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Theme B — The Particulate Nature of Matter

B.5.9 Electrical Power — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 50 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.5.9 Electrical Power — IB PHYSICS SL notes: Use P = VI, P = I²R and P = V²/R and calculate energy transferred and its cost.Power — the rate of energy transfer, P = E/t = W/tWhat the symbols mean: P = power (W = J s−1) · W or E = work done / energy transferred (J) · t = time (s)…

  • Substitution
  • Series circuit
  • Parallel circuit
  • Useful heating
  • Wasteful heating
  • High voltage

Part 1 of 8Power in a circuit

Power
  • Power the rate of energy transfer, P = E/t = W/t
    What the symbols mean: P = power (W = J s−1) · W or E = work done / energy transferred (J) · t = time (s) · F = force (N) · v = velocity (m s−1)
  • The p.d. is the work done per unit charge and the current is the rate of flow of charge.
  • Power is measured in watts, 1 W = 1 J s−1.
Electrical power
  • P = VI
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
  • 1 W = 1 V × 1 A.
  • It follows from W = VIt divided by t.
  • It holds for any component, whether or not it is a resistor.
Energy transferred
  • E = VIt = Pt
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)

Part 2 of 8Resistive heating equations

Substitution
  • Substitution using V = IR gives P = I2R and P = V2/R
  • Choose the form for which the quantities are known and constant.
Series circuit
  • Series circuit the current is common, so P = I2R shows the largest resistance dissipates the most power
Parallel circuit
  • Parallel circuit the p.d. is common, so P = V2/R shows the smallest resistance dissipates the most power
Doubling
  • Doubling doubling the current or the p.d. in a fixed resistor makes the power four times greater

Part 3 of 8Why conductors heat up

Mechanism
  • Mechanism electrons gain kinetic energy from the field and transfer it to the lattice in collisions
  • Electrical energy is dissipated as thermal energy in the surroundings.
  • The heat spreads by conduction, convection and radiation.
Factors
  • Factors the heat produced grows with the current and with the resistance for a given current
  • Lowering the resistance can raise the current, which may increase the heat produced.
Useful heating
  • Useful heating kettle and toaster elements have high resistivity
Wasteful heating
  • Wasteful heating power is lost as heat in transmission cables

Part 4 of 8Power transmission

1High voltage
  • High voltage for a fixed power P = VI, a high V means a low I
2Cable loss
  • Cable loss the power wasted in the cable is I2R, so a low current makes the loss small
3Step-down
  • Step-down transformers reduce the voltage again before use

Part 5 of 8Energy and the kilowatt-hour

Kilowatt-hour
  • Kilowatt-hour the energy used by a 1 kW device in one hour
  • 1 kWh = 1000 W × 3600 s = 3.6 × 106 J = 3.6 MJ.
  • It is a unit of energy, not of power.
Units used
  • Units used number of kWh = power in kW × time in hours
Cost
  • Cost cost = units used × price per unit
Efficiency
  • Efficiency useful output power divided by total input power

Part 6 of 8Appliance ratings

Rating
  • Rating a label such as 230 V, 2.0 kW gives the power transferred at the rated p.d.
Current
  • I = P/V
Resistance
  • R = V2/P
Lower p.d.
  • Lower p.d. the resistance is almost unchanged, so the power falls with the square of the ratio of the p.d.s
Fuses
  • Fuses a fuse is chosen with a rating just above the normal working current

Part 7 of 8Measuring power and energy

Direct method
  • Direct method measure the p.d. across the component with a voltmeter and the current with an ammeter, then multiply
Heating experiment
  • Heating experiment heat water with a known V and I for a time t, or use a joulemeter
  • Check that the thermal energy gained by the water matches VIt.
  • Allow for energy lost to the surroundings.
Power–time graph
  • Power–time graph the area under the graph is the energy transferred
Heating calculations
  • Heating calculations combine VIt with mc∆T for the energy needed to warm a substance
  • A real kettle is typically 85–90% efficient because the casing and air also gain thermal energy.

Part 8 of 8More from the specification

1Electrical power in resistive circuits
  • Electrical power dissipated in a circuit component is calculated from potential difference and current: P = VI = I2 R = V2 / R.
2Internal resistance and terminal potential difference
  • Power sources possess internal resistance, causing terminal potential difference to fall below electromotive force under load.
3Combinations of electrical cells
  • Connecting cells in series adds their emfs,
  • whereas connecting identical cells in parallel reduces total internal resistance and extends supply duration.
4Conventional current
  • Current drawn from the positive to the negative terminal.
5Battery
  • Two or more cells joined together.
6Thermistor
  • Resistor whose resistance falls as temperature rises.
7Temperature
  • Measure of the average kinetic energy of the particles.
8Terminal
  • Connection point of a cell, labelled + or −.
9Source
  • Reservoir at TH providing heat QH
10Internal resistance
  • Resistance inside a cell that causes lost p.d.
11Circuit diagram
  • Drawing of a circuit using standard symbols.
12Light intensity
  • How bright the light falling on a surface is.

Key terms

TermMeaning
PowerRate of energy transfer, P = E/t, in watts.
WattOne joule per second.
Kilowatt-hourEnergy used by a 1 kW device in one hour; 3.6 MJ.
EfficiencyUseful output power divided by input power.
Heating effectEnergy transferred to the lattice by colliding charge carriers.

Avoid these mistakes

1
Using P = I^{2}R to compare parallel resistors with different currents — use P = V^{2}/R when the p.d. is common.
2
Mixing kW with joules — convert kWh to J using 3.6 × 10^{6}.
3
Saying the kilowatt-hour is a unit of power — it is a unit of energy.
4
Forgetting that doubling the p.d. across a fixed resistor quadruples, not doubles, the power.
5
Using V = IR for a diode or other non-ohmic component without checking its operating point.

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