1. Electrical Circuits Varying Resistance
Physics (IGCSE)
Electrical Circuits Varying Resistance
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2. Current
Electricity effectively travels at the speed of light in a conductor (186,000 miles/second) but slower in an insulating material.
An electric current is a flow of charge carried by tiny particles known as electrons.
Movement of charge is caused by the force from a voltage supply such as a battery or mains supply acting on electrons which are free to move.
Note: The electrons flow in the opposite direction to that of the current.

3. Current
Electricity effectively travels at the speed of light in a conductor (186,000 miles/second) but slower in an insulating material.
An electric current is a flow of charge carried by tiny particles known as electrons.
Movement of charge is caused by the force from a voltage supply such as a battery or mains supply acting on electrons which are free to move.
Note: The electrons flow in the opposite direction to that of the current.

4. Current Electric current is measured in units called amps (A).
The electric current is not used up by the components in a circuit but it transfers energy from the voltage source to the various components making up the circuit.
Electric current is measured using an ammeter.

5. Current Electric current is measured in units called amps (A).
The electric current is not used up by the components in a circuit but it transfers energy from the voltage source to the various components making up the circuit.
Electric current is measured using an ammeter.
The ammeter is placed in series within the circuit.

6. Voltage
Voltage is supplied by a battery, cell or power supply and is measured in volts (V).
The voltage of a power supply (converting other forms of energy into electrical) is an electromotive force (emf).
The voltage across a circuit component, e.g. a bulb converting electrical energy into heat and light is a potential difference (p.d.).
Potential difference is also measured in volts (V). The potential difference is a measure of the energy carried by the charge (current) flowing within the circuit.

7. Voltage
Within an electrical circuit, the difference in electrical potentials across different components drops as each component converts electrical energy into other forms of energy.
Potential difference is measured using a voltmeter. The voltmeter is placed in parallel to the circuit.

8. Voltage
Within an electrical circuit, the difference in electrical potentials across different components drops as each component converts electrical energy into other forms of energy.
Potential difference is measured using a voltmeter. The voltmeter is placed in parallel to the circuit.
Note: the switch does not need to be closed to take a voltage reading across a battery.

9. Resistance
Resistance is the property of a material which restricts the flow of electricity. Energy is converted into other forms (e.g. light, heat), as the voltage across the component drives the current through it.
The unit of resistance is the Ohm (Ω) .
What causes resistance? As an electric current flows, charged particles called electrons move through a conductor. These moving electrons collide with the atoms of the conductor, making it more difficult for the current to flow. This causes the resistance.
What factors affect resistance? Longer wires increases resistance. The electrons collide with atoms more often in a long wire than in a short wire. Thin wires increases resistance. A thin wire has fewer electrons to carry the current than a thick wire.

10. Georg Ohm
In 1826, based on his work on conduction in metal wires, Georg Ohm formulated a law relating to the current passing through a wire, and the potential difference (voltage) applied.
Georg Simon Ohm ( )
He found that for a fixed temperature, the current flowing through a resistor is directly proportional to the potential difference across the resistor.
V = I x R
This is known as Ohm’s Law.
Ohm also introduced the SI unit of resistance, as the Ohm (Ω).

11. Ohm’s Law
Using Ohm’s Law, the resistance of any component can be calculated by measuring the current in the circuit, and the potential difference across the component.

12. Ohm’s Law
Using Ohm’s Law, the resistance of any component can be calculated by measuring the current in the circuit, and the potential difference across the component.

13. Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any one of the other three values within the triangle.
A resistor is placed in a circuit with a potential differences of 12 volts and a current flow of 0.1A.
What is the resistance?
R = V I
R = 12V A
R = 120Ω
The resistance is 120Ω

14. Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any one of the other three values within the triangle.
A 10Ω resistor is placed in a circuit with a potential difference of 5 volts.
What is the current?
I = V R
I = 5V Ω
I = 0.5A
The current is 0.5A

15. Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any one of the other three values within the triangle.
A 20Ω resistor is placed in a circuit with a current of 0.1 A.
What is the voltage across the resistor?
V = I x R
V = 0.1A x 20Ω
V = 2V
The voltage is 2V

16. Electron Flow in a Circuit
To appreciate how a flow of electrons can carry an electric charge, a simple electric circuit can be compared with a domestic central heating system.
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17. From the results you should see that:
Ohm’s Law
Drag each of these resistors into the circuit to find out how they affect the current flowing in the circuit.
From the results you should see that:
For a fixed potential difference within the circuit, the greater the resistance, the smaller the current.

18. Current v Potential Difference Graphs
Current against potential difference graphs can be used to show how the current varies through a given component, dependant upon the potential difference. .

19. Thermistor
The resistance of a thermistor is dependant upon the temperature.
Turn up the temperature on the thermistor (drag the red slider on the thermometer) and see how it affects the resistance in the circuit. (If the current goes down, the resistance has gone up.)
A thermistor’s resistance decreases as the temperature increases.

20. Light-dependant Resistor (LDR)
The resistance of a light-dependant resistor (LDR) is dependant upon the levels of light.
Turn up the light on the LDR and see how it affects the resistance in the circuit. (If the current goes down, the resistance has gone up.)
The LDR’s resistance decreases as the light intensity increases.

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