P2.4 Mains Electricity

  • Created by: Fiona S
  • Created on: 01-03-15 15:50

Safety Devices

Fuses: circuit symbol (http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Fuse_symbol_Europe.svg/120px-Fuse_symbol_Europe.svg.png)

Usually...  3A   5A  13A

The power of an appliance:

Power = current x voltage
P        =      I    x   V
Watts  = Amps  x Volts
W       =    Hz    x  V

For appliances, always use the current one above e.g. 13A

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Circuit Breakers

Circuit Breakers

A circuit breaker is an electromagnetic device that breaks a circuit when the current goes above a certain value.
When the current in a circuit increases, the strength of the electromagnet will also increase. This will pull the electromagnet.
As a result, spring 1 pulls apart the contact and disconnecting the circuit immediately, stopping current flow.
The reset button can be pushed to bring the contact back to it's original position to reconnect the circuit.


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Fuses vs. Circuit Breakers

Both can prevent fire by limiting the current flowing through a cable or appliance.

Fuses are simple and cheap to replace

Fuses - Slower, must replace and cheap

Circuit Breakers act more quickly than fuses and can be reset.

RCCB - Residual Current Circuit Breakers --> Faster, can reset but expensive

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Energy Transfer

The power tells you how fast an appliance will transfer energy.

Electrical Power(W) = Energy transferred(J) / Time taken(s)

1 Watt = 1 Joule per second

P = E/t            E = P x t           t = E/p

P = Power(W)
E = Energy(J)
t = time(s)

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Alternating Current

  • Potential difference cycles smoothly between positive and negative values (+V and -V)
  • Current is changing direction back and forth
  • Electrons move back and forth around the same spot in the circuit
    Examples: TV, Toaster, Oven, Computer, Kettle, Fridge


Direct Current

  • Constant potential difference (V)
  • Current always flows in the same direction
  • Electrons flow around the circuit 
    Example: Torch, Phone, Laptop, Watch, Alarm Clock, Remote


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AC + DC graph


Time Period = number of seconds per cycle

Frequency = number of cycles per second

Time Period (s) = 1 / frequency

Frequency (Hz) = 1 / time period

Frequency of UK supply is 50Hz, so the time period is 0.02s

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Energy and Charge in Electric Current

V = I x R
P = V x I
P = E / T
E = V x Q
Q = I x t

V = Voltage
I = Current
R = Resistance
P = Power
E = Energy
T/t = Time Period
Q = Charge

Voltage is Energy per unit charge.
Current is the rate of flow of charge.

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3-pin Plug

Outside is plastic, Wires are copper (good conductor), Pins are brass(hard metal).

3 terminals and 3 Wires - Live wire (Brown), Earth Wire (Yellow/Green) and Neutral Wire (Blue).


Cable Clamp - holds wires together.

Plastic Casing around wires - reduces chance of electrocution from the wires inside.

Fuse melts if currents too high.

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