Rough Notes

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Static Electricity

The build-up of static electricity is caused by friction.

When certain insulating materials are rubbed together, negatively charged

electrons will be scraped off one and dumped onto the other. This will leave a

positive static charge on the one and a negative charge on the other. Which way

the electrons are transferred depends on the two materials involved. Good

examples are polythene and acetate rods being rubbed with a cloth duster. With

the polythene rod, electrons move from the duster to the rod. With the acetate

rod, electrons move from the rod to the duster.

Only electrons move, never the positive charges. A positive

charge is caused when electrons move elsewhere. A negative charge is caused

when something gains electrons.

When two things with electrical charges are brought

together, they exert a force on each other. Two things with the same charge

will repel each other. Two things will opposite charges will attract each

other.

Charges can move easily through some materials. These

materials are known as conductors.

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Current & Potential Difference

Current is the flow of electric charge around the circuit. Current

will only flow through a component if there is a potential difference across

that component. Current is measures in amperes (A).

Potential difference is the

driving force that pushes the current around. Potential difference is measured

in volts (V).

Resistance is a measure of how much obstruction there is to the current.

Resistance is measured in ohms (Ω). The greater the resistance, the

smaller the current.

The total charge is dependent on current and time. The

overall charge is the current multiplied by the time. The current is measured

in amperes, charge is measured in coulombs and time is measured in seconds.

Potential difference is the work done per unit of charge. To

find this out, you multiply the potential difference by the charge.

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Circuits - The Basics

Seite 4

Circuits - The Basics

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Circuits - The Basics

The ammeter measures the current flowing through the

component. Must be placed in series. Can be put anywhere in the main circuits,

but never in parallel like the voltmeter.

The voltmeter measures the potential difference across the

component. Must be placed in parallel around the component under test, not

around the variable resistor or the battery.

The previous circuit is used for testing components and for

getting V-I graphs from them. The component, the ammeter and the variable

resistor are all in series, which means that they can be put in any order in

the main circuit. The voltmeter, however, must be placed in parallel around the

component under test. As you vary the variable resistor, it alters the current

flowing through the circuit. This allows you to take several pairs of readings

from the ammeter and the voltmeter.

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Resistance and V = I x R

The current through a resistor (at a constant temperature)

is directly proportional to potential difference. Different resistors have

different resistances.

As the temperature of a filament lamp increases, so does the

resistance.

Current will only flow through a diode in one direction. The

diode has a very high resistance in the other direction.

Resistance increases with temperature. Some of the

electrical energy that flows around a circuit is transferred into heat energy.

This heat energy causes the ions to vibrate more, which as a result makes it

harder for the current to flow, so the resistance increases.

Most resistors have a limit to the amount of current that

can flow, as more current means an increase in temperature, so the current will

eventually decrease again.

Ohm's Law states that to find the potential difference, you multiply the resistance by the current.

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Circuit Devices

Currents only flow in one direction through a diode. A diode

is a special device made from semiconductor material such as silicon. Diodes

are used to regulate the potential difference in circuits. It lets the current

flow freely in one direction, but not in the other.

Light-emitting diodes are very useful. A light emitting

diode emits light when a current flows through it in the forward direction.

LEDs are being used more frequently as they use a much smaller current than

other forms of lighting. LEDs also indicate the presence of current in a

circuit, for example, in TVs to let you know that they are switched on.

A light dependent resistor is dependent on the light

intensity. The dimmer the light, the higher the resistance. Light dependent

resistors are at their most effective when it is night time. They are used in

automatic night lights and burglar detectors.

A thermistor is a temperature dependant resistor. If it is

hot, the resistance is low. If it is cold, the resistance is high. Thermistors

are useful temperature detectors.

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Series Circuits

In series circuits, the different components are connected

in a line, end to end, to the power supply. If you remove one component, then

the circuit will not be able to function.

In series circuits, the total potential difference of the

supply is shared between all of the components. The voltages always add up to

equal the voltage that is supplied to the circuit.

The current is the same all throughout the series circuit.

The size of the current is determined by the total potential difference of the

circuit.

In a series circuit, the total resistance is the sum off all

the resistances. The bigger the resistance of the component, the higher its share

of the total potential difference.

When more cells are in a series circuit, the more potential

difference is supplied.

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Parallel Circuits

In a parallel circuit, each component is separately

connected to the power supply. If you remove one, it will hardly affect the

other components.

All components get the full potential difference of the

source in a parallel circuit. Identical bulbs that are connected by parallel

circuits will be of the same brightness.

The current is shared by all components. The total current

flowing around the circuits is equal to the total of all currents that flow

through each component. In parallel circuits, there are junctions where the

current splits or re-joins. The total current going into the junction is equal

to the total current leaving.

Ammeters are always connected in series, even in a parallel

circuit.

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Mains Electricity

Mains supply is AC; battery supply is DC. The UK mains

supply is approximately 230 volts. As it is an AC supply (alternating current),

this means that the current is constantly changing direction. The frequency of

the AC mains supply is 50 cycles per second (50Hz). However, cells and

batteries supply direct current, which means that the current flows in the same

direction.

Electricity supplies can be shown on an oscilloscope screen.

If you plug an AC supply into an oscilloscope, you get a trace on the screen

that shows how the voltage of the supply changes with time. The trace goes up

and down in a regular pattern. If you plug in a DC supply, you get a straight

line. The vertical height of the AC trace at any point shows the input voltage

at that point. For DC, the voltage is the distance from the straight line trace

to the centre line.

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Electricity in the Home

There are various electrical hazards in the home. These

include; long cables, frayed cables, cables in contact with something hot/wet,

water near sockets, ramming things into sockets, damaged plugs, too many plugs

in a single socket, lighting sockets without bulbs in and appliances without

covers on.

Most electrical appliances are connected to the mains supply

by three core cables (cables that have three wires inside them). These wires

are the live wire, which is brown and alternates between a high positive and

negative voltage. The neutral wire, which is blue, is always at OV. Electricity

normally flows in and out through live and neutral wires. The earth wire, which

is green and yellow, is for protecting the wiring and for safety. It works

together with a fuse to prevent fire and shocks.

The right coloured wire is connected to each pin and firmly

screwed in. There are no bare wires inside the plug. The cable grip is tightly

fastened over the cable outer layer. Different electrical appliances may

require more energy; so thicker cables have less resistance so they are able to

carry more current.

The metal parts of the plug are made of brass or copper as

they are very good conductors. The case, cable grip and cable insulation are

made of rubber or plastic because they’re good insulators. This keeps all

electricity flowing where it should.

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Earthing and Fuses

Fuses and earthing prevent electrical overloads. If a fault

develops where the live wire somehow touches the metal case, then because the

case is earthed, too great a current flows in through the live wire, through

the case and down the earth wire. This surge in current melts the fuse as the

current is greater than the fuse rating. This breaks the circuit. This isolates

the whole appliance, making it impossible to get an electric shock from the

case. It also prevents the risk of a fire caused by the heating effect of a

large current.