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Theme B — The Particulate Nature of Matter

B.5.2 Electric Current — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 43 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
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Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.5.2 Electric Current — IB PHYSICS SL notes: Define current, use I = Δq/Δt and the drift-speed equation, and explain how charge carriers move in a conductor.Electric current = the rate of flow of charge carriers, I = Δq/Δt.I is the current in amperes (A).Δq is the charge in coul…

  • Greater flow
  • Conventional current
  • Direct current (d.c.)
  • Electrolytes and ionised gases
  • Semiconductors
  • Quantised charge

Part 1 of 8What electric current is

1Electric current
Electric current the rate of flow of charge carriers, I = Δq/Δt.
  • I is the current in amperes (A).
  • Δq is the charge in coulombs (C).
  • Δt is the time in seconds (s).
2Ampere
  • 1 A = 1 C s−1, and the coulomb is a derived unit, 1 C = 1 A s.
3Greater flow
  • Greater flow the greater the flow of charge, the greater the current.
4Scalar
  • Scalar the sign only indicates the direction of the current.
5Ammeter
  • Ammeter it measures the current, in series.

Part 2 of 8Direction of current

1Conventional current
  • Conventional current the flow of positive charge from the positive terminal of a cell to the negative terminal.
2Electrons
  • Electrons they flow from the negative terminal towards the positive terminal, opposite to conventional current.
3History
  • History conventional current was described before the nature of electric current was understood.
4Direct current (d.c.)
  • Direct current (d.c.) it flows in one direction with a single value and is produced by cells and batteries.
5Cause
  • Cause the potential difference in the circuit makes the current flow.

Part 3 of 8Charge carriers

1Metals
  • Metals the carriers are free (conduction) electrons with charge −e, where e = 1.60 × 10−19 C.
2Electrolytes and ionised gases
  • Electrolytes and ionised gases positive and negative ions carry current in opposite directions.
3Semiconductors
  • Semiconductors the carriers are electrons and holes.
4Quantised charge
  • Quantised charge the charge passing a point is a whole number of elementary charges, Δq = ne.

Part 4 of 8Drift speed

Random motion
  • Random motion free electrons move randomly at high speed (about 106 m s−1), colliding constantly with lattice ions.
Applied p.d.
  • Applied p.d. an electric field is set up along the wire and the electrons acquire a very small net velocity, the drift speed v.
Equation
  • I = nAvq.
  • n is the number density of free charge carriers in m−3.
  • A is the cross-sectional area in m2.
  • v is the drift speed in m s−1.
  • q is the charge per carrier in C.
Typical size
  • Typical size fractions of a millimetre per second in copper, of order 10−4 m s−1.
Lamp lights instantly
  • Lamp lights instantly the electric field travels at close to the speed of light and sets all the carriers moving together.

Part 5 of 8Consequences of I = nAvq

1Thinner wire
  • Thinner wire for the same current, a smaller A needs a larger drift speed.
2Few free carriers
  • Few free carriers a semiconductor, with small n, needs a much larger drift speed than a metal.
3Narrowing wire
  • Narrowing wire the drift speed increases where the wire narrows, because the current is the same throughout a series loop.

Part 6 of 8Current in a circuit

Conservation of charge
  • Conservation of charge at a junction the total current entering equals the total current leaving.
Series loop
  • Series loop the current is the same everywhere; a lamp does not use up current.
Graphs
  • Graphs a plot of charge against time has gradient equal to the current.
Varying current
  • Varying current the charge passed is the area under the current–time graph.
Battery capacity
  • 2000 mA h can supply 2.0 A for one hour, which is 7200 C.

Part 7 of 8Using current in problems

1
Start from the quantity given: a current and time give the charge.
2
From the charge find the number of electrons, or with a p.d. the energy transferred.
3
With a wire's dimensions use I = nAvq for the drift speed.
4
Convert units first: mA to A, minutes to seconds and mm2 to m2.
Sense check
  • Sense check household appliances carry amps and calculators microamps; a drift speed in m s−1 signals a slip.
Definition
  • Definition the ampere is defined through the force between current-carrying conductors, not the charge of electrons.

Part 8 of 8More from the specification

More from the specification
Resistors in Series and Parallel Combinations
  • When resistors are connected in series, their equivalent resistance equals the sum of resistances; in a parallel network, total resistance decreases according to 1 / Rp = 1 / R1 + 1 / R2.
    • (Rs = R1 + R2)
Electrical Resistivity of a Material
  • The resistance R of a uniform conductor is proportional to its length and inversely proportional to its cross-sectional area, governed by the resistivity equation R = rho * L / A.
Temperature-Dependent Thermistor
  • An NTC thermistor is a non-ohmic sensing component whose electrical resistance decreases significantly as temperature increases, altering voltage distribution in divider circuits.
Electrical Power and Internal Resistance
  • The electrical power dissipated in a circuit component is calculated from P = I2 * R = V * I;
    • real voltage sources also possess internal resistance that dissipates power internally as thermal energy.
Source
  • Reservoir at TH providing heat QH
Internal resistance
  • Resistance inside a cell that causes lost p.d.
Combined resistance
  • Single resistance that could replace several resistors.
Circuit diagram
  • Drawing of a circuit using standard symbols.
Light intensity
  • How bright the light falling on a surface is.
Fixed resistor
  • Resistor with a resistance that cannot change.
Resistance
  • Opposition to current, R = V / I, in ohms (Ω).
Parallel
  • Components on separate branches, each with the full e.m.f.
Component
  • Part of a vector along one axis.
Resistivity
  • Property of a material defined by ρ = RA/L.
Thermistor
  • Resistor whose resistance falls as temperature rises.
Components
  • Perpendicular parts of a vector that add to give the vector.
Temperature
  • Measure of the average kinetic energy of the particles.
Voltmeter
  • Instrument that measures p.d., connected in parallel.

Key terms

TermMeaning
Electric currentThe rate of flow of charge
Ampere (A)SI base unit of current, 1 C s−1
Coulomb (C)Unit of charge, 1 A s
Drift speedSmall average velocity of charge carriers along a conductor
Conventional currentFlow of positive charge from positive to negative
Direct currentCurrent that flows in one direction only

Avoid these mistakes

1
Saying electrons move from positive to negative — conventional current is positive to negative, electrons the opposite way.
2
Thinking electrons travel from the switch to the lamp quickly — the field propagates, the drift is slow.
3
Saying current is used up in a component — it is the same in a series loop.
4
Forgetting to convert mA to A — convert before substituting.
5
Using the electron speed instead of the drift speed in I = nAvq — use the drift speed.

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