Current and Mains

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  • Created by: Amylyford
  • Created on: 06-06-18 13:43

What are the differences between alternating and d

In direct current (DC), the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.

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What type of current do mains electricity in the U

Electrical supplies can be direct current (DC) or alternating current (AC).The UK mains electricity supply is about 230V at 50 Hz. It can kill if not used safely. Electrical circuits, cables, plugs and appliances are designed to reduce the chances of receiving an electric shock.

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What frequency and voltage do the mains supply use

The mains supply is alternating current (ac). This means that the current from a mains supply constantly changes direction in a circuit. The frequency of the mains supply is 50 hertz (50 Hz). The declared value of the mains supply in Britain is 230 volts (230 V).

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What is direct potential difference?

If the current flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V. The diagram shows an oscilloscope screen displaying the signal from a d.c. supply.

The electric current flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for this type of current was galvanic current. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.

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What is alternating potential difference?

If the current constantly changes direction, it is called alternating current, or a.c.. Mains electricity is an a.c. supply, with the UK mains supply being about 230V. It has a frequency of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.

The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero. The diagram shows an oscilloscope screen displaying the signals from the mains supply. The red trace is the live terminal and the blue trace the neutral terminal. Note that, although the mean voltage of the mains supply is about 230V, the peak voltage is higher.

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What are the difference between the two potential

The potential difference between any two points in a circuit is the energy transferred to, or from, a given amount of charge as it passes between those points. In the circuit above, the charges gain energy in the cell, and then transfer that same amount of energy into light and heat in the lamp

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How do you wire a plug?

The plug

The features of a plug are:

  • The case is made from tough plastic or rubber, because these materials are good electrical insulators.
  • The three pins are made from brass, which is a good conductor of electricity.
  • There is a fuse between the live terminal and the live pin.
  • The fuse breaks the circuit if too much current flows.
  • The cable is secured in the plug by a cable grip. This should grip the cable itself, and not the individual wires inside it.

Where does each wire go?

There is an easy way to remember where to connect each wire. Take the second letters of the words blue, brown and striped. This reminds you that when you look into a plug from above:

blue goes left, brown goes right and striped goes to the top.

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What are the functions and colour are each wire?

ColourWireFunction Blue Neutral Completes the circuit Brown                                                       Live Carries the high voltage Green and yellow stripes Earth                                   A safety wire to stop the appliance becoming live

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Why do we have outer insulation on the wires?

A mains electricity cable contains two or three inner wires. Each has a core of copper, because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator.

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Why would we need a fuse?

The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

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Why do electrical items need to be earthed?

If there is a fault in your electrical installation you could get an electric shock if you touch a live metal part. This is because the electricity may use your body as a path from the live part to the earth part.

Earthing is used to protect you from an electric shock. It does this by providing a path (a protective conductor) for a fault current to flow to earth. It also causes the protective device (either a circuit-breaker or fuse) to switch off the electric current to the circuit that has the fault.

For example, if a cooker has a fault, the fault current flows to earth through the protective (earthing) conductors. A protective device (fuse or circuit-breaker) in the consumer unit switches off the electrical supply to the cooker. The cooker is now safe from causing an electric shock to anyone who touches it.

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How can you calculate Power of appliances?

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.

E = P × t

  • E is the energy transferred in kilowatt-hours, kWh
  • P is the power in kilowatts, kW
  • T is the time in hours, h.

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.

For example, 2,000 W = 2,000 ÷ 1,000 = 2 kW.

Also note that time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.

For example, 7,200 s = 7,200 ÷ 3,600 = 2 h.

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Describe what happens in the national grid.

The National Grid is the high-voltage electric power transmission network in Great Britain, connecting power stations and major substations and ensuring that electricity generated anywhere in England, Scotland and Wales can be used to satisfy demand elsewhere.

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Explain what a transformer does.

A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. A varying current in one coil of the transformer produces a varying magnetic field, which in turn induces a voltage in a second coil.

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Why do we need step up and step down transformers?

A step down transformer has less turns on the secondary coil that the primary coil. The induced voltage across the secondary coil is less the applied voltage across the primary coil or in other words the voltage is “stepped-down”.

Step-up transformers at power stations produce the very high voltages needed to transmit electricity through the National Grid power lines. This is because high voltages improve efficiency by reducing heat loss in the power lines. But high voltages are too dangerous for use in the home, so step-down transformers are used locally to reduce the voltage to safe levels. Power lines and substations are potentially dangerous as an electric shock can kill someone who gets too close to such a high voltage supply.

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How can power loss be reduced?

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How can power loss be reduced?

To reduce energy loss,a step uptransformer at the power station is used to raise the voltage.

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