Electrical Components (Edexcel GCSE Physics)

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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

Filament lamp IV graph, downloadable AS & A Level Physics revision notes

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

Examiner Tip

Make sure to practice drawing which current-voltage graph is for which component, as this is a common exam question!

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, downloadable IGCSE & GCSE Physics revision notes

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

LDR & Light Intensity, downloadable IGCSE & GCSE Physics revision notes

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-symbol, IGCSE & GCSE Physics revision notes

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

Thermistor, IGCSE & GCSE Physics revision notes

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

Digital Thermometer, downloadable IGCSE & GCSE Physics revision notes

A digital thermometer uses a thermistor

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.