• As the potential difference across a component is increased, the current also increases
  • This is because potential difference and current are?proportional

   

• The precise relationship between voltage and current is different for different components and can be shown on an?IV?graph, including in:
  • Fixed resistors & wires
  • Filament lamps
  • Diodes

   

Fixed Resistors & Wires

• The?current?through a fixed resistor or a wire?increases?as the?potential difference (or voltage)?across it?increases
• In other words, current is?directly proportional?to the potential difference for a fixed resistor (or a wire)
  • This relationship is true because the resistance of the fixed resistor (or wire) stays?constant

   

• An?IV?graph shows that the line is?straight?and goes through the?origin,?as shown in the image?below:

IV graph for a fixed resistor. The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin

Filament Lamps

• For a filament lamp, current and voltage are?not?directly proportional
  • This is because the resistance of the filament lamp?increases?as the?temperature?of the filament?increases

   

• The?IV?graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference

IV 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

   

• The?IV?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

     

   

IV graph for a semiconductor diode

Investigating IV Graphs Experimentally

• In order to investigate the relationship between?current?and?voltage?different components, the following equipment is required:
  • An?ammeter?- to measure the current through the component
  • A?voltmeter?- to measure the voltage across the component
  • A?variable resistor?- to vary the current through the circuit
  • Power?source?- to provide a source of potential difference (voltage)
  • Wires?- to connect the components together in a circuit

   

• The image below shows the circuits set up to obtain?IV?graphs for a filament lamp and a diode

These circuits enable the investigation of current and voltage for a filament lamp or diode to be investigated

• The?current?is the?independent?variable
  • The?variable resistor?is used to change the current flowing through the filament lamp / diode

   

• The?voltage?is the?dependent?variable
  • The?voltmeter?is used to measure the voltage across the filament lamp / diode

   

• Recording measurements of current and voltage as the current increases enables an?IV?graph to be plotted for each component

• Resistance is the?opposition?to the flow of?current
  • The?higher?the resistance of a circuit the?lower?the current

   

• Resistors come in two types:
  • Fixed?resistors
  • Variable?resistors

   

• Fixed resistors have a resistance that remains?constant
• Variable resistors can?change?the resistance by changing the?length?of wire that makes up the circuit
  • A?longer?length of wire has?more?resistance?than a shorter length of wire

   

Fixed and variable resistor circuit symbols

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