P2b

Seite 1

Static Electricity

Some insulating materials become electrically charged when they are rubbed together

When you rub two different insulating materials against each other they become electrically charged

Negatively charged electrons will be scraped off on and onto the other

It will leave a positive charge on one and on a negative charge on the other

The material that loses electrons becomes positively charged

The material that gains electrons becomes negatively charged

Which way the electrons are transferred are depends on the 2 materials used

Both materials gain an equal amount of charge, but the charges are opposite

Charges that are the same repel, while unlike charges attract

When they are brought together they exert a force on one another

The forces get weaker the further apart the objects are

This only works for insulated objects

Electrical charges move easily to earth through conductors such as metals (they have free electrons)

Only the electrons move

This will only cause positive or negative charges

Produced by the movement of electrons

Positive charges don't move

Current

The flow of electric charge round a circuit

No current can flow if the circuit is broken - for example, when a switch is open

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

Unit = ampere, A

Potential Difference

The driving force that pushes the current around

also called voltage

Cells or batteries often provide the potential difference needed

Unit = volt, V

More charge passes around the circuit when a bigger current flows

The potential difference is the work done per coulomb of charge that passes between 2 points in an electric circuit

Potential difference across an electrical component is the amount of energy that is transferred by that electric component per unit of charge

Ammeter measures current (in amps)

Must be placed in series

Can be put anywhere in the series in the mains circuit

But never parallel like the voltmetre

Voltmeter measures voltage/potential difference (in volts) across a component

Must be placed in parallel around the component under test

Very basic

Used for testing components

Used for getting V-I graphs

The component, variable resistor and ammeter are all in series

They can be put in any order in the main circuit

As you vary the variable resistor the current flowing through the circuit changes

Allowing you to take several different readings of the ammeter and voltmeter

You can plot these values on a V-I graph to find the resistance

V-I Graphs

Shows how current varies as you change potential difference

Different Resistors

The current through a resistor is directly proportional to the potential difference

Different resistors have different resistances (hence different slopes)

Filament Lamp

As the temperature of the filament increases the resistance increases

Hence the curve

Diode

Current will only flow through a diode in one direction (only in positive quadrant)

The diode has high resistance in the opposite direction

Resistance increases with Temperature

When electrical charge flows through a resistor

Some of the electrical energy gets transferred to heat energy

So the resistor gets hot

This causes the ions in the conductor to vibrate more

When the ions are moving so much it's hard for the charge-carrying electrons to get through the resistor

The current can't flow

The resistance increases

Most resistors have a limit to how much current can flow

More current = increased temperature

Increased temperature= increased resistance

Increase in resistance = decrease in current

(this is why the graph for the filament levels off at high currents)

The steeper the graph the lower the resistance

If it curves the resistance is changing

Diode

Current only flows in 1 direction

Made from a semiconductor material like silicon

Used to regulate potential difference

Light-Emitting Diode (LED)

Emits light when current flows in a forward direction

Used more and more for lighting

Use less current

They indicate current

Often used in TVs, traffic lights etc...

Light Dependant Resistor (LDR)

A resistor dependant on the intensity of light

bright light = falling resistance

darkness= high resistance

e.g. automatic outdoor lights

Thermistor

Resistance decreases as temperature increases

Temperature dependant

Hot=resistance drops

Cool=increased resistance

Make useful temperature detectors

e.g. electronic thermoststats

Series Circuit

Different components are connected in a line

Between + and - power supply

(Except voltmeters that are always connected in a parallel)

If one thing is disconnected the circuit is broken

Has to be complete

Not really used for much

Potential difference is shared

V1+V2=V

Current is the same everywhere

A1=A2

The size of current is determined by the total PD

Resistance adds up

R=R1+R2+R3

The bigger the resistance of a component the bigger the share of total PD

Cell voltages add up

Parallel Circuit

Each component is separately connected to the + and - power supply

If you remove or disconnect one

It won't really affect the rest of the circuit

Most things are connected this way

PD is the same across all components

V=V1=V2

Identical bulbs in parallel have the same brightness

Current is shared

A=A1+A2

Current splits equally at junctions

Ammeters are always connected in series

Voltmeters are always connected in parallel

Everything in a car is connected din Parallel

Means that:

Everything can be turned on/off separately

Everything gets full voltage of the battery

Though it means that the current is divided and lights are dimmer (slightly)

Mains Supply

In the UK it's about 230 volts

It's AC, alternating current

Current is constantly changing direction

Frequency = 50Hz

Cells and Batteries supply DC, direct current

The current always flows in one direction

Oscilloscope Screen

Cathode ray oscilloscope (CRO) fancy voltmeter

If you plug in an AC supply to an Oscilloscope you get a trace

Shows how the voltage of supply changes with time

Trace goes up and down in a regular pattern

Some positive, some negative

The vertical height of the AC show the input voltage

By measuring the height you can find the PD of the AC supply

DC supply will give you a straight line

Voltage is distance between centre line and trace

Reading an Oscilloscope

period of an AC supply is the time taken for one complete oscillation.

Time between 2 peaks

Frequency of an AC supply is the number of oscillations per second

Frequency = 1 ÷ period

Remember to convert to seconds if needed

UK mains frequency is 50 Hz but you may get other frequencies

Hazards

Long cables

Frayed cables

Cables in contact with water

Water near sockets

Damaged plus

etc...

Cables have 3 wires

Each has a copper core (good conductor)

And a plastic coating (good insulator)

Safety of Wiring

Right colour connected in each pin

Firmly screwed

Cable strip tightly fastened over cable outer layer

Different appliances need different amounts of electrical energy

Thicker wire = less resistance = greater current

Plug features

Metal parts made of copper and brass

They're good conductors

Casing and outer wires are made of plastic

Good conductors

Earthing and Fuses

Prevent electrical overloads

Static Electricity, Current and voltage

Circuits

Circuit Devices, Parallel and Series

AC and DC, and home electricity

Fuses and Earthing