- All solids are made up of vibrating atoms
- The higher the temperature, the faster these atoms vibrate
- Electric current is the flow of free electrons in a material
- The electrons collide with the vibrating atoms which impedes their flow, hence the current decreases
- So, if the current decreases, then the resistance will increase (from V = IR)
- Therefore, an increase in temperature causes an increase in resistance
I–V Graphs (AQA GCSE Physics: Combined Science)
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Ohm's Law
- Ohm's Law states that:
The current through a conductor is directly proportional to the potential difference across it
- Electrical conductors that obey Ohm's Law are referred to as ohmic conductors
- Examples of ohmic conductors are:
- Fixed resistors
- Wires
- Heating elements
- Ohm's Law is represented by the equation V = IR
- If V and I are directly proportional, this means that the resistance R remains constant
- Ohm's Law is relevant only at constant temperatures
- An ohmic conductor will have a current-voltage (I–V) graph that is a straight line through the origin
An I-V graph for an ohmic conductor is a straight line graph through the origin
- Reversing the potential difference (and direction of the current) makes no difference to the shape of the line, the potential difference and current values will just be negative
- If the axis labels are swapped around (the current on the x-axis and p.d on the y-axis) the graph will still be a straight line through the origin
Filament Lamps
- A filament lamp is an example of a non-ohmic conductor
- This means that the 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
Resistance & Temperature
Diodes
- A diode is a non-ohmic conductor that 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 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
- When the current is in the direction of the arrowhead symbol, this is forward bias
I-V graph for a semiconductor diode
- An LED is a specific type of diode that emits light and works the same way as a normal diode
Linear & Non-Linear Graphs
- Circuit elements can be 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
- Linear elements include:
- Fixed resistors
- Wires
- Heating elements
- Non-linear elements include:
- Filament lamps
- Diodes & LEDs
- LDRs
- Thermistors
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