9.3.5Electricity and D.C. circuits
Resistance in Sensory Resistors
5 min read 8 parts Lesson 12 of 26 1 quiz
Lesson summary
9.3.5 Resistance in Sensory Resistors — AS PHYSICS 9702 notes: Describe how the resistance of an LDR and a thermistor changes and where they are used.LDR — a non-ohmic conductor and sensory resistor.Behaviour — its resistance decreases as the light intensity increases.In the dark…
- In bright light
- Light sensor
- Temperature sensor
- Constant conditions
- Series circuit
- Resistivity
Section 1 of 8Light-dependent resistor
LDR
- LDR a non-ohmic conductor and sensory resistor.
Behaviour
- Behaviour its resistance decreases as the light intensity increases.
In the dark
- In the dark its resistance is very large, millions of ohms.
In bright light
- In bright light its resistance is small, tens of ohms.
Section 2 of 8Uses of an LDR
Light sensor
- Light sensor used in circuits that automatically switch on lights when it gets dark.
Examples
- Examples street lighting and garden lights.
Section 3 of 8Thermistor
Thermistor
- Thermistor a non-ohmic conductor and sensory resistor.
Behaviour
- Behaviour for an NTC thermistor, resistance decreases as temperature increases.
Hotter
- Hotter the resistance decreases.
Cooler
- Cooler the resistance increases.
Section 4 of 8Uses of a thermistor
- Temperature sensor
- used in ovens, fire alarms and digital thermometers.
Section 5 of 8Using them in circuits
Constant conditions
- Constant conditions at a fixed light intensity or temperature, the component obeys Ohm’s law and has a straight-line I–V graph.
Series circuit
- Series circuit if the thermistor cools, its resistance rises, the total resistance rises and the current falls.
- The thermistor’s share of the total p.d. increases because its resistance is larger relative to the fixed resistor.
Section 6 of 8More from the specification
More from the specification
Transducer sensor calibration
- Transducer sensor calibration monitoring resistance changes in thermistors or light-dependent resistors provides a responsive means to sense environmental parameters.
Resistivity of Conductors
- Electrical resistivity ρ is an intrinsic material property defined by ρ = R·A / L;
- highly conducting materials like copper exhibit low resistivity due to high free conduction electron density.
Filament Lamp I-V Characteristics
- In a filament lamp,
- increasing potential difference across the component accelerates electrons,
- causing increased lattice vibrations; the resulting temperature rise causes resistance to increase,
- producing a curved I-V graph with decreasing gradient.
Ohm's Law and I-V Graphs
- For an ohmic conductor at constant temperature, current is directly proportional to the potential difference across it,
- resulting in a straight-line I-V graph whose inverse gradient gives electrical resistance.
Proportional
- Changing by the same factor as another quantity.
Potential difference
- Work done by a unit charge passing through a component.
Semiconductor diode
- Component that lets current flow in one direction only.
Resistivity
- Property of a material defined by ρ = RA/L.
Copper
- Metal widely used for electrical wires.
Gradient
- Slope of a graph line.
Characteristic
- Graph of current against p.d. for a component.
Filament
- Thin wire in a lamp that gets hot and glows.
Electron
- Negatively charged particle in an atom.
Q = It
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| LDR | Light-dependent resistor whose resistance decreases as light intensity increases. |
| NTC thermistor | Thermistor whose resistance decreases as temperature increases. |
| Sensory resistor | Resistor whose resistance changes with an environmental condition. |
| Non-ohmic | Not having a constant ratio of p.d. to current. |
Section 8 of 8Exam technique
1
Saying an LDR resistance increases with light — it decreases.
2
Saying a thermistor’s current rises when it cools in series — the current falls.
3
Thinking sensory resistors never obey Ohm’s law — at constant conditions they do.
4
Confusing LDR with thermistor — LDRs respond to light, thermistors to temperature.