IGCSE Physics Electricity and Magnetism [CIE/TIS] v1.2

transistor

amplify signals and act as switches

reverse bias

when current flows the wrong way in a diode and is blocked by the very high resistance

What are basic factors of a SERIES CIRCUIT?

- Components connected in a line, end to end - If one component is removed the circuit brakes and all will stop.

Explain the basic factors of PARALLEL CIRCUITS

- Each component is separately connected to the positive and negative of each supply - If one component is removed, it will hardly affect the rest - Used most commonly in real life, e.g. cars and houses

When can LDRs be used?

- Outside night lights - Burglar detectors

What are the uses of Thermistors?

- Temperature detectors - Car engines - Thermostats

In an electric circuit...

...it is more usual to measure the current through the appliance and the potential difference across it rather than the energy transferred and the time. So we can use current and potential difference to calculate the power of the appliance using the equation: P = I x V

Facts about a circuit breaker and a residual current circuit breaker (RCCB)

A circuit breaker can be used in place of a fuse. This is an electromagnetic switch that opens and cuts of the supply if the current is bigger than a certain value. A RCCB cuts off the current in the live wire if it is different to the current in the neutral wire. It works faster than a fuse or an ordinary circuit breaker.

series circuit

A circuit with only one path for an electric current to flow.

What is a Current and what is it measured in?

A current is the flow of electric charge round the circuit. Current will only flow through a component if there is a potential difference across that component. It is measured in AMPERES (A)

How would the lines look on a potential difference graph showing current through a FILAMENT LAMP? Why is there a curved "S" shape?

A curved "S" shape going through the centre point Because as the temperature of the filament increases, the resistance also increases.

digital electrical signals

A digital signal has only two possible values. In computer and communication systems these values are 0 V and 5 V.

Facts about a Fuse

A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows. - The rating of the fuse should be slightly higher than the normal working current of the appliance. - If it is much higher, it will not melt soon enough. - If it is not higher than the normal current, it will melt as soon as the appliance is switched on.

fuse

A length of wire designed to melt when a specified current value is exceeded, thus breaking the circuit.

resistance

A measure of how difficult it is for current to pass through a part of a circuit. Measured in ohms.

voltage

A measure of the energy converted per unit charge passing through a component. also a measure of the amount of energy transferred to electrical form per unit by an electrical power supply, like a battery.

voltmeter

A measuring instrument for measuring the voltage between two points in a circuit.

Explain potential difference in a PARALLEL CIRCUIT

All components get the full source of potential difference so the voltage is the SAME across all components

A.C.

Alternating Current - a value in which the current (and potential difference) oscillates between two values, one of which is positive and the other negative.

Electrical energy and charge

An electric current is the rate of flow of charge. The equation relating charge, current and time is: Q = I x t

Electrical power and potential difference

An electrical appliance transfers electrical energy into other forms of energy. The rate at which it does this is called the power. Power can be calculated using the equation: P = E/t.

ammeter

An instrument used to measure the size of current in a circuit.

Why don't appliances with plastic cases need to be earthed?

Appliances with plastic cases do not need to be earthed. They are said to be double insulated and are connected to the supply with two-core cable containing just a live wire and a neutral wire.

Why does the graph for the filament lamp level off at high currents?

Because most resistors have a limit to the amount of current that can flow. More current means an increase in temperature, which means an increase in resistance, which means the current decreases again.

Why does the resistance of a filament increase as the temperature increases?

Because the ions in the metal filament vibrate more as the temperature increases so they resist the passage of electrons through the filament more.

Complete the sentence. "The greater the resistance across a component..."

"...the smaller the current that flows."

half-wave rectification

Converting AC to DC - but losing half of the current.

extension

In springs this is the increase in length that results from applying a force to stretch the spring.

What happens when certain insulating materials are rubbed together?

Negatively charged electrons will be scraped off of one and transferred on the other leaving a positive static charge on one and a negative static charge left on the other.

amp

The SI unit of electric current.

joule

The SI unit of energy. 1 joule is the amount of work done (energy transferred) when a force of 1 newton is applied through a distance of 1 metre.

Ohm's Law

The resistance of a circuit component is constant unless outside conditions (specifically temperature) are changed.

What do Thermistors do?

They are temperature dependant resistors. Their resistance decreases in hot conditions and increases in cool conditions

What do diodes do?

They're used to regulate the potential difference in circuits and let current flow freely in one direction but not the other.

Ohm's Law Formulae

V = I x R

Q = I x t

Where: Q is the charge in coulombs, C I is the current in amps, A t is the time in seconds, s

capacitor

stores electric charge

microchips

tiny circuits with many components, may be as small as a few millimetres across

Why are Ammeters and Voltmeters an exception in PARALLEL and SERIES CIRCUITS?

- Ammeters are always connected in series, even in a parallel circuit - Voltmeters are always connected in parallel with a component, even in a series circuit

The UK mains supply is approximately...

...230 Volts

Frequency

1/time period

How would the lines look on a potential difference graph showing current through a DIODE? Why is there a backwards curved "L" shape?

A backwards curved "L" shape on one quarter of the graph Because current only flows through the diode in one direction as the diode has very high resistance in the opposite direction.

parallel circuit

A circuit with two or more conducting paths between any two points in the circuit.

alternating current

A current that continually changes direction.

efficiency

A measure of how effectively energy is transformed into a useful form.

neutral

Having no overall electric charge. Neutrons are electrically neutral because there is a balance between the number of negative charges on electrons and the number of positive charges on the protons which make up part of the nucleus.

current

The rate of flow of electric charge.

power

The rate of transfer or conversion of energy.

positive electric charge

The type of charge possessed by the proton.

E = V x Q

Where E is the energy in joules, J V is the potential difference in volts, V Q is the charge in coulombs, C.

D.C.

Direct Current - a value in which the current (and potential difference) is a constant amount.

Electrical Issues

Electrical faults may occur as a result of damage to the sockets, plugs, cables or appliances.

analogue electrical signals

Electrical signals, usually voltages, that have continuously variable values.

transformers

Electromagnetic devices used to increase or decrease the size of alternating voltage electricity supplies.

free electrons

Electrons which are not bound to any particular atom in a solid. These are free to move and enable charge to move through a material forming an electric current.

energy

Energy exists in many forms - heat, light etc; it is required to do work.

electron

Extremely small particle carrying negative charge and making up the outer 'shell' or 'shells' of an atom.

earthed

Having a very low resistance connection to the general mass of the earth, taken as always being a 0 V.

double insulated

Having an outer casing which is an electrical insulator; having no exposed metal casing.

amplified

Increased in size or power.

What do LDRs do?

Light Dependant Resistors are dependant on the intensity of light. Their resistance falls in bright light and rises in darkness.

conductors (electrical)

Materials that allow electricity to pass through them easily. Most metals are good electrical conductors.

soft magnetic materials

Materials that are easy to magnetize and demagnetize.

insulators (electrical)

Materials that electricity cannot pass through.

hard magnetic materials

Materials that retain their magnetism well.

P = E/t

P is the power in watts, W E is the energy transferred in joules, J t is the time in seconds, s.

magnetic

Possessing the ability to attract iron and its compounds.

Explain Potential Difference in a SERIES CIRCUIT?

Potential Difference is SHARED between the various components so the voltages round a series always add up to equal the source voltage.

What is Potential Difference and what is it measured in?

Potential Difference is the driving force that pushes the current round. It is measured in VOLTS (V)

full-wave rectification

Redirecting the current so it will flow in the same direction during both halves of the cycle.

Smoothing

Removing the peaks from rectified current.

Equation which links Resistance, Current and Potential Difference

Resistance = Potential difference / currents

What is Resistance and what is it measured in?

Resistance is anything in the circuit which slows the flow down. It is measured in OHMS (Ω)

How would the lines look on a potential difference graph showing current through DIFFERENT RESISTORS?

Straight diagonal lines passing through the centre point at different angles.

Energy, potential difference and charge

The amount of energy transferred can be calculated using the equation: E = V x Q

circuit breakers

The modern equivalent of fuses, designed to break the conducting path in a circuit when a set current is exceeded. They may be reset by the push of a switch once the fault causing them to operate is remedied.

What are the pins made of and why?

The pins are made out of brass. Brass is: a good electrical conductor, hard and will not rust or oxidise.

electric charge

The property of particles that causes electric effects.

Motor Rule

The rule devised by Fleming to predict the direction of the force produced on a wire when it carries current in a magnetic field (provided the direction of current is perpendicular to the magnetic field).

Explain Current in a SERIES CIRCUIT

The same current flows through all parts of the circuit so it is the same everywhere. The size of the current is determined by the total Potential Difference of the cells and the total Resistance of the circuit

Explain current in a PARALLEL CIRCUIT

The total current flowing around the circuit is equal to the total of all currents through the separate components. There are junctions where the current either splits or rejoins. The total current going into the junction has to equal the total current leaving. If two identical components are connected in parallel the same current will flow through each component.

Explain Resistance in a SERIES CIRCUIT

The total resistance is just the sum of all the resistances. The bigger the resistance of a component, the bigger its share of the total potential difference

Series Circuit (Kirchoff's law)

The total resistance is the sum of each component in the circuit. Total resistance = r1 + r2 so the current is the same at every point in the circuit.

negative electric charge

The type of charge possessed by the electron.

watt

The unit of power equal to a rate of transfer of energy of 1 joule per second.

volt

The unit of voltage. 1 volt is equal to 1 joule of energy per coulomb of charge passed through a component.

Parallel Circuit (Kirchoff's law)

The voltage is the same across each branch. V(total) = v1 = v2 The total current through the circuit is the sum of the current through each component. I(total) = i1 + i2

Explain Cell Voltages in a SERIES CIRCUIT

There is a bigger potential difference when more cells are in series, provided the cells are all connected in the same way. For example, when 2 batteries of voltage 1.5V are connected in series they supply 3V between them

ohm (Ω)

Unit of resistance; the resistance of a conductor that passes a current of 1 amp when a voltage of 1 volt is applied across it.

Explain how RESISTANCE increases with TEMPERATURE

When charge flows through a resistor, some of the electrical energy is transferred to heat energy and the resistor gets hot causing the ions in the conductor to vibrate more making it more difficult for the charge to carry electrons to get through the resistor - the current can't flow as easy and the resistance increases.

P = I x V

Where: P is the power in watts, W I is the current in amps, A V is potential difference in volts, V

diodes

allow current to flow in only one direction

rectification

changing AC current to DC

relay

electromagnetic switching device allowing low-current circuit to control high-current circuit (as in a car ignition)

LED

light-emitting diode, usually to indicate current flows

resistor

modifies current and voltage

Peak potential difference

number of squares (up) x xv/div

potential divider

set-up of resistors which splits the voltage

uses of capacitors

smoothing output, time delay circuits

reed switch

switch operated by a magnetic field

semiconductor

usually silicon, diodes

forward bias

when current flows through a diode in the direction of low resistance