© boardworks ltd 2005 1 of 41 ks4 physics resistance and power

© Boardworks Ltd 20051 of 41

KS4 Physics

Resistance and Power



Resistance

Calculating resistance

Electrical power

Summary activities

Resistance components

Contents


Electron flow in a wire


Electricity is the flow of electrons along a wire.

As the electrons move along the wire they collide with the metal atoms.

These collisions 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.

All wires and devices have some resistance, which is why electrical appliances always waste some energy as heat.

Electron flow and resistance


Material – Different materials have different resistances because some materials are better conductors. Nichrome wire has a higher resistance than a copper wire of the same size.

Length – The longer a wire is the higher its resistance. When electrons travel down a long wire they can collide with more metal ions than in a short wire.

Thickness – A thin wire has a higher resistance than a thick wire.

Temperature – The higher the temperature of a wire the higher its resistance. Metal ions vibrate more at higher temperatures and so collisions with electrons are more likely to happen.

Factors that affect resistance

The resistance of a wire depends on several factors:


Investigating current and voltage


Set up this circuit with a resistor and a variable resistor.

Slowly move the variable resistor so that the voltage increases by 0.5 V and record the current for each setting.

Plot a current-voltage graph of the results.

voltage(V)

current(A)

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

0.6

1.1

1.8

2.5

3.0

3.5

4.2

V

A

How are current and voltage related for a resistor?


Ohm’s Law The current flowing through a wire is proportional

to the potential difference (voltage) across it provided the temperature remains constant.

Cu

rre

nt

/ A

Potential difference / VSo if voltage doubles then the current doubles and so on.

xx

xx

x xx

x

Plot the current and voltage readings for a resistor on a graph and draw a line of best fit.

The graph is a straight line so the two quantities current and voltage are proportional.

This is called Ohm’s Law after the scientist Georg Ohm.

Current-voltage graph for a resistor Current-voltage graph for a resistor


xx

xx

x xx

x

Plot the current and voltage readings for nichrome and copper wires of the same size.

x

x

x

x

x

The points produce straight lines with different gradients.

The copper wire has a steeper gradient and so has a lower resistance than the nichrome.

The steeper the gradient of a current-voltage graph, the lower the resistance of the wire.

Current-voltage graphs for different wires

nichrome

copper

At the same potential difference, a copper wire lets a larger current flow than a nichrome wire of the same length and thickness.

Cu

rre

nt

/ A

Potential difference / V


x

xx x

x x x

x

Plot the current and voltage readings for a filament bulb.

The higher the temperature of a wire, the higher its resistance.

The graph produced is not a straight line but a curved line.

Increasing the voltage across the filament in the bulb causes this wire to get very hot and give out light.

Current-voltage graphs for a bulb

As the wire gets hotter, its resistance gets higher, which means the current flow is less. So as the temperature rises the current is not proportional to the voltage.

Cu

rre

nt

/ A

Potential difference / V


1. A wire or resistor

Which of the components obeys Ohm’s Law?

I

V

I

V

I

V

1 2 3

Current-voltage graphs and Ohm’s Law

2. A filament lamp

3. Wires of different materials


Resistance – true or false?



Resistance


Electrical power

Summary activities


Contents


Resistance is a measure of how hard it is for electrons to move in an electrical circuit.

The connection between current, voltage and resistance was discovered in 1827 by Georg Ohm, a German physics and maths teacher.

The equation R = V/I is known as Ohm’s Law. It was such an important discovery in electricity that the unit of resistance is called the ohm, which is represented by the symbol .

The irresistable Georg Ohm!


What are the units of voltage, current and resistance?

The resistance of a conductor can be calculated using:

Potential difference is measured in volts (V). Current is measured in amps (A). Resistance is measured in ohms ().

Resistance formula

This equation can also be written as:

voltage = current x resistance V = I x R

resistance = R =

voltagecurrent V I


V

R

x

A formula triangle helps you to rearrange a formula. The formula triangle for V = IR 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 are trying to find current (I)...

…cover up I…

…which gives the formula…

Resistance formula triangle

II =

VR


Using the resistance formula triangle


A filament bulb has a current of 20 A running through it, with a potential difference of 100 V across it.

What is the resistance of the filament in the bulb?

V = IR

R = V I

= 100 V 20 A= 5

Calculating the resistance of a bulb


Resistance calculations


4 2

Total resistance = R1 + R2

What is the total resistance for this circuit?

Total resistance

= R1 + R2

= 4 + 2

= 6

Resistors in series

When resistors are connected in series, the total resistance can be calculated using:



Resistors in series

Total resistance = R1 + R2

= 6 + 34

= 40

6 34


2

4

= 1.33

Resistors in parallel

When resistors are connected in parallel, the total resistance can be calculated using:

Total resistance = R1 x R2

R1 + R2


Total resistance

= R1 x R2

R1 + R2

= 4 x 2 4 + 2



= 3.4

Resistors in parallel

6

8

Total resistance = R1 x R2

R1 + R2

= 8 x 6 8 + 6



Resistance


Electrical power

Summary activities


Contents


Component Circuit symbol

resistor

variable resistor

thermistor

light dependent resistor

diode

Different types of resistors


A thermistor is a component that has a high resistance when cold but a low resistance when hot.

The graph shows how the resistance of a thermistor decreases as its temperature increases.

This is the reverse of the normal trend for resistance.

This property makes thermistors useful in circuits that control and monitor temperatures.

Re

sis

tan

ce /

Temperature / C

xx

xx x

x x

x

x

Thermistors


The resistance of a light dependent resistor (LDR) is not fixed and depends on the light intensity.

A LDR has a high resistance in the dark but a low resistance in the light.

The graph shows how the resistance of an LDR decreases as the light intensity increases.

This makes LDRs useful in circuits that are controlled by light intensity.

Re

sis

tan

ce /

k

Light intensity

xx

xx x

x x

x

x

Light dependent resistors


A diode is a component that allows a current to flow in one direction only.

It has a low resistance in one direction and a very high resistance in the other.

Current flows in the direction with low resistance but is not proportional to the voltage.

If the voltage is reversed or the diode is connected the other way around, the high resistance of the diode ‘blocks’ the flow of current.

Diodes

Cu

rre

nt

/ A

Voltage / Vx x x x

xx

x

x

x

No current flows


A diode has a current of 5 A running through it and a resistance of 5 .

What is the potential difference across the diode?

V = IR

= 5 A x 5

= 25 V

Calculating the current through a diode



Resistance


Electrical power

Summary activities


Contents


The relationship between power, current and voltage is shown by the equation:

Power is measured in watts (W).

Current is measured in amps (A).

Voltage is measured in volts (V).

Electrical power

power = current x voltage

P = I x V

What are the units of power, current and voltage?


P

V

x

A formula triangle helps you to rearrange a formula. The formula triangle for P = IV 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 are trying to find current (I)...

…cover up I…

…which gives the formula…

Power formula triangle

II =

P V


A filament bulb has a potential difference of 200 V across it and a current of 0.2 A running through it.

What power is the filament bulb operating at?

Calculating power

P = IV

= 0.2 A x 200 V

= 40 W


A filament bulb operates at a power of 60 W and has a potential difference of 240 V across it.

What is the current running through the bulb?

Calculating current

P = IV

I = P V

= 60 W 240 V= 0.25 A


1 kV = 1000 V

1 kJ = 1000 J

1 kW = 1000 W

How many volts in 6 kV? _________ V

How many joules in 12.3 kJ? _________ J

How many watts in 0.6 kW? _________ W

6 000

12 300

600

Converting units


How many kilovolts in 9 000 V? _________ kV

How many kilojoules in 23 500 J? _________ kJ

How many kilowatts in 325 W? _________ kW

9.0

23.5

0.325

Converting units

1 kV = 1000 V

1 kJ = 1000 J

1 kW = 1000 W


Power calculations



Resistance


Electrical power

Summary activities


Contents


Glossary

diode – A device that allows current to flow in one direction only.

ohm – The unit of electrical resistance, named after Georg Ohm.

power – The rate at which energy is transferred.resistance – The reduction in the flow of electrons

through a piece of wire or a device.resistor – A component that opposes the flow of electrons. thermistor – The resistance of this device decreases

as its temperature increases.variable resistor – The resistance of this device can

be changed.watt – The unit of power. 1 watt = 1 joule of energy

transferred every second.


Anagrams


Multiple-choice quiz

© boardworks ltd 2005 1 of 41 ks4 physics resistance and power

Documents

x x x x x x x x

wire slide

resistance slide

voltage slide

higher resistance

nichrome wire

currentvoltage graphs

power resistance