Resistance

ResistanceResistance

GCSE Physics

ResistanceResistance

• Book ReferenceResistance- Pg 123Ohm’s Law (the statement and

equation)- page 126

Learning IntentionsLearning Intentions

• Describe the relationship between voltage and current

• State what is meant by the resistance of a circuit

• State Ohm’s Law• Recognise how a device obeys Ohm’s

Law• Recall how to measure the

resistance of a device in a circuit

Voltage and CurrentVoltage and Current

• For many electrical devices there is a relationship between the current flowing through it and the voltage across it

• As the current flows through it faster, what happens to the voltage?

• Why?

• What other words can we use What other words can we use to explain resistance?to explain resistance?

Impede Impede

ChallengChallenge e

ObstructObstruct

OpposOpposee

Slow Slow downdown

ResistanceResistance

Georg Simon Ohm 1789-1854

ResistanceResistanceResistance is anything that will

RESIST a current. It is measured in Ohms, a unit

named after me.The resistance of a component can be calculated using the equation of Ohm’s Law:

V

RI

VoltageResistanc

e Current=

VR

I=

Volt, VOhm, Ω

Amp, A

=

Symbol

Unit

ResistanceResistance

• The current in a metallic conductor is directly proportional to the voltage between its ends (provided that the temperature and other physical conditions are the same).

Give it to me straight…Give it to me straight…

V R I= X

VR

I=

Re-arranged

If the resistance is constant the

voltage increases at the

same proportion as

the current

If we measure the gradient, we get the resistance.

Notice that the graph goes through the origin

Change in V

Change in I

VV

IIRR==

For a component in a circuit which obeys Ohm’s law, measuring the current as the voltage increases will give a graph with the shape shown below…

An example question:An example question:

V

A

1) What is the resistance across this bulb?

2) Assuming all the bulbs are the same what is the total resistance in this circuit?

Voltmeter reads 10V

Ammeter reads 2A

R = V / I = 10 / 2 = 5Ω

Total Voltage = 10 x 3 = 30 VR = V / I = 30 / 2 = 15Ω

Questions from the Questions from the bookbook

• Pg 126 and 127• Questions 17 - 22

Learning IntentionsLearning Intentions

• State how the resistance of a wire or device can change

• Plot and interpret a graph for the voltage and current passing through a device

Ohm’s LawOhm’s Law

The Stony Tube with water…

ResistanceResistance

In an electrical circuit RESISTANCE is the measure of how the material opposes or

impedes the flow of electrons• Glass and synthetic polymers have

great resistance because so many of their electrons are not free to move from atom to atom

• Silver and copper atoms offer only slight resistance to an electric current because many of their electrons are free to move from atom to atom.

What affects What affects resistance?resistance?

Here we have a piece of electrical wire with a set EMF pushing the electrons (large spheres) through it.

The small spheres are the atoms making up the material

If the thickness of the wire is reduced then the resistance increases.

The electrons obviously don’t have as much space to flow down the wire.

1. Cross sectional 1. Cross sectional AreaArea

- +

What affects What affects resistance?resistance?

2. Length of the material2. Length of the material

Comparing the ‘journey’ of an electron down a longer piece of wire, we can see that it has to deal with more friction from the sides of the wire.

Therefore the longer the wire the higher the resistance of the material.

TemperatureTemperature

What affects What affects resistance?resistance?

When the temperature of a material is turned up it becomes hotter. This is because the atoms are vibrating more vigorously and bouncing off each other and giving off heat energy.

The quicker the atoms are moving the more difficult it is for the electrons to get to where they want to go.

The resistance increases.

Website HelpWebsite Help

• http://micro.magnet.fsu.edu/electromag/electricity/resistance.html

QuestionsQuestions

• Page 128 Questions 23 and 24