Ohm's Law & I-V Characteristics (OCR A Level Physics)

Ohm's Law & I-V Characteristics

• Ohm’s law states:

  For a conductor at a constant temperature, the current through it is proportional to the potential difference across it

• Constant temperature implies constant resistance

• Ohm's law is represented in the equation below:

• Measuring the variation of current with potential difference through a fixed resistor will produce a straight line graph, such as the one below

Circuit for plotting graphs of current against voltage

• Since the gradient is constant, the resistance R of the resistor can be calculated by using 1 ÷ gradient of the graph

• An electrical component obeys Ohm’s law if its graph of current against potential difference is a straight line through the origin

  • A resistor does obey Ohm’s law

  • A filament lamp does not obey Ohm’s law

• This applies to any metal wires, provided that the current isn’t large enough to increase their temperature

I–V Characteristics

• 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

• For an ohmic conductor eg. a fixed resistor:

  • The I–V graph is a straight line through the origin

• For a non-ohmic conductor, such as

  • A semiconductor diode, the I–V graph is a horizontal line that goes sharply upwards

  • An LED (light-emitting diode), the I–V graph is similar to the semiconductor diode, since it is a specific diode that emits visible light

  • A filament lamp, the I–V graph has an 'S' shaped curve

  • A thermistor, the I–V graph is a curved line with increasing gradient through the origin

I–V characteristics for an ohmic conductor (e.g. resistor), semiconductor diode, filament lamp, LED and thermistor

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 straight-line graph through the origin

Semiconductor Diode

• The I–V graph for a semiconductor 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 which then goes steeply vertically down

• For an LED, the I–V graph is identical, except the sharp increase in p.d is further away from the origin as the frequency of the light increases

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