Current, voltage and resistance

Current, voltage and resistanceRevise the following:

• Circuit symbols• Circuit diagrams• Current and voltage in series circuits• Current and voltage in parallel

circuits

Circuit symbols

Current, voltage and resistance• Current = the rate of flow of electrons in a

circuit

• Voltage/potential difference = the difference in electrical potential energy between two points in a circuit. Potential difference is also called voltage

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

A A

A

1 A

1 A

1 A

Current in a series circuitIn a series circuit, the current flows

through one continuous pathway so the current is the same in all parts of a series

circuit.

In a series circuit, the voltage supplied by the battery is shared by the components so the sum of the potential difference across the components equals the battery

voltage.

V

VV

6 V

3 V 3 V

As more bulbs are added in series, each bulb has less potential difference and so the bulbs become

dimmer.

Voltage in a series circuit

In a parallel circuit, the current divides at the point where the circuit branches and then recombines to

complete the circuit so the current is not the same in all parts of a parallel circuit.

A A

A

A

1 A

1 A

2 A 2 A

Current in a parallel circuit

• In a parallel circuit, the potential difference across each bulb is the same as the potential difference across the battery. V

V

V6 V

6 V

6 V

• This means that all the bulbs have the same brightness, and they are brighter than the same number of bulbs in a series circuit. • This also means that the battery will run down faster in a parallel circuit.

Voltage in parallel circuits

Georg Ohm (Ohm’s Law)

• It was such an important discovery in electricity that the unit of resistance is called the ohm.

• The formula used to calculate Resistance is called Ohm’s Law

• This unit is represented by the symbol Ω.

• Resistance is a measure of how hard it is for electrons to move in an electrical circuit.• In 1827 a German physics and maths

teacher Georg Ohm discovered a connection between current, voltage and resistance

Ohm’s law is usually written as:

Voltage is measured in volts (V). Current is measured in amps (A).

Resistance is measured in ohms ().

This formula can also be written as:

voltage = current x resistance V = I x R

resistance = R =

voltagecurrent V I

What is the formula/equation for Ohm’s law?

Current-voltage graphs

A resistor at constant temperature

Potential difference isdirectly proportional to

current.As pd/voltage doubles,

current doublesAs pd/voltage triples,

current triples

Filament lamp

A smooth curved line which levels

off

DiodeNo current when

pd/voltage is negative. When

pd/voltage reaches a certain positive voltage, current increases and is

directly proportional to pd

Resistance...