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14.3Chapter 14: Electrical Circuits, Resistance and Power

Resistance — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 6 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision
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Lesson summary

Physics 14.3 Resistance — Ohm's Law (R = V / I), Variable Resistors and Ohms (Ω) — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how resistance affects current, calculate resistance and know the circuit symbols for fixed and variable resistors.Resistance =…

  • Depends on components
  • More resistance
  • Particle picture
  • Comparing components
  • Fixed resistor
  • Same voltage

Part 1 of 6What resistance is

What resistance is
Resistance
Resistance a measure of how difficult it is for current to flow through a component, measured in ohms (Ω).
Depends on components
  • The current in a circuit depends on the components as well as on the battery.
More resistance
  • Two lamps give more resistance than one, so less charge flows per second.
  • A buzzer in place of a lamp can reduce the current because it has a bigger resistance.
Particle picture
  • Flowing electrons are slowed down by obstacles such as vibrating atoms.

Part 2 of 6The rope model

Current
  • It is like the speed of the rope sliding through your hands.
Voltage
  • It is like how hard the "battery" person pulls the rope.
Resistance
  • It is like how hard the "component" person squeezes the rope.
Result
  • More resistance gives a smaller current for the same voltage.

Part 3 of 6The resistance equation

The resistance equation
Current
  • Current (A) = voltage (V) ÷ resistance (Ω), or I = V ÷ R.
Resistance
  • Resistance (Ω) = voltage (V) ÷ current (A), or R = V ÷ I.
Voltage
  • V = I × R.
    What the symbols mean: V = potential difference (V) · I = current (A) · R = resistance (Ω)
Comparing components
  • At the same voltage, the component with the smaller current has the bigger resistance.
QuantitySymbolUnitInstrument
VoltageVvolt (V)Voltmeter, in parallel
CurrentIampere (A)Ammeter, in series
ResistanceRohm (Ω)Calculated

Part 4 of 6Fixed and variable resistors

Fixed resistor
  • Its resistance does not change.
Variable resistor
  • Its resistance can be changed.
Effect
  • Increasing the resistance reduces the current, so a lamp becomes dimmer.
Uses
  • Dimmer switches, volume controls and motor controls.

Part 5 of 6Parallel branches

Same voltage
  • Each branch has the full battery voltage.
Current in branches
  • The current from the battery equals the sum of the branch currents, so an unknown branch current is found by subtraction.
Comparing branches
  • The branch with the smaller current has the bigger resistance.
More branches
  • Adding a branch gives the current another path, so the overall resistance of the circuit falls.

Part 6 of 6Measuring resistance

1
Connect the component in series with an ammeter and a variable resistor to a low-voltage supply.
2
Connect a voltmeter across the component.
3
Change the variable resistor to obtain several pairs of readings.
4
Calculate R = V ÷ I for each pair and take the mean.
1Metal wire at constant temperature
  • A graph of voltage against current is a straight line through the origin, showing V is proportional to I (Ohm's law).
2Filament lamp
  • The line curves because resistance rises as the filament heats up.

Key terms

TermMeaning
ResistanceHow difficult it is for current to flow, in ohms.
Ohm (Ω)The unit of resistance.
Fixed resistorA resistor with a constant resistance.
Variable resistorA resistor whose resistance can be changed.
Ohm's lawFor a metal at constant temperature, voltage is proportional to current.

Avoid these mistakes

1
Thinking a bigger resistance gives a bigger current — it gives a smaller current.
2
Thinking a battery alone decides the current — the components matter too.
3
Using the wrong unit — resistance is in ohms, not amperes.
4
Thinking adding a parallel branch increases the overall resistance — it decreases it.

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