Mains electricity

What are use the following units used for: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V) and watt (W)?

current : Ampere (A)

charge : coulomb (C)

energy : joule (J)

resistance : ohm (Ω)

time : second (s)

potential difference : volt (V)

power : watt (W)

How the use of insulation, double insulation, earthing, fuses and circuit breakers protects the device or user in a range of domestic appliances?

Electrical safety

• Common electrical safety hazards include:

  • Damaged Insulation – if someone touches an exposed piece of wire, they could be subjected to a lethal shock

  • Overheating of cables – passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulation, exposing live wires

  • Damp conditions – if moisture comes into contact with live wires, it could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk

• To protect the user or the device, there are several safety features built into domestic appliances, including:

  • Double insulation

  • Earthing

  • Fuses

  • Circuit breakers

Insulation & double insulation

• The conducting part of a wire is usually made of copper or some other metal

  • If this comes into contact with a person, this poses a risk of electrocution

• To improve electrical safety wires are covered with an insulating material, such as rubber

Insulating electrical wires to improve electrical safety

The conducting part of a wire is covered in an insulating material for safety

• Some appliances do not have metal cases, so there is no risk of them becoming electrified

• Such appliances are said to be double insulated, as they have two layers of insulation:

  • Insulation around the wires themselves

  • A non-metallic case that acts as a second layer of insulation

• Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing

Earthing

• Many electrical appliances have metal cases

• This poses a potential electrical safety hazard:

  • If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

• The earth wire is an additional safety wire that can reduce this risk

The earth wire is an electrical safety feature

A diagram showing the three wires going to a mains powered appliance: live, neutral and earth

• If this happens:

  • The earth wire provides a low resistance path to the earth

  • It causes a surge of current in the earth wire and hence also in the live wire

  • The high current through the fuse causes it to melt and break

  • This cuts off the supply of electricity to the appliance, making it safe

Fuses & circuit breakers

• Fuses and circuit breakers are electrical safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

  • As explained in the Selecting fuses revision note a fuse consists of a glass cylinder containing a metal wire

  • A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value

A circuit breaker is the most important electrical safety feature in houses

The main circuit breaker can quickly shut off electricity to the whole house. The branch circuit breakers can shut off electricity to specific areas of the house

• A circuit breaker has a major advantage over a fuse as an electrical safety device because:

  • It doesn't melt and break, hence it can be reset and used again

  • It works much faster

• For these reasons, circuit breakers are used in mains electricity in homes as the most important electrical safety device

  • Sometimes they are misleadingly named "Fuse boxes"

Why does a current in a resistor result in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts?

• When electricity passes through a component, such as a resistor, some of the electrical energy is turned into heat therefore increasing its temperature

• This is because energy is transferred as a result of collisions between:

  • Electrons flowing in the conductor, and

  • The lattice of atoms within the metal conductor

• Electricity, in metals, is caused by a flow of electrons

  • This is called the current

• Metals are made up of a lattice of ions

• As the electrons pass through the metal lattice they collide with ions

  • The ions resist the flow of the electrons

As electrons flow through the metal, they collide with ions, making them vibrate more

• When the electrons collide, they lose some energy by giving it to the ions, which start to vibrate more

  • As a result of this, the metal heats up

• This heating effect is utilised in many appliances, including:

  • Electric heaters

  • Electric ovens

  • Electric hob

  • Toasters

  • Kettles

The heating effect of current can be used for many applications such as electric hobs

What is the the relationship between power, current and voltage?

power = current × voltage

P = I × V

What is the relationship between energy transferred, current, voltage and time?

energy transferred = current × voltage × time

E = I × V x t

What is the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery?

Direct Current

• A direct current (d.c.) is defined as

A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative

• The potential difference across a cell in a d.c. circuit travels in one direction only

  • This means the current is only positive or only negative

• A d.c. power supply has a fixed positive terminal and a fixed negative terminal

• Electric cells, or batteries, produce direct current (d.c.)

Circuits powered by cells or batteries use a d.c. supply

Alternating Current

• An alternating current (a.c.) is defined as

A current that continuously changes its direction, going back and forth around a circuit

• An a.c. power supply has two identical terminals that switches between positive and negative

  • The current is therefore defined as positive or negative, depending on which direction it is flowing at that time

• The frequency of an alternating current is the number of times the current changes direction back and forth each second

• In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V

• On an oscilloscope, direct current and alternating current are represented in the following way:

Two graphs showing the variation of current with time for alternating current and direct current

Comparing AC & DC

• The following table summarises the differences between d.c. and a.c.

Direct Current vs. Alternating Current Table