What we will do today

• Define the term ‘potential difference’.

• Look at the ratio V/I for a resistor.• Define an equation relating

resistance, voltage and current.

Potential difference

• The ‘potential difference’ across a component in a circuit is the difference in voltage across that component.

• It is measured in volts (V) using a voltmeter.

Voltage to Current ratio (READ)

• What apparatus will be needed to investigate the ratio V/I for a series resistor circuit as the voltage is altered?

• How will the ammeter be set up in the circuit?

• How will the voltmeter be set up?• What will our circuit diagram look like?

Voltage to Current ratio:Circuit diagram

Voltage to Current ratio• Copy the following results table:

Potential difference

across resistor (V)

Current through resistor (mA)

Ratio V/I

0.51.01.52.02.5

Voltage to Current ratio• We can now state that the ratio V/I for a

resistor remains approximately constant for different currents.

• Voltage and current are directly proportional

Relationship between V, I and R

• Potential difference, current and resistance are all related through the following equation:

• Potential difference = Current x Resistance

V = IR

• This is known as Ohm’s Law

What have we learned today?

• The ‘potential difference’ across a component in a circuit is the difference in voltage across that component.

• The ratio V/I for a resistor remains constant.

• We can now state a relationship between voltage, current and resistance, V = IR

Example 1• V = ?• R = 10 Ω• I = 0.5 A

Example 2• V = 6 V• R = ?• I = 200 mA

Example 3• V = 12 V• R = 0.12 kΩ • I = ?

2003 Qu: 12

2007 Qu: 8

2007 Qu: 8