other cards in this set

### Card 2

#### Front

What is alternating current?

#### Back

Current that repeatedly reverses direction

### Card 3

#### Front

What is the frequency of UK mains supply?

### Card 4

#### Front

How much PD does a power station generate?

### Card 5

#### Front

How is electricity transferred from station to national grid?