P5 Electricity in the Home Flashcards
What is the difference between d .c and a.c
Direct current only flows in one direction.
On a voltage-time graph this would appear as a straight horizontal line at a constant voltage.
Alternating current
The electricity supply to your home is alternating current.
The National Grid distributes electricity throughout the country and this is referred to as “mains” electricity.
The voltage and current change size and direction in a regular way which repeats as time goes on.
On a voltage-time graph, this would appear as a curve alternating between positive and negative voltages - the positive and negative values indicate the direction of current flow.
What are the mains electricity delivered at in the UK?
In the UK, the mains electrical supply is generated at a frequency of 50 Hz and is delivered to houses at an average of 230 V.
What unit do electrical companies use?
A joule is much too small a unit of energy and so the electricity companies use units called kilowatt-hours, kWh.
What is the equation for electricity cost?
total cost = number of units used × cost per unit
An electricity bill has two important numbers: present meter reading and previous meter reading.
The number of units used is the difference between these two readings.
The cost per unit is set by the electricity company.
Which equation helps you find the number of units?
Number of units used = power rating in kW x time in hours
e.g
Calculate the cost of using a 2 kW heater for 6 hours if the price of a unit is 13p.
Number of units used = power rating in kW x time in hours
Time = 6 hours
Power rating = 2 kW
Number of units used = 2 kWh x 6 h = 12 kWh
total cost = number of units used × cost per unit
= 12 kWh x 13p
= 156p
= £1.56
The cost of using the heater is £1 .56
Difference between a step up and a step down transformer?
A step-up transformer beside the power station to step-up the voltage and consequently step-down the current reducing electrical energy wasted as heat energy in the overhead cables. Electricity is sent through these at 400,000 V.
A step-down transformer
close to domestic consumers to step the voltage down to 230 V for safe use in the home.
Why are plugs made from copper and plastic?
The cable between the device and the three-pin plug contains three copper wires that are coated with plastic.
copper wires are good conductors;
plastic is a good insulator.
What are the features of a plug?
Outer insulation -All three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety.
Cable grip-This holds the cable tightly in place so that wires do not become loose.
Live wire-Copper wire coated with brown plastic along which the current enters the device.
Fuse-A glass or ceramic canister containing a thin wire that melts if the current gets too high.
Neutral wire-Copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit.
Earth wire-Copper wire coated in striped green and yellow plastic that provides a path for current to flow from the case of the device to the ground if there is a fault.
What are the safety features of Plugs?
Earth Wire(Earthing)
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the metal case.
The next person who uses the appliance could get electrocuted.
The earth pin on the three pin plug is connected by a wire to the metal body of the appliance.
This wire is connected to earth via the plug socket to a metal plate or water pipe underground.
As the wire is made of copper, the earth wire provides a low resistance path to the ground.
In the event of a fault, the live cFuses
A fuse provides a built-in fail-safe to the electrical circuit for a device.
The fuse contains a thin wire that will melt if the current gets too high.
If there is a fault that causes the casing of the device to become live, a large current will flow through the live wire and low-resistance earth wire.
This high current will cause the fuse to melt.
Once the fuse has melted, the circuit is broken and no more current flows through the device.
This means the case of the device is no longer live and there is no more risk of electrocution.
A circuit breaker can serve the same function as a fuse but can be reset without the need for replacement if it trips.
The fuse or circuit breaker must be connected in the live wire side of a domestic circuit to ensure that it keeps high voltage from reaching the user, or surroundings, if a fault develops.
A fuse protects the appliance but does not protect the person using it.
It can take up to 2 s for a fuse to melt which is long enough for the user to be electrocuted.
Currents as small as 50 mA (milliamp) can cause electrocution and a fuse would not prevent this from flowing along the live wire.urrent passing through the case will follow this path to the ground instead of passing through a person.
As the earth wire has virtually no resistance, a large current flows.
This causes the fuse to blow, preventing any further current flow.
What are the sizes for fuses?
Fuses come in various ratings, but the common sizes are 3 A, 5 A and 13 A.
What is the formula for power involving voltage and current.
Electric power P = voltage V x current I
P = VI
Question
A bed side lamp is rated 60 W, 240 V.
Calculate the size of fuse that should be fitted to the lamp for it to operate safely.
The fuse available are 3 A, 5 A and 13 A.
Answer:
P = VI
P = 60 W
V = 240 V
60 W = 240 V x I
I = 60/240
I = 0.25 A
The normal current for the lamp is 0.25 A. Hence a 3 A fuse would be selected.
Where is a switch or fuse positioned?
The purpose of a switch or a fuse is to break or connect an electric circuit, but they must be correctly connected in the home.
A switch or a fuse in an electrical circuit is always connected to the live wire so that the socket or appliance is not live when switched off.
If the switch or fuse is placed in the neutral wire, the electrical appliance is still connected to the high voltage live wire even when the switch is open, or the fuse is blown.
This could cause the user to get an electric shock if they touched the inside of the appliance.
What is double insulation?
Appliances encased in an insulating plastic case or body, such as hair dryers, mobile phone chargers and hand whisks do not need an Earth wire.
Such appliance are designed so that there are two levels of insulating material between the electrical parts of the appliance and any parts on the outside that can be touched.
These two layers are normally the insulating plastic case and the insulated cable carrying the live and neutral wires.
A metal attachment, such as a whisk blade, is fitted into a plastic case so that it does not make direct contact with the internal electrical parts.
Even though the device contains a live wire at mains voltage, there are two layers of plastic separating the user from this live wire.
The symbol for double insulation on an appliance is:
What is the formula for Power with resistance?
P= I squared (I^2) x R
power = current squared x resistance
