10.3.4Electricity and D.C. circuits
Potential Divider Components
5 min read 8 parts Lesson 24 of 26 1 quiz
Lesson summary
10.3.4 Potential Divider Components — AS PHYSICS 9702 notes: Use LDRs and thermistors in potential dividers to give an output that depends on light or temperature.Purpose — they vary the output voltage of a potential divider.Effect — the changed output could switch an external co…
- Sensory resistors
- Resistance
- Other resistor
- Galvanometer
- Potentiometer
- Uses — street and security lights
Section 1 of 8Variable and sensory resistors
Purpose
- Purpose they vary the output voltage of a potential divider.
Effect
- Effect the changed output could switch an external component on or off, such as a heater switching off at room temperature.
Sensory resistors
- Sensory resistors light-dependent resistors (LDRs) and thermistors.
Section 2 of 8LDR circuits
Resistance
- Resistance the higher the light intensity, the lower the resistance of an LDR.
Uses
- Uses street and security lights.
Section 3 of 8Thermistor circuits
Resistance
- Resistance the hotter the thermistor, the lower its resistance.
Uses
- Uses fire alarms, ovens and digital thermometers.
Section 4 of 8Predicting the output
1Proportional to resistance
- Proportional to resistance from V = IR, the p.d. across a resistor in a divider is proportional to its resistance.
2Sensor resistance decreases
- Sensor resistance decreases its p.d. decreases.
3Sensor resistance increases
- Sensor resistance increases its p.d. increases.
4Other resistor
- Other resistor if one p.d. decreases, the p.d. across the other resistor must increase, and vice versa.
Section 5 of 8Example reasoning for a cooled thermistor
1
The thermistor and resistor are in series, so they have the same current.
2
Lower temperature increases the thermistor resistance.
3
A larger share of the supply p.d. is across the thermistor.
4
The p.d. across the fixed resistor decreases, so the total stays equal to the e.m.f.
Section 6 of 8More from the specification
1Sensor-controlled voltage division
- Sensor-controlled voltage division combining fixed resistors with light-dependent resistors provides an automated means to control switching thresholds in dark-activated circuits.
2Potential divider equation
- A series arrangement of two resistors divides input voltage in proportion to their resistances: Vout = Vin * (R2 / (R1 + R2)).
3Potentiometers and null methods
- A potentiometer wire with a sliding contact provides a continuously variable output voltage, measured precisely using a sensitive galvanometer in a null-deflection balance.
4Sensor circuits using thermistors and LDRs
- Replacing one divider resistor with an LDR or NTC thermistor produces temperature- or light-dependent output voltages for automated control circuits.
5Potential divider equation
- A series arrangement of two resistors divides input voltage in proportion to their resistances: Vout = Vin * (R2 / (R1 + R2)).
6Potentiometers and null methods
- A potentiometer wire with a sliding contact provides a continuously variable output voltage, measured precisely using a sensitive galvanometer in a null-deflection balance.
7Sensor circuits using thermistors and LDRs
- Replacing one divider resistor with an LDR or NTC thermistor produces temperature- or light-dependent output voltages for automated control circuits.
8Potential difference
- Work done by a unit charge passing through a component.
9Sliding contact
- Movable connection that divides the resistance into two parts.
10Input voltage
- P.d. applied across the series resistors.
11Circuit symbol
- Standard picture for a component.
12Galvanometer
- Sensitive centre-zero meter used to detect zero current.
13Potentiometer
- Variable resistor used as a potential divider.
14Variable
- A factor that can change in an investigation.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| LDR | Light-dependent resistor whose resistance falls as light intensity rises. |
| Thermistor | Resistor whose resistance changes with temperature; NTC resistance falls as temperature rises. |
| Sensory resistor | Resistor used to sense an environmental condition. |
| Output voltage | P.d. taken from a potential divider. |
Section 8 of 8Exam technique
1
Saying a thermistor’s resistance rises as it heats — an NTC thermistor’s resistance falls.
2
Forgetting that the p.d.s across the two resistors always add up to the supply p.d.
3
Reading Vout across the wrong component.
4
Mixing up which sensor responds to light and which to temperature.