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