I–V Graphs (OCR GCSE Physics A (Gateway))

Linear & Non-Linear 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 an I-V graph

  • This is known as the I-V characteristic of the component

• Circuit components can have I-V characteristic graphs that are linear or non-linear

  • In maths, linear means the graph is a straight line

• To know whether a circuit element is linear or non-linear, check whether its I-V graphs is a straight line or not

  • Linear components have an I-V graph that is a straight line through the origin

  • Non-linear components have an I-V graph that is not a straight line

• Linear components are said to obey Ohm's Law and have a constant resistance, whilst non-linear do not

• Some components may be linear at low currents, then become non-linear as the current increases (and therefore a change in temperature)

  • For example, a fixed resistor at room temperature is linear, but when it becomes very hot it will become non-linear

Linear and non-linear I-V graphs

• Components with linear I-V characteristics include:

  • Fixed resistors

  • Wires

  • Heating elements

• Components with non-linear I-V characteristics include:

  • Filament lamps

  • Diodes & LEDs

  • LDRs

  • Thermistors

I-V Characteristics

Fixed Resistors

• The current through a fixed resistor increases as the potential difference across it increases

• In other words, current is directly proportional to the potential difference for a fixed resistor

• An I-V graph shows that the line is straight and goes through the origin, as shown in the I-V graph below:

I-V graph for a fixed resistor. The current is directly proportional to the potential difference as the graph is a straight line through the origin

• This relationship is true because the resistance of a fixed resistor is constant

Filament Lamps

• For a filament lamp, current and potential difference are not directly proportional

  • This is because the resistance of the filament lamp increases as the temperature of the filament increases

• The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference

I-V graph for a filament lamp

• This is because:

  • As the current increases, the temperature of the filament in the lamp increases

  • The higher temperature causes the atoms in the metal lattice of the filament to vibrate more

  • This causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through

  • 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

• Reversing the potential difference reverses the current and makes no difference to the shape of the curve

Diodes

• A diode allows current to flow in one direction only

  • This is called forward bias

• In the reverse direction, the diode has very high resistance, and therefore no current flows

  • This is called reverse bias

I-V graph for a semiconductor diode

• The I–V graph for a diode is slightly different:

  • 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, this is reverse bias

    • This is shown by a zero reading of current or potential difference on the left side of the graph