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Theme B — The Particulate Nature of Matter

B.5 Current and Circuits — IB Physics SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 32 of 44 1 quiz

📚 IB Physics SL Hub / Topical Revision
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Lesson summary

B.5 Current and Circuits — IB SL Notes — IB PHYSICS SL notes: Analyse d.c. circuits using current, potential difference, resistance, resistivity, power, e.m.f.

  • Charge carriers
  • Ohmic conductor
  • Filament lamp
  • What the symbols mean
  • Electrical power
  • Conservation

Part 1 of 9Charge, current and potential difference

Current
  • Current the rate of flow of charge, I = Δq/Δt, with 1 A = 1 C s−1.
Charge carriers
  • Charge carriers electrons in metals and ions in electrolytes.
Direction
  • Direction conventional current flows from positive to negative while electrons drift the opposite way.
Potential difference
  • Potential difference the work done per unit charge, V = W/q, with 1 V = 1 J C−1.
E.m.f.
  • E.m.f. the energy transferred per unit charge from other forms to electrical energy in a source.
  • Chemical cells and solar cells are common sources.

Part 2 of 9Resistance and resistivity

Resistance
  • R = V/I.
Ohmic conductor
  • V/I is constant at constant temperature, giving a straight I–V line through the origin.
Filament lamp
  • Filament lamp it is non-ohmic because its resistance rises as it heats up.
Resistivity
  • ρ = RA/L, in Ω m.
    What the symbols mean: R = resistance (Ω) · ρ = resistivity (Ω m) · L = length (m) · A = cross-sectional area (m2)
  • It is a property of the material and depends on temperature.

Part 3 of 9Power

  • Electrical power
    • P = IV = I2R = V2/R.
      What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Part 4 of 9Series and parallel

FeatureSeriesParallel
CurrentSame in each componentShared between branches
Potential differenceShared between componentsSame across each branch
Total resistanceR = R1 + R2 + …1/R = 1/R1 + 1/R2 + …; less than the smallest branch
One component fails (open)All stopOther branches continue
  • Conservation
    • charge is conserved at junctions and energy round any loop.

Part 5 of 9Meters

Ideal ammeter
  • Ideal ammeter it has zero resistance and is connected in series.
Ideal voltmeter
  • Ideal voltmeter it has infinite resistance and is connected in parallel.

Part 6 of 9Potential divider and sensors

Potential divider
  • Potential divider two resistors in series share the supply p.d. in the ratio of their resistances, V1 = V R1/(R1 + R2).
LDR
  • LDR its resistance falls as light intensity rises.
NTC thermistor
  • NTC thermistor its resistance falls as temperature rises.
Potentiometer
  • Potentiometer a variable resistor that gives a p.d. adjustable from zero.

Part 7 of 9Internal resistance

Equation
  • ε = I(R + r), where ε is the e.m.f., R the load and r the internal resistance.
Terminal p.d.
  • V = ε − Ir, which falls as the current rises.
    What the symbols mean: E or ε = e.m.f. (V) · I = current (A) · R = external resistance (Ω) · r = internal resistance (Ω) · V = terminal p.d. (V)
Graph
  • V against I has intercept ε and gradient −r.

Part 8 of 9Method for circuit problems

1
Redraw the circuit and mark current directions.
2
Combine series and parallel groups to find the total resistance.
3
Find the total current from ε = I(R + r).
4
Work back through the circuit for the p.d. across and current in each component.
5
Check that currents add at junctions and p.d.s add round each loop.

Part 9 of 9More from the specification

More from the specification
Circuit Diagrams and Component Symbols
  • Standard circuit diagrams use recognized graphic symbols to represent electrical components, showing how power sources, meters,
  • and resistors are interconnected in series and parallel.
Potential Dividers and Sensing Components
  • In a potential divider combining a fixed resistor and a thermistor, warming the sensor causes its resistance to decrease,
  • which decreases the voltage dropped across it and increases the voltage across the fixed resistor.
Internal Resistance and Lost Volts
  • When current is flowing from a battery, charge passes through the internal resistance of the cells,
  • causing lost volts (I * r) so that the terminal voltage decreases below the open-circuit electromotive force.
Current and Potential Difference in Series Loops
  • In a simple series circuit, adding a component that increases total resistance causes the overall current flowing through the circuit to decrease according to Ohm's law.
Circuit Diagrams and Component Symbols
  • Standard circuit diagrams use recognized graphic symbols to represent electrical components, showing how power sources, meters,
  • and resistors are interconnected in series and parallel.
Potential Dividers and Sensing Components
  • In a potential divider combining a fixed resistor and a thermistor, warming the sensor causes its resistance to decrease,
  • which decreases the voltage dropped across it and increases the voltage across the fixed resistor.
Internal Resistance and Lost Volts
  • When current is flowing from a battery, charge passes through the internal resistance of the cells,
  • causing lost volts (I * r) so that the terminal voltage decreases below the open-circuit electromotive force.
Current and Potential Difference in Series Loops
  • In a simple series circuit, adding a component that increases total resistance causes the overall current flowing through the circuit to decrease according to Ohm's law.
Battery
  • Two or more cells joined together.
Circuit diagram
  • Drawing of a circuit using standard symbols.
Fixed resistor
  • Resistor with a resistance that cannot change.

Key terms

TermMeaning
Electric currentThe rate of flow of charge
Potential differenceThe work done per unit charge in moving charge between two points
Electromotive forceThe energy per unit charge transferred to electrical energy by a source
ResistanceThe ratio of the potential difference across a component to the current in it
ResistivityA material property equal to RA/L
Internal resistanceThe resistance of the materials inside a source of e.m.f.
Ohmic conductorA conductor for which current is proportional to p.d. at constant temperature
Potential dividerA series arrangement of resistors that provides a fraction of the supply p.d.

Avoid these mistakes

1
Connecting an ammeter in parallel — it goes in series.
2
Assuming the current is used up in components — it is the same in a series loop.
3
Adding resistances in parallel — add the reciprocals.
4
Forgetting the internal resistance of the source — terminal p.d. is ε − Ir.
5
Assuming every component is ohmic — a filament lamp is not.

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