© boardworks ltd 2003 igcse electricity – resistance

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IGCSE Electricity – Resistance

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Resistance

• State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current

• Recall and use the equation R = V/I• Describe an experiment to determine

resistance using a voltmeter and an ammeter

• Relate (without calculation) the resistance of a wire to its length and to its diameter

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Aims

• Know the relationship between resistance, p.d. and current

• Recall and use R=V/I• Describe an experiment to determine

resistance• Understand the concept of resistivity• Use R=***l/A

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Resistance

• Supplement• Recall and use quantitatively the

proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire

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How are current and voltage related for a resistor?

V

A

Set up the circuit as shown below:

Slowly move the variable resistor and for each setting record the current in amps and the voltage in volts.

Plot a graph of your results.

P.D.(V)

I(A)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

0.61.1

1.8

2.5

3.03.5

4.2

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Ohm’s Law:

The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant. Georg Ohm 1826

Cu

rrent,

I /

am

ps

Potential difference / V

What does proportional mean?

Current and voltage

If you double the voltage then the current doubles.x

xx

x

x xx

x

Plot your points on the graph.Draw a line of best fit for your graph.What does your graph look like?If you get a straight line it means that the two quantities current and voltage are proportional.

This fact was put into a law:

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Electron flow and Resistance

Electricity in wires is a flow of electrons along the wire. As the electrons move along the wire they collide with the metal atoms. These collision make the atoms vibrate more…which makes the metal hotter.

Resistance is a measure of how much a material tries to stop electricity passing through it.

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V=IR

We can express Ohm’s Law mathematically using the equation:

Voltage = Current x Resistance

V=IR

Voltage measured in Volts (V)

Current measured in Amps (A)

Resistance measured in Ohms ()

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Formula triangles

V

I R

x

Formula triangles help you to rearrange formula, the triangle for the Ohm’s Law is shown below:

Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.

So if you were trying to find current, I…..

…you would cover I up…

…and you are left with the sum…

I = V

R

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Formula triangles

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Resistance for a bulb

If you have a filament bulb and it has a current of 20A running through it, with a potential difference of 100V across it, what is the resistance of the bulb?

V = IR

R = V/I

R = 100V/20A

R = 5

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Current for a diode

A diode has a current of 5A running through it, and a resistance of 5. What is the potential difference across the diode?

V = IR

V = 5A x 5

V = 25V

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To do

• P179 answer all questions.

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Resistivity

• Resistance of a wire depends on 3 things.– Length– Cross sectional area– Material (resistivity)

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Length

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Area

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Resistivity

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To do

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Use a textbook or other resource to fill in the table below:

Component Circuit symbol

Diode

Light Dependent Resistor

Variable Resistor

Thermistor

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Voltage/current graphs

1. A wire or resistor.

2. A filament lamp.

3. Wires of different materials.

4. A diode.

Which of the components obeys Ohms Law?

I

V

I

V

I

V

I

V

1 2 3 4

..partly

x

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To do

• Sketch I-V graphs for• An Ohmic conductor at constant

temperature.• A filament bulb• A diode

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What are the units of resistance?

A. Amps

B. Ohms

C. Volts

D. Watts

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What does the circuit symbol shown represent?

A. Voltmeter

B. Variable resistor

C. Light dependent resistor

D. Thermistor

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What causes resistance?

A. Inertia

B. Friction

C. Buoyancy

D. Viscosity

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If two resistors of 6 and 4 are placed in parallel, what is their total resistance in the circuit?

A. 10

B. 2

C. 2.4

D. 24

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If a resistor that obeys Ohm’s Law has a potential difference of 10V across it and a current of 5A running through it. What is its resistance?

A. 50

B. 2

C. 0.5

D. 15