Electrical Components (Edexcel GCSE Combined Science) : Revision Note

I-V Graphs

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

LDRs

• A light-dependent resistor (LDR) is a type of sensory resistor

  • This means it is a resistor which has a resistor that changes with its surroundings

• The circuit symbol for an LDR is:

LDR circuit symbol

• The resistance of an LDR changes depending on the light intensity on it

  • As the light intensity increases the resistance of an LDR decreases and vice versa

The resistance of an LDR is dependent on the amount of light intensity on it

• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights

Thermistors

• A thermistor is also a type of sensory resistor

• It is represented by the following circuit symbol:

Thermistor circuit symbol

• The resistance of a thermistor changes depending on its temperature

  • As the temperature increases the resistance of a thermistor decreases and vice versa

The resistance through a thermistor is dependent on temperature

• Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers

  • As the thermistor gets hotter, its resistance decreases

  • As the thermistor gets cooler, its resistance increases

A digital thermometer uses a thermistor