1. Resistance components & Power
HSC Physics
Resistance components & Power

2. Resistance components & Power
Contents
Resistance components & Power
Resistance components
Electrical power
Summary activities

3. Different types of resistors
Component
Circuit symbol
resistor
variable resistor
thermistor
light dependent resistor
diode

4. Thermistors
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. x x x x
Resistance /  x x x x x
Temperature / C

5. Light dependent resistors
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. x x x x
Resistance / k x x x x x
Light intensity

6. Diodes
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. x x
Current / A x x x x x x x
No current flows
Voltage / V

7. Calculating the current through a diode
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

8. Resistance components 7 Power
Contents
Resistance components 7 Power
Resistance components
Electrical power
Summary activities

9. P = I x V Electrical power power = current x voltage
The relationship between power, current and voltage is shown by the equation:
power = current x voltage
P = I x V
What are the units of power, current and voltage?
Power is measured in watts (W).
Current is measured in amps (A).
Voltage is measured in volts (V).

10. Power formula triangle
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)...
…which gives the formula… P  
I =
P V
…cover up I… I V x

11. Calculating power P = IV = 0.2 A x 200 V = 40 W
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?
P = IV
= 0.2 A x 200 V
= 40 W

12. Calculating current P = IV I = P V = 60 W 240 V = 0.25 A
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?
P = IV
I = P V
= 60 W
240 V
= A

13. Converting units 1 kV = 1000 V 1 kJ = 1000 J 1 kW = 1000 W 6 000
How many volts in 6 kV? _________ V
6 000
How many joules in 12.3 kJ? _________ J
12 300
How many watts in 0.6 kW? _________ W
600

14. Converting units 1 kV = 1000 V 1 kJ = 1000 J 1 kW = 1000 W 9.0 23.5
How many kilovolts in V? _________ kV
9.0
How many kilojoules in J? _________ kJ
23.5
How many kilowatts in 325 W? _________ kW
0.325

15. Power calculations

16. Resistance and Power Resistance Calculating resistance
Contents
Resistance and Power
Resistance
Calculating resistance
Resistance components
Electrical power
Summary activities

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

18. Anagrams

19. Multiple-choice quiz