Electricity

Atomic Structure

Atoms are made of three main sub-atomic particles:

● Electrons

● Protons

● Neutrons

The Bohr model of the atom, which is the currently accepted view of the atom, has a dense nucleus at the centre containing the neutrons and the protons. The electrons are found in orbits around this heavy nucleus. All the mass of the atom is found in the nucleus, as electrons are very low in mass, so low that for all practical purposes they are considered to have no mass.

Sub-atomic particle properties

Protons:	Mass: 1u	Charge: +1	Location: Nucleus
Neutron:	Mass: 1u	Charge None	Location: Nucleus
Electron:	Mass: None	Charge: -1	Location: Orbits around nuclei

Note: ‘u’ is the units of atomic mass equal to 1/12th of the mass of a carbon 12 atom.

In an atom the number of protons equals the number of electrons. The number of positive charges equals the number of negative, this results in atoms being neutral in charge overall.

Electricity is the flow of charged particles, which can be electrons, protons or a charged atom called an ion. In most circuits the particles that are moving are the electrons. These are easier to move as they are very low in mass and on the outside of the atom. In metals, which are good conductors of electricity, the electrons are held in their orbits by weak forces. This makes them easier to move and hence easier for electricity to flow through metals.

Current

An electrical current is caused by the movement of charged particles. In metals that make up the conductors in electrical circuits it is always the electrons that are moving to create the electricity. The rate at which these electrons move is measured as the electrical current, measured in amperes or amps (A).

Each electron carries a small electrical charge, so current measures the amount of this charge moving in a given amount of time.

Current (Amps) =Charge (coulombs) ÷ time (s)

1 Amp is equal to1 coulomb per second

This is also written as Charge = current x time. Measuring current and time are easy compared to measuring charge in a circuit.

Charge = current x timeorQ = I x t

where Q is charge,I is current and t is time

Charge is measured in coulombs, Current in Amps and time in seconds.

Measuring Current

A device called an Ammeter is used to measure the current flowing in an electrical circuit. It is important that an ammeter is placed in series with the main circuit as it measures the charge flowing through it.

Types Of Current

There are two types of electrical currents in circuits. Alternating Current (AC) and Direct Current (DC). In both cases the electrons (charge) is moving, but in two very different ways.

Direct Current: The electrons are flowing around the circuit slowly so the current always flows in one direction, from the positive to the negative side of the circuit. This is the sort of current produced by a battery or a solar panel.

Alternative Current: The electrons are constantly changing their direction of movement, the flow of charge is alternating. This means that no one side of a circuit is always positive or negative, this keeps changing. In the UK this happens 50 time each second, giving a frequency for mains electricity of 50 Hz. This sort of current is found in the mains electricity supply.

Voltage

For the charge carrying electrons in a circuit to move and produce the electricity, there must be a force to produce the movement. This ‘orce’ is the voltage or potential difference of the circuit. Most people are familiar with voltage from using batteries, 1.5 V, 5 V etc. The ‘V’ is the measure of the voltage and is short or Volts.

What is Potential Difference (voltage)?

Within a battery a chemical reaction moves electrons from one end to the other. This makes one end more negative than the other, the negative electrons will be pulled (by an electrostatic force) from the negative to the positive, however the design of the battery means they can only do this when the two ends are connected by an external conductor (a wire), they cannot flow directly through the battery.

The amount of charge at each end of the battery is unequal, there is a difference in the electrical charge or potential. ___Voltage is a measure of this difference ___in electrical potential in different parts of a circuit. The greater the difference in potential, the greater the force applied to the charge carrying electrons to make them move at a faster rate. Recall that current is the rate at which the charge (electrons) moves, so the greater the potential difference (pd) the greater the current.

Voltage is the force to move the charge, therefore, it is also a measure of the energy given to each electron, a bigger force (higher voltage) produces faster movement (more energy).

Energy = Charge moved x Voltage

E = Q xV where E is energy in Joules, Q is charge in coulombs and V is voltage in Volts

1 Volt is equal to 1 joule of energy per coulomb of charge moved.

Measuring Voltages

A device called a voltmeter is used to measure the voltage. As voltage is a measure of the __difference __in electrical potential in different parts of a circuit, it is important to place the voltmeter in parallel with the circuit, this is so the device can compare the potential in two part of the circuit and report the difference.

Charge

Within the atom only the protons and the electrons carry a charge. Although the charge per particle is very small, (approximately 1.6x10^-19 Coulombs per electron) there are billions of them in a single wire. In current electricity these ‘packets’ of charge are moving from areas of high potential to areas of lower potential taking energy with them. In static electricity these ‘packets’ of charge are building up (accumulating) in one place, creating areas of high potential.

Charge = Current x time Q= It

Example 1: What is the charge in a circuit where a current of 3 Amps flows for 15 seconds.

Q = It _= 3_A x 15s = 45 Coulombs (C)

Example 2: What is the current in an electric cooker where 108,000C are transferred in 2 hours?

First convert the time to seconds, 2 hours x 3,600s per hours = 7,200s

Q = It therefore I =Q ÷ t _ =108,000 _C ÷ 7200s = 15 Amps (A)

Energy Transfer

In any circuit where a current is flowing electrical energy is being transferred from the battery or mains supply to the components in the circuit, these might be bulbs, heating elements, motors or logic gates in computer chips. The amount of energy available for transfer depends upon the voltage and the current in the circuit as well as the amount of time that the circuit is switch on.

Energy = Charge x Voltage but Charge = Current x time

Therefore Energy = (Current x time) x Voltage or E = ItV

Example:

An electrical cooker takes 2 hours to roast a chicken, if the cooker is a standard UK design requiring a 15A supply, how much energy has been transferred by the cooker?

First convert time to seconds, 2 hours x 3,600s per hours = 7,200s

UK main supply is set at at 230V. (It is worth remember this figure for the exam)

E = ItV = 15A x 7,200s x 230 V = 24,840,000J or 24.84 MJ

What causes a current to flow in a circuit and how would you measure this current?

Your answer should include: electrons / ammeter
Explanation: The current is the flow of charged particles, normally electrons, these are made to move by an electrostatic force created by a difference in electrical potential in different parts of the circuit. The potential difference can be achieved from a battery or a mains supply. To measure this current you can use an ammeter, placed in series in the circuit.

A 1.5 A bulb is connected to a 6 V battery for 15 minutes, calculate the amount of energy transferred to the bulb. (in kJ)

Your answer should include: 81 / 81kJ
Explanation: E = ItV 1.5A x (15 x 60)s x 6V = 81,000 J or 81 KJ

1 MJ of energy is transferred from the mains in a UK household to a 3A TV. How long was the TV used for? (give your answer in minutes and seconds.)

Your answer should include: 24 / 9.3 / 24m / 9.3s
Explanation: UK mains = 230 V E = ItV therefore t =E ÷ (IV) = 1,000,000J ÷ (3A x 230 V) = 1,449.28 s ≣ 24 minutes and 9.3 seconds