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Theme B — The Particulate Nature of Matter

B.5.12 Variable Resistance — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 53 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.5.12 Variable Resistance — IB PHYSICS SL notes: Explain and use rheostats, potential dividers, potentiometers, thermistors and LDRs as variable resistance and sensors.Construction — a sliding contact on a resistance track lets the resistance be altered smoothlyUse in series — i…

  • Comparing p.d.s
  • At fixed illumination
  • LDR with fixed resistor
  • NTC thermistor
  • Applications
  • Describing a response

Part 1 of 8Variable resistor (rheostat)

Construction
  • Construction a sliding contact on a resistance track lets the resistance be altered smoothly
Use in series
  • Use in series it controls the current in a loop, and a larger resistance gives a smaller current
  • It is used in dimmer controls for small lamps.
Limit
  • Limit the load stays in the loop, so the current is never zero and the p.d. across the load cannot be reduced to zero

Part 2 of 8Potential divider

Potential divider
  • Potential divider two resistors, or a resistor and a sensor, in series across a supply with the output taken across one of them
Rule
  • Rule the current is the same in both, so the p.d. divides in proportion to the resistances
  • Vout = Vin × R2 / (R1 + R2)
  • R2 is the resistor across which the output is taken.
Range
  • Range the output is a fraction between 0 and 1 of the input, and the larger resistor takes the larger share
Condition
  • Condition the formula holds only if the device on the output draws negligible current

Part 3 of 8Potentiometer

Potentiometer
  • Potentiometer a resistive track across a supply with a sliding contact that selects the output
  • It is a variable resistor connected as a potential divider.
  • It gives a continuously variable output voltage.
Symbol
  • Symbol a resistor fitted with a sliding contact
Operation
  • Operation the slider splits the track into an upper and a lower part with different resistances
  • Moving the slider up raises the resistance and p.d. of the lower part.
Range
  • Range the output varies from zero to the full supply p.d.
    • (zero resistance in the output part)
Uses
  • Uses a volume control, and I–V experiments where the p.d. across a component must start at 0 V
Comparing p.d.s
  • Comparing p.d.s it can compare potential differences in different parts of a circuit

Part 4 of 8Thermistor

Thermistor
Thermistor
  • Thermistor a non-ohmic sensory resistor whose resistance varies with temperature
NTC type
  • NTC type most thermistors have a negative temperature coefficient
  • Resistance decreases as temperature increases.
  • Resistance increases as temperature decreases.
Uses
  • Uses temperature sensors in ovens, fire alarms and digital thermometers
Cause
  • Cause heating releases more charge carriers in the semiconductor

Part 5 of 8Light-dependent resistor

LDR
  • LDR a non-ohmic sensory resistor whose resistance changes with the light falling on it
Behaviour
  • Behaviour resistance decreases as light intensity increases
  • In the dark it is very large, millions of ohms.
  • In bright light it is small, tens of ohms.
At fixed illumination
  • At fixed illumination it behaves as an ohmic conductor, so its I–V graph is a straight line
Uses
  • Uses automatic street lights and garden lights that switch on when it gets dark

Part 6 of 8Sensors in a divider

Signal
  • Signal if one resistor of a divider is a sensor, the output p.d. becomes a signal for the environment
LDR with fixed resistor
  • LDR with fixed resistor in bright light the LDR resistance falls, so its p.d. falls and the p.d. across the fixed resistor rises
  • Swapping the positions of the two reverses the effect.
NTC thermistor
  • NTC thermistor heating lowers its resistance and the share of the supply p.d. across it
Applications
  • Applications thermostats, automatic lights and fire alarms, with the output feeding a switching circuit
Describing a response
  • Describing a response state what happens to the sensor resistance, then to the ratio of resistances, then to the output p.d.

Part 7 of 8Effect of loading

Load on the output
  • Load on the output a load RL across R2 forms a parallel combination, which lowers the resistance and so lowers Vout
Remedy
  • Remedy make the load resistance much larger than R2
Voltmeter
  • Voltmeter a voltmeter is also a load, and a low-resistance meter disturbs the reading it takes
Design
  • Design choose the ratio R2/(R1 + R2) = Vout/Vin first, then the sizes
  • Large resistances waste little power but are affected more by loading.
Complex dividers
  • Complex dividers replace any parallel parts by their equivalent resistance before using the divider formula

Part 8 of 8More from the specification

1Internal Resistance of Cells
  • The natural resistance to current flow within the electrolyte and electrodes of a battery, converting a portion of electrical energy into internal thermal dissipation.
2Terminal Potential Difference
  • Expressed by the equation V = epsilon - Ir, showing that the available voltage delivered to the load decreases linearly as the circuit current measured by an ammeter increases.
3Electron Drift and Current Direction
  • Conduction electrons move away from the negative terminal towards the positive terminal through the external circuit,
  • which is directly opposite to the conventional current direction.
4Potential Divider Circuit
  • An arrangement of variable resistors used to continuously adjust the output potential difference supplied to an electrical component from zero to the full source voltage.
5Circuit diagram
  • Drawing of a circuit using standard symbols.
6Conventional current
  • Current drawn from the positive to the negative terminal.
7Electron
  • Negatively charged particle in an atom.
8Battery
  • Two or more cells joined together.
9Ammeter
  • Component that measures current.
10Terminal
  • Connection point of a cell, labelled + or −.
11Source
  • Reservoir at TH providing heat QH
12Internal resistance
  • Resistance inside a cell that causes lost p.d.
13Electrical energy
  • Energy carried by electrons in a circuit.
14Energy transferred
  • W = V Q, in joules.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)

Key terms

TermMeaning
Potential dividerSeries resistors that share the supply p.d. in proportion to their resistances.
PotentiometerResistive track with a sliding contact used as a variable divider.
RheostatVariable resistor used in series to change the current.
ThermistorSensor whose resistance changes with temperature.
LDRSensor whose resistance falls as light intensity rises.
LoadingThe reduction of a divider output caused by a load connected across it.

Avoid these mistakes

1
Using the formula with the wrong resistor in the numerator — the numerator is the resistor across which the output is measured.
2
Forgetting that a divider output falls when a load is connected.
3
Believing a rheostat in series can reduce the p.d. across the load to zero.
4
Saying the resistance of an LDR increases in bright light — it decreases.
5
Ignoring that the divider formula only holds when the output current is negligible.

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