P2.3.1 – Static Electricity

• Objectives, to understand that:– When certain electrical insulators are

rubbed together they become electrically charged.

– Objects can become charged when electrons are transferred from one to another.

– Similarly charged objects repel one another.

Electrical Charges

Explain how a charged rod can attract a neutral piece of paper.

How large a piece of paper can you pick up?

Explain how a charged rod can attract a neutral piece of paper.

How large a piece of paper can you pick up?

• When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and onto the other.

• The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

• When two electrically charged objects are brought together they exert a force on each other.

• What is electric current?

• What is current measured in?

• What is the difference between a series and parallel circuit?

• How many circuit symbols can you draw?

P2.3.2 Electric Circuits

• By the end of today:– You should be able to draw circuit

symbols for common components.

– Describe what these things actually do.

– Some will be able to use these symbols to draw circuit diagrams.

Electric Circuits

Can you light a bulb with just one wire?

• By the end of today:– You will know where to put an ammeter

and a voltmeter in a circuit.

– You will be able to describe how to measure the resistance of a component.

– You can state Ohm’s law.

– Some will be able to rearrange the resistance equation.

Resistance

Ohm’s Law

The current through a resistor at a constant temperature is directly

proportional to the potential difference across the resistor.

This means if you double the current you double the voltage over a component.

It also means that the resistance of the component does not change when you put more current through it.

Starter

• How many different circuits can you draw with 3 cells and 4 bulbs?

• 10?

Series and Parallel Circuits

Series Circuit

The components in the circuit are lined up in series, one after each other.

Parallel Circuit

The components in the circuit are lined up in parallel, in parallel lines to one another.

Series Circuit

The current (electrons) can only go one way so the current is the same everywhere in the circuit.

But the energy it has given to it by the battery is shared equally amongst all the bulbs.

Parallel Circuit

The components in the circuit are lined up in parallel, in parallel lines to one another.

Resistance of Components

• By the end of today:– You should be able to recognise the

graph of current against voltage for a diode, filament lamp, thermistor and LDR

5.3 Resistance of Components

A

V

•An ammeter measures the current in the circuit•A voltmeter measures the potential

difference across a component.

A

V

A

Vbit of wire

http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

How to measure resistance

Current (A) Potential Difference (Volts)

Cur

rent

(A

)

Potential Difference (Volts)

Cur

rent

(A

)

Potential Difference (Volts)

R = V

I• If the gradient is

constant…• …then the ratio of

V to I is constant….

• …so the resistance is constant

A Resistor

A Filament Lamp

The filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This heats up when an electric current passes through it, and produces light as a result.

A Filament Lamp

The filament lamp does not follow Ohm’s Law. Its resistance increases as the temperature of its filament increases. So the current flowing through a filament lamp is not directly proportional to the voltage across it. This is the graph of current against voltage for a filament lamp.

The diode

If the diode is this way round, no current can flow so the lamp stays unlit.

The diode has a very high resistance in one direction. This means that current can only flow in the other direction.

This is the graph of current against potential difference for a diode.

Conventional Current

A

By scientific convention the current goes from the positive end of the battery to the negative end.

Conventional Current

A

•But current in an electronic circuit is the flow of electrons.•Electrons are negatively charged.•So the electrons actually go the other way around the circuit to what ‘conventional current’ says.

What is negative current and voltage?

A

•Consider conventional current (+ve to –ve)+

ve r

eadi

ng

What is negative current and voltage?

A

•If we turn the battery around we send the current the other way around the circuit, this gives us a negative reading on the ammeter.

-ve

read

ing

Thermistors

Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases as the temperature increases:

•At low temperatures, the resistance of a

thermistor is high and little current can flow through them. •At high temperatures, the resistance of a

thermistor is low and more current can flow through them.

Light Dependent Resistor

LDRs are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

•In the dark and at low light levels, the resistance

of an LDR is high and little current can flow through it. •In bright light, the resistance of an LDR is low and

more current can flow through it.

Extension Tasks

• Collect your equipment from my desk.

• Can you set up the multimeter to find the resistance of a component?

• Draw a superb, artistic, clear diagram of what you have just done.

• Measure the resistance of a thermistor and an LDR. – How can you change their resistance? – Does that agree with what you have written in

your notes?

The total resistance (known as RT) of a series circuit is equal to the sum of the resistance of each individual component.

RT = R1 +R2

R1 R2

Resistance of a Circuit