Using Mains Electricity Safely And The Power Of Electrical Appliances Flashcards Preview

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what is a direct current (d.c.)?

Cells and batteries supply currents that always passed in the same direction


What is an alternating current (a.c)?

It is one that is constantly changing direction


What is mains electricity?

it is an a.c. supply. In the UK is has a frequency of 50 cycles per second (50 hertz) and is about 230 V


What are most appliances connected to?

to the mains using cable and a three-pin plug


How does a three-pin plug work?

It has a live wire (brown)
a neutral wire (blue)
an earth wire (green and yellow)
When we use electrical appliances with metal bodies, we require protection against possible electric shock. If there is a fault current (leakage) inside the appliance, the whole metal case becomes live. If you happen to touch it, a current will flow through you to the earth (ie. electric shock). Hence, to prevent electric shock, we need to ensure that the system is properly earthed (= 'grounded'). This way if you touch a faulty appliance, you don't get electrocuted!


What is the structure of an electrical cable?

It was 2 or 3 insulated copper wires surrounded by an outer layer of flexible plastic material


How does the earth pin work and what does it do?

The earth pin on a plug is longer than the live and neutral pins. This means the earth pin is the first to connect and the last to disconnect.
When inserting the plug, the earth connection is made before the current carrying contacts of the plug become live.
When withdrawing the plug, the current carrying contacts shall separate before the earth connection is broken.
Thus, the earthing connection is always maintained to improve safety.


What happens if an electrical fault causes too great a current?

The circuit is disconnected by a fuse or a circuit breaker in the live wire


What happens when the current in a fuse wire exceeds the rating of the fuse?

it will melt, breaking the circuit


How are some circuits protected by?

Residual Current Circuit Breakers (RCCBs)


How do RCCBs operate?

They operate by detecting a difference in the current between the live and the neutral wires. These operate much faster than a fuse


Appliances with metal cases are usually....

Earthed. Some appliances a double insulated, and therefore have no earth wire connection


What protects the wiring of te circuit?

The earth wire and the fuse together


What happens when an electrical charge flows through a resistor?

the resistor gets hot. A lot of energy is wasted in filament bulbs by heating. Less energy is wasted in power saving lamps such as CFLs


What is power?

It is the rate at which energy is transfered by an appliance


P = ------ What do these symbols mean?

P = power in watts
E = energy in joules
t is time in seconds


P = I x V What do these symbols mean?

P = power in watts
I = current in amps
V = potential difference in volts


E = V x Q What do these symbols mean?

E = energy in joules
V = potential differece in volts
Q = charge in coulombs