Flashcards in Mains Electricity Deck (36):
How is the electricity around a home delivered?
Via a ring main which consists of 3 main wires, the live wire, the neutral wire and the earth wire
The UK mains voltage supply is 230V a.c at a frequency of 50Hz
Describe the purpose of the live wire
The live wire provides the path along which the electricity energy from the power station travels at alternating current and is the BROWN wire
Alternates between - and + voltage
Describe the purpose of the neutral wire
The neutral wire completes the circuit as once the energy has been transferred the current returns along the neutral wire and is the BLUE wire, electricity normally flows in though the live wire and out through the neutral wire
Always at 0V
Describe the purpose of the earth wire
The earth wire is a low resistance connection to the Earth, it is there for safety and no current should normally flow in the earth wire and is the YELLOW and GREEN wire
What is an example of a good insulator and a good conductor?
Plastic is a good insulator (case, cable grip and cable insulation) and metal is a good conductor
Which path will current always take?
Current is lazy and so will always take the path will the least resistance
What hazards of electricity are there?
1. Frayed cables
2. Damaged plugs
3. Water around socket, as water is a good conductor
4. Pushing metal objects into sockets
ALL lead to electrocution
5. Long cables create a tripping hazard
-The right coloured wire is connected to each pin and firmly screwed in
-No bare wires are showing inside the plug e.g. live parts could be exposed and you could get a shock
-The cable grip is tightly fastened over the cable outer layer
What is a short circuit?
This is when a large current flows for a short time before safety devices are activated, results in hating and fire. When a path of low resistance between the live and neutral or earth wire develops, causing a surge of current.
What is double insulation?
-This is when both cable and socket are insulated (appliance and wiring), with usually plastic and so if there is nothing metallic on the exterior of the appliance it can be touched and there is no need for an earth connection
-The plastic is an insulator, so it stops a current flowing
What is the earth wire?
1. This creates a path of low resistance to the earth and all the metal parts (i.e. metal case) connected to the earth wire
2. If the metal case goes 'live' due to a fault a large current will:
-flow to Earth and blow the fuse or activate a circuit breaker if the appliance is Earthed
-electrocute you if touched because the fuse will not blow as only 0.05A can electrocute you as the current would rather go through you than the neutral wire in order to get to the ground because you have a lower resistance if the appliance is not Earthed
The Earth connection = ⏚
What is a fuse?
1. A fuse is a safety device that prevents electrical shock and fires.
2. A fuse contains a thin wire with a low melting point, so when the current exceeds a certain value the wire will melt as high current means a high temperature, thus breaking the circuit.
3. Fuses are contained in ceramic or glass (as they are insulators) cylinders to contain the heat as the fuse 'blow'
4. The metal end caps act as connectors
5. When the fuse 'blows' it must be replaced
-6. hey are connected to the live terminal as that is the one carrying the current from the power supply
Fuse = ⏛
Do fuses come in different ratings?
-Fuses come in three different ratings - 3A, 5A and 15A and so when choosing which fuse to purchase you must choose one slightly bigger than the rating of your electrical appliance to ensure that all the current is killed and not just part.
What are circuit breakers?
-Circuit breakers use the heating effect or the magnetic effect of a current to control a switch that will break the circuit
-They are re-settable (unlike fuses)
-They must be connected to the live terminal
What is the heating effect?
-Wiring in house has low resistance so that current can pass through easily
-Electrical appliances have a high resistance (less current will flow or a greater voltage will be needed to produce the same current) so that a large amount of electrical energy is needed, thus more current flows through the element meaning that electrical energy is transferred into heat energy, heating the element up.
Mostly used for heating and cooking e.g. a toaster, fan heaters and hairdryers
What is direct current?
-Is where the current is always travelling in the same direction, though it does not have to be the same value all the time
-Cells and batteries use d.c. current
What is alternating current?
-Is when the flow of electricity is constantly changing direction
-Mains supply uses a.c. current
-Although it goes up and down the line meaning that it doesn't always have a 'full current' the change is so small and quick we don't realise, but it is still moving forward like a d.c. current and the majority of the time it does meet the positive peak value
(Markscheme: current changing direction, continuously!)
Do some appliances work better with a.c. current than d.c. current?
Many components such as bulbs and heaters, operate equally well on d.c. and a.c. supplies, while others such as anything involving digital electronics (e.g. logic circuits, microprocessors etc.) must have a low voltage d.c. supplies
a.c. current can be turned into d.c. current by a diode (or a collection of diodes)
How do you work our power?
power = voltage x current
W = V x A
How do you work our energy transferred?
energy = power x time
energy = voltage x current x time
J = W x s
E = Pt
Why are circuit breakers better than fuses?
-Can easily be reset by flicking a switch so are more convenient
What is an RCCB? What are its advantages?
-Residual Current Circuit Breaker
1. Normally exactly the same current flows through the live and neutral wires. If somebody touches the live wire a small but deadly current will flow through them to the Earth. This means that the neutral wire carries less current than the live wire
2. The RCCB is able to detect the difference in current and quickly cuts off the power by opening a switch
3. They work for small current changes that might not be large enough to melt a fuse and since even small current change could be fatal, this means that RCCBs are more effective at protecting against electrocution
4. They operate much faster than fuses as so as soon as there is a current surge they break the circuit and there is no waiting for the current to melt a fuse, hence they are safer
What does current in a resistor do?
Results in the electrical transfer of energy and an increase in temperature
Describe how an earth wire acts as a safety feature (mark scheme)
1. Earth connected to metal casing
2. If casing becomes live
3. Provides low resistance path to the Earth
4. So large/surge current in earth wire
5. Hence fuse breaks/melts/blows
6. So circuit switches off or current stops or supply cuts off
Explain why this heater should be fitted with an earth wire (mark scheme)
1. It has a metal case
2. Metal/the case conducts electricity
3. To prevent user getting a shock
How do you wire a plug correctly?
1. The right coloured wire is connected to each pin, and firmly screwed in
2. No bare wires are showing inside the plug
3. The cable grip is tightly fastened over the cable outer layer
-The metal parts are made of copper or brass because these are very good conductors
-The case, cable grip and cable insulation are made of tubber or plastic because they are really good insulators. The plastic or rubber used for the cable insulation is flexible too
What does current in a resistor result in?
The electrical transfer of energy and an increase in temperature
Describe the heating effect and its uses
1. When there is an electric current in a resistor there is an energy transfer which heats the resistor
2. This heating effect increases the resistor's resistance, so less current will flow or a greater voltage will be needed to produce the same current
3. This heating effect can cause components in the circuit to melt, which means the circuit will stop working, or not work properly. Fuses use this effect to protect circuits, they melt and break the circuit if the current gets too high
4. The heating effect of an electric current can have other advantages for example if you want to heat something:
-Toasters contain a coil of wire with a really high resistance
-When a current passes though the coil, its temperature increase so much that it glows and gives off infrared (heat) radiation which cooks the bread
-Old style light bulbs work in a similar way
-Kettle, dishwasher, cooker and washing machine also sue this and it is used in fan heaters and hair dryers
What is electrical power?
-Is the rate at which an appliance transfers energy
-An appliance with a high power rating transfers a lot of energy in a short time
-This energy comes from the current flowing through it. This means that an appliance with a high power rating will draw a large current from the supply
What do electrical appliances do
What is current?
The rate of flow of charge, current will only flow though a component if there is a voltage across that component
What is a voltage?
The driving force that pushes the current around, like an electrical pressure
What happens if you increase the voltage?
More current will flow
What happens if you increase the resistance
Less current will flow, or more voltage will be needed to keep the same current flowing
What is resistance?
-Is anything in the circuit which slows the flow down, if you add more components to the circuit one after another there will be a higher overall resistance
-The voltage is trying to push the current round the circuit and the resistance is opposing it, the relative sizes of the voltage and resistance decide how big the current will be
What happens when you vary the variable resistor?
It alters the current going though the circuit