CurrentPotential Difference Graphs
 As the potential difference (voltage) across a component is increased, the current also increases (by Ohm’s law)
 The precise relationship between voltage and current is different for different components and can be shown on a currentpotential difference or IV graph
 For an ohmic conductor, the I–V graph is a straight line through the origin
 For a semiconductor diode, the I–V graph is a horizontal line that goes sharply upwards
 For a filament lamp, the I–V graph has an 'S' shaped curve
I–V characteristics for an ohmic conductor (e.g. resistor), semiconductor diode and filament lamp
Ohmic Conductor
 The I–V graph for an ohmic conductor at constant temperature e.g. a resistor is very simple:
 The current is directly proportional to the potential difference
 This is demonstrated by the straightline graph through the origin
Diode
 The I–V graph for a diode is slightly different.
 A diode is used in a circuit to allow current to flow only in a specific direction:
 When the current is in the direction of the arrowhead symbol, this is forward bias. This is shown by the sharp increase in potential difference and current on the right side of the graph
 When the diode is switched around, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph
 The threshold voltage at which a diode starts to conduct is typically around 0.6V
Filament Lamp
 The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
 This is because:
 As the current increases, the temperature of the filament in the lamp increases
 Since the filament is a metal, the higher temperature causes an increase in resistance
 Resistance opposes the current, causing the current to increase at a slower rate
 Where the graph is a straight line, the resistance is constant
 The resistance increases as the graph curves
 The filament lamp obeys Ohm's Law for small voltages
Thermistor
 The I–V graph for a thermistor is a shallow curve upwards
 The increase in the potential difference results in an increase in current which causes the temperature of the thermistor to rise
 As its temperature rises, its resistance decreases
 This means even more current is able to flow through
 Since the current is not directly proportional to the potential difference (the graph is still curved), the thermistor does not obey Ohm's Law
 The I–V graph for a thermistor shows the current increasing at a proportionally slower rate than the potential difference
 This is because:
 As the current increases, the temperature of the filament in the lamp increases
 Since the filament is a metal, the higher temperature causes an increase in resistance
 Resistance opposes the current, causing the current to increase at a slower rate
Worked Example
The I–V graph of two electrical components X and Y are shownWhich statement is correct?
A. The resistance of X increases as the current increases
B. At 2 V, the resistance of X is half the resistance of Y
C. Y is a semiconductor diode and X is a resistor
D. X is a resistor and Y is a filament lamp
ANSWER: C
Step 1: Consider the characteristics of graph X

 The I–V graph X is linear
 This means the graph has a constant gradient. I/V and the resistance is therefore also constant (since gradient = 1/R)
 This is the I–V graph for a conductor at constant temperature e.g. a resistor
 The I–V graph X is linear
Step 2: Consider the characteristics of graph Y

 The I–V graph Y starts with zero gradient and then the gradient increases rapidly
 This means it has infinite resistance at the start which then decreases rapidly
 This is characteristic of a device that only has current in one direction e.g a semiconductor diode
 The I–V graph Y starts with zero gradient and then the gradient increases rapidly
Step 3: Compare this information with the statements AD

 A. Resistance is constant
 therefore this statement is incorrect
 B. At 2V the current of X is 0.5 A and the current of Y is 0 A.
 therefore this statement is incorrect
 D. X is an ohmic component such as a resistor, however Y is the graph for a diode not a filament bulb.
 therefore this statement is incorrect
 A. Resistance is constant
Step 4: State the correct answer

 The correct answer is C
Exam Tip
Make sure you're confident in drawing the I–V graphs for different components, as you may be asked to sketch these from memory in exam questions
You may get a question asking you to explain why resistance in a metal increase with temperature. This is usually two marks given for:
 The vibrations of metal atoms are faster and of greater displacement from equilibrium
 Therefore there are more collisions between the conduction electrons and the atoms