Edexcel iGCSE Physics -2.11–2.12 Resistance of LDRs and Thermistors; Lamps and LEDs- Study Notes- New Syllabus

Edexcel iGCSE Physics -2.11–2.12 Resistance of LDRs and Thermistors; Lamps and LEDs- Study Notes- New syllabus

Edexcel iGCSE Physics -2.11–2.12 Resistance of LDRs and Thermistors; Lamps and LEDs- Study Notes -Edexcel iGCSE Physics – per latest Syllabus.

Key Concepts:

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Edexcel iGCSE Physics -Concise Summary Notes- All Topics

Variation of Resistance in LDRs and Thermistors

Some electrical components are designed so that their resistance changes in response to environmental conditions. Two important examples are light-dependent resistors (LDRs) and thermistors.

These components are widely used as sensors in automatic control systems.

Key Relationship

The current in a circuit depends on resistance according to:

\( \mathrm{I = \dfrac{V}{R}} \)

This means that any change in resistance will change the current if the voltage is constant.

Light-Dependent Resistors (LDRs)

A light-dependent resistor is a resistor whose resistance depends on the intensity of light falling on it.

  • In bright light → low resistance.
  • In dim light or darkness → high resistance.

Qualitative Explanation

  • Light provides energy to release charge carriers.
  • More light allows more charge to flow.
  • Resistance decreases as illumination increases.

Resistance–Light Graph

  • Resistance decreases as light intensity increases.
  • The graph is curved, not straight.

Uses of LDRs

  • Automatic street lighting.
  • Security lights.
  • Camera exposure control.

Thermistors

A thermistor is a resistor whose resistance depends on temperature.

At IGCSE level, thermistors are usually NTC thermistors.

  • As temperature increases → resistance decreases.
  • As temperature decreases → resistance increases.

Qualitative Explanation

  • Heating provides energy to charge carriers.
  • More charge carriers reduce resistance.
  • Resistance falls as temperature rises.

Resistance–Temperature Graph

  • Resistance decreases rapidly as temperature increases.
  • The graph is non-linear.

Comparison of LDRs and Thermistors

  • LDRs respond to light.
  • Thermistors respond to temperature.
  • Both are non-ohmic components.

Key Idea

  • LDR resistance decreases with increasing light.
  • Thermistor resistance decreases with increasing temperature.
  • These changes affect current in a circuit.

Important Points to Remember

  • Changes are qualitative, not linear.
  • Voltage is usually kept constant.
  • LDRs and thermistors are used as sensors.

Example

An LDR is used in a street light circuit. Explain what happens to the resistance of the LDR when it gets dark.

▶️ Answer / Explanation

When it gets dark, the light intensity decreases.

The resistance of the LDR increases.

Example

A thermistor is placed in hot water. Describe how its resistance changes.

▶️ Answer / Explanation

The temperature increases.

The resistance of the thermistor decreases.

Using Lamps and LEDs to Indicate Current

Lamps and light-emitting diodes (LEDs) can be used as indicators to show whether an electric current is flowing in a circuit.

If current flows, the lamp lights or the LED emits light. If no current flows, they remain off.

Why Lamps and LEDs Indicate Current

  • Both lamps and LEDs emit light only when current passes through them.
  • Light output provides a clear visual indication.
  • This makes them useful for testing and monitoring circuits.

Lamps as Current Indicators

A lamp contains a thin metal filament.

  • When current flows, the filament heats up.
  • The hot filament glows, producing light.
  • No light means no current.

Lamps work with both a.c. and d.c. supplies.

LEDs as Current Indicators

A light-emitting diode (LED) emits light when current flows through it in the correct direction.

  • Current flows only from anode to cathode.
  • No light if connected the wrong way round.
  • Produces light with very little energy loss.

LEDs are commonly used as power and status indicators.

Key Relationship

The brightness of a lamp or LED depends on the current:

\( \mathrm{I = \dfrac{V}{R}} \)

  • Higher current → brighter light.
  • Lower current → dimmer light.

Comparing Lamps and LEDs

  • Lamps: work in either direction, but use more energy.
  • LEDs: one direction only, very energy efficient.
  • LEDs require a resistor to limit current.

Uses in Circuits

  • Power-on indicators.
  • Fault detection.
  • Testing whether a circuit is complete.

Key Idea

  • Light output shows that current is flowing.
  • No light means no current.
  • LED direction matters.

Important Points to Remember

  • LEDs must be connected with correct polarity.
  • LEDs require a series resistor.
  • Lamps respond to current by heating.

Example

A lamp in a circuit is not glowing. State what this shows about the current.

▶️ Answer / Explanation

This shows that no current is flowing in the circuit.

Example

An LED connected to a battery does not light up. Give one possible reason.

▶️ Answer / Explanation

The LED may be connected the wrong way round.

This prevents current from flowing through it.

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