4.2Electricity and Magnetism

Electrical Quantities — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

30 min read 44 parts Lesson 9 of 65 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

0%read

Lesson summary

4.2 Electrical Quantities (Part 1) — IGCSE PHYSICS notes: Explain how insulators become charged by friction through electron transfer and predict attraction or repulsion between charges.Positive and negative — there are two types of electric charge.Inside an atom — electrons carr…

Unit (Extended)
Not magnetism
What happens
Only electrons move
Polythene rod and cloth
Acetate rod and cloth

Section 1 of 44Two kinds of charge

Two kinds of charge

Positive and negative

Positive and negative there are two types of electric charge.

Inside an atom

Inside an atom electrons carry negative charge, protons carry positive charge and neutrons are neutral.
An atom has equal numbers of protons and electrons, so the charges cancel and the atom is neutral overall.

Unit (Extended)

Unit (Extended) electric charge is measured in coulombs (C).

Section 2 of 44Forces between charges

Effect

Effect two charged objects close together exert a force on each other; it is either attractive or repulsive.

Rule

Rule opposite charges attract and like charges repel.
The saying "opposites attract" is a quick way to remember this.

Not magnetism

Not magnetism electrostatic forces are similar to magnetic forces but they are a different phenomenon and must not be confused.

Section 3 of 44Charging insulators by friction

1What happens

What happens when two different insulating solids are rubbed together they can become charged.
The charges stay on the insulators because they cannot flow away.
One insulator ends with a net positive charge and the other with a net negative charge.

2Only electrons move

Only electrons move an object that gains electrons becomes negatively charged;
- one that loses electrons becomes positively charged.
Positive charge never moves in a solid, so always explain charging in terms of electrons.

3Polythene rod and cloth

Polythene rod and cloth electrons transfer from the cloth to the rod.
The rod becomes negative and the cloth becomes positive, so they attract each other.

4Acetate rod and cloth

Acetate rod and cloth electrons transfer from the rod to the cloth.
The rod becomes positive and the cloth becomes negative.

Section 4 of 44Experiment: charge by friction with suspended rods

Rub both ends of a polythene rod with a cloth while holding it at its centre.

Hang the rod from a nylon string and cradle on a wooden stand, without touching its ends.

Rub one end of a second polythene rod with a cloth and bring it close to the hanging rod.

Record whether the hanging rod is attracted or repelled.

Repeat using acetate and rods of other materials (acrylic, glass, wood).

Variables

Variables the independent variable is the rod material and the dependent variable is the charge on the rod.
Controlsrubbing time, type of cloth and length of rod.

Conclusions

Conclusions polythene repels polythene because they have the same charge;
- polythene attracts acetate because the charges are opposite.

Improving the experiment

Improving the experiment close windows to remove draughts, do not touch charged ends because that discharges them,
- and rub for the same longer time to transfer more charge.

Section 5 of 44More from the specification

Section 6 of 44The gold-leaf electroscope

The gold-leaf electroscope

Parts

Parts a metal plate, a metal stem and a thin gold leaf, enclosed in an airtight case to stop draughts.

Purpose

Purpose it detects electric charge: the leaf rises when the stem and leaf carry the same charge, because like charges repel.

Setting up

Setting up touch the plate with a finger so the leaf hangs straight down next to the stem.

Section 7 of 44Experiment: detecting charge

Make sure the electroscope is uncharged by touching the plate with a finger.

Charge a polythene rod by rubbing it with a cloth.

Bring the rod towards the plate without touching it and note the leaf.

Take the rod away and note the leaf.

Touch the rod to the plate and note the leaf.

Repeat with an acetate rod.

Section 8 of 44Explaining the observations

Action	Leaf	Reason
Charged rod brought near	Rises	A negative rod pushes electrons down the stem and leaf, making them negative; a positive rod pulls electrons up to the plate, leaving stem and leaf positive. The leaf and stem repel.
Rod moved away	Falls	Electrons redistribute and stem and leaf become neutral again.
Rod touches plate	Stays up	Charge transfers to the plate, stem and leaf, which now have the same charge and repel; the electroscope is charged.
Finger touches plate	Falls	Charge flows through the person to earth, so the electroscope is discharged.

Section 9 of 44Evaluating the electroscope experiment

1Contact charging

Contact charging if the leaf does not stay up, repeat and draw the rod along the edge of the plate pressing harder.

2Do not touch charged rods

Do not touch charged rods touching their ends discharges them.

Section 10 of 44Electric fields (Extended)

Definition

Definition an electric field is a region of space in which an electric charge experiences a force.

Direction

Direction the direction of the field at a point is the direction of the force on a positive charge there.

Vector

Vector an electric field has both size and direction, and every charged object creates one, as magnets do.

Section 11 of 44Electric field patterns (Extended)

1Field lines

Field lines show direction and size of the field and always run from positive charge to negative charge.

2Point charge

Point charge the lines are radial: outwards for a positive charge and inwards for a negative charge.

3Charged conducting sphere

Charged conducting sphere the pattern is like that of a point charge.
The like charges repel and spread evenly over the conducting surface, and the lines meet the surface at right angles.
A Van de Graaff generator can demonstrate this pattern.

4Two parallel plates

Two parallel plates between oppositely charged plates the field is uniform.
The lines are straight, parallel and equally spaced, running from the positive to the negative plate.

Section 12 of 44Conductors and insulators (introduction)

Conductor

Conductor a material that lets charge, usually electrons, flow easily;
- metals such as silver, copper, aluminium and steel are the best.

Why metals conduct

Why metals conduct they contain a lattice of positive metal ions with delocalised outer electrons that are free to move.

Insulator

Insulator a material with no free charges, such as rubber, plastic, glass and wood, that does not let charge flow easily.

Section 13 of 44More from the specification

Section 14 of 44Testing conductors and insulators with an electroscope

Touch the plate of the electroscope to make sure it is uncharged.

Charge a polythene rod by rubbing it with a cloth.

Touch the rod to the plate; the leaf should stay up.

Touch the plate with the material under test and watch how fast the leaf falls.

Repeat for metal, plastic, glass, graphite, wood, fabric and rubber.

Interpreting

Interpreting a good conductor lets charge flow away, so the leaf falls quickly.
A poor conductor lets the leaf fall slowly; if it does not move, the material is a good insulator.

Expected results

Expected results metals and graphite are good conductors;
- plastic, glass, wood, fabric and rubber are poor conductors (insulators).

Improvement

Improvement an electrometer gives numerical readings to compare conductors.

Section 15 of 44What current is

Definition

Definition electric current is the rate of flow of electric charge.

Complete circuit

Complete circuit current flows only when a circuit is formed, for example when a wire joins the two terminals of a cell.

Metal wires

Metal wires current is a flow of negatively charged electrons through the lattice of metal ions when a potential difference is applied.

Section 16 of 44Measuring current with an ammeter

Connection

Connection an ammeter is always connected in series with the component being measured.

Analogue ammeter

Analogue ammeter has a needle and scale, with typical ranges of 0.1–1.0 A and 1.0–5.0 A.
Check for a zero error and subtract it from every reading if the needle is not on zero.
Read from directly in front of the scale to avoid parallax error.

Digital ammeter

Digital ammeter shows digits and can read very small currents in mA or µA, so it is easier to read and more precise.
The display may flicker between values, so a judgement is needed, and the zero error should still be checked.

Section 17 of 44Calculating current (Extended)

Definition

Definition current is the charge passing a point in a circuit per unit time.

Equation

I = Q / t, or Q = I × t.
What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
I is current in amperes (A), Q is charge in coulombs (C) and t is time in seconds (s).

Unit

1 A = 1 C/s, so one ampere is one coulomb passing each second.

Prefixes

Prefixes convert milliamps before calculating, for example 10 mA = 10 × 10⁻³ A.

Section 18 of 44Conventional current (Extended)

1Direction

Direction conventional current is defined as the flow of positive charge, from the positive terminal to the negative terminal of a cell.

2Electron flow

Electron flow electrons are negative, so they flow the opposite way, from the negative terminal to the positive terminal.

3In diagrams

In diagrams arrows for current always point from positive to negative.

Section 19 of 44More from the specification

1Mutual electrostatic repulsion of charges

Mutual electrostatic repulsion of charges charge carriers with identical sign repel each other,
- producing uniform surface charge distributions on metallic conductors.

2Electrical Potential Difference (Voltage)

Defined as the electrical energy transferred per unit charge passing between two points in a circuit (V = W/Q), measured in volts.

3Electric Field Lines

Electric fields are regions where an electric charge experiences a force;
- field lines indicate the direction of force acting on a positive test charge.

4Electrostatic Charging by Friction

Rubbing an insulating acetate rod with a cloth transfers electrons, leaving the rod with a net electrostatic charge and strong surrounding electric field.

5Electrical Power and Resistance

The power dissipated in a circuit component is related to current and resistance by P = IV = I²R, applying to both direct and alternating current circuits.

6Electromotive force

Energy transferred from other forms to electrical energy per unit charge.

7Gold-leaf electroscope

A device in which a charged gold leaf falls when photoelectrons discharge the plate.

8Mains supply

Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.

9Electrical power

Rate of energy transfer, P = I V.
What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

10Resistance

Opposition to current, R = V / I, in ohms (Ω).

11Energy 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)

12Electric field

Region of space in which a charge experiences a force.

13Alternating current

Current that continually reverses direction.

14Field lines

Lines showing the direction and strength of a magnetic field.

Section 20 of 44Direct current

Definition

Definition a direct current (d.c.) is a steady current that always flows in the same direction round a circuit, from positive to negative.

Source

Source cells and batteries produce d.c.; their terminals are fixed as positive and negative.

Graph

Graph on a current-time graph d.c. is a horizontal line.

Section 21 of 44Alternating current

Definition

Definition an alternating current (a.c.) continually changes direction, going back and forth round a circuit.

Supply

Supply the two terminals are identical and keep swapping between positive and negative, so the current reverses.

Frequency

Frequency the number of complete back-and-forth changes each second.
UK mains is a.c. with a frequency of 50 Hz and a potential difference of about 230 V.

Graph

Graph on a current-time graph a.c. is a repeating wave that goes above and below zero.

Section 22 of 44Comparing d.c. and a.c.

Feature	Direct current (d.c.)	Alternating current (a.c.)
Direction	Continuous, in one direction	Constantly changing direction
Source	Cells and batteries	Electrical generators (mains)
Terminals	One positive and one negative	Two identical terminals that swap polarity

Section 23 of 44Electromotive force

Definition

Definition e.m.f. is the electrical work done by a source in moving a unit charge around a complete circuit.

Meaning

Meaning it is the potential difference of the power source and is measured in volts (V), for example 12 V.

Equation (Extended)

E = W / Q.
E is e.m.f. in volts, W is energy transferred to the charges by the source in joules and Q is charge in coulombs.

Section 24 of 44Potential difference

Definition

Definition potential difference (p.d.) is the work done by a unit charge passing through a component, measured in volts.

Energy idea

Energy idea charges gain energy passing through a cell and lose it in the components of the circuit.
- (in a 12 V cell each coulomb gains 12 J)

Equation (Extended)

V = W / Q, so 1 V = 1 J/C.
W = V × Q is the rearranged form for energy transferred.

Section 25 of 44Measuring potential difference

Voltmeter in parallel

Voltmeter in parallel p.d. is the difference between two points, so the voltmeter joins to both sides of the component.

Building a circuit

Building a circuit complete the circuit first and connect the voltmeter last.

Analogue voltmeters

Analogue voltmeters typical ranges 0.1–1.0 V and 0–5.0 V;
- check for a zero error and read directly in front of the scale to avoid parallax error.

Digital voltmeters

Digital voltmeters read mV or µV precisely, but the display can flicker between values and the zero error must still be checked.

Terminology

Terminology potential difference and voltage mean the same, but p.d. is the preferred term since it describes the difference between two points.

Section 26 of 44Comparing e.m.f. and potential difference

Quantity	Where it applies	Meaning
E.m.f.	Across the source	Energy given to each coulomb of charge by the source
Potential difference	Across a component	Energy transferred by each coulomb as it passes through the component

Both are measured in volts and both use the relationship energy = voltage × charge.

Section 27 of 44More from the specification

1Electrostatic Charging of Insulators

Rubbing a polythene rod with a woollen cloth transfers negative electrons from cloth to rod,
whereas rubbing an acetate rod leaves it with a positive charge.

2Gold-Leaf Electroscope Operation

When a charged rod approaches the metal plate of an uncharged gold-leaf electroscope, electrostatic induction repels like charges into the gold leaf and stem,
causing the leaf to diverge.

3Electric Fields and Field Lines

An electric field is a region where an electric charge experiences a force;
- field lines emanate radially from positive charges and terminate on negative charges.

4Conductors vs Insulators

Electrical conductors contain free delocalized conduction electrons that move freely,
- (like copper and aluminium)
whereas insulators hold valence electrons tightly, offering immense electrical resistance.

5Electric field

Region of space in which a charge experiences a force.

6Field lines

Lines showing the direction and strength of a magnetic field.

7Electromotive force

Energy transferred from other forms to electrical energy per unit charge.

8Gold-leaf electroscope

A device in which a charged gold leaf falls when photoelectrons discharge the plate.

9Resistance

Opposition to current, R = V / I, in ohms (Ω).

10Electroscope

Instrument with a metal cap, stem and gold leaf used to detect charge.

11Conductor

Material that allows current to flow.

12Insulator

Material that does not allow current to flow.

Section 28 of 44What resistance is

Definition

Definition resistance is the opposition to current.

Cause

Cause free electrons collide with the metal ions in the wire, which slows them down.

Effect

Effect the higher the resistance of a circuit, the lower the current.
Good conductors have low resistance and insulators have very high resistance.

Resistors

Resistors adding a resistor or variable resistor raises the circuit resistance;
- wires and cells are assumed to have negligible resistance in exam questions.

Section 29 of 44Ohm’s law

Equation

R = V / I, where R is resistance in ohms (Ω), V is p.d. in volts and I is current in amps.

Rearranged

V = I × R and I = V / R.

Inverse proportion

Inverse proportion at constant p.d., doubling the resistance halves the current.

Uses of resistors

Uses of resistors to control the current in a branch or the p.d. across a component.
For a constant p.d., current falls as resistance rises;
- for a constant current, p.d. across a conductor rises as resistance rises.

Section 30 of 44Ohmic and non-ohmic components (Extended)

Type	I-V graph	Resistance	Examples
Ohmic	Straight line through the origin	Constant	Fixed resistor and wire at constant temperature
Non-ohmic	Curve	Changes	Filament lamp, diode, LDR, thermistor

Section 31 of 44I-V graph for a filament lamp

Shape

Shape an S-shaped curve; the current rises more slowly as the voltage rises, so the slope flattens.

Reason

Reason the larger current heats the filament, so the metal ions vibrate more.
Free electrons then collide with the ions more often, so the resistance increases.

Section 32 of 44I-V graph for a diode

Forward bias

Forward bias in the direction of the arrowhead the current rises sharply once the voltage is large enough, showing very low resistance.

Reverse bias

Reverse bias in the opposite direction the resistance is very high and the current is zero for all negative voltages.

Use

Use a diode lets current flow in one direction only.

Section 33 of 44Resistance of a wire (introduction)

1Electron collisions

Electron collisions electrons collide with metal ions, losing kinetic energy which makes the ions vibrate more,
- so the wire warms up and resistance rises.

2Length

Length a longer wire means more collisions per electron, so resistance is greater.

3Thickness

Thickness a thicker wire gives electrons more space, so resistance is smaller.

4Proportionality (Extended)

Proportionality (Extended) resistance is directly proportional to length (R ∝ L) and inversely proportional to cross-sectional area (R ∝ 1/A).

Section 34 of 44Investigating resistance in the laboratory

Circuit

Circuit a cell, variable resistor and component in series with an ammeter, and a voltmeter in parallel with the component.

Method

Method change the variable resistor to alter the p.d., record V and I, and plot an I-V graph.

Reading the graph

Reading the graph a straight line through the origin means constant resistance, and the resistance at any point is V divided by I at that point.

Section 35 of 44Resistance in everyday terms

Heating

Heating resistance makes components warm because electrons transfer energy to the metal ions,
- which is why filament lamps glow and toaster elements heat up.

Choosing wires

Choosing wires connecting wires are thick and short so their resistance is negligible.

Section 36 of 44More from the specification

Section 37 of 44Energy transfer in a circuit

Energy transfer in a circuit

Source to component

Source to component the power supply transfers energy to the charges, and the charges transfer it to each component as they pass through.
Components often dissipate some of that energy to the surroundings.

Chemical stores

Chemical stores mobile phones, laptops and remote controls transfer energy from the chemical store of cells and batteries.

Mains-powered appliances

Mains-powered appliances larger appliances take energy electrically from the mains supply.

Section 38 of 44Motors and heaters in household appliances

Type	Examples	Energy transfer
Motor	Vacuum cleaner (suction), washing machine (rotating drum), refrigerator (compressing the refrigerant)	Electrically from the mains to the kinetic store of the motor
Heater	Toaster, kettle, central-heating boiler	Electrically from the mains to the thermal store of the heating element

Section 39 of 44The electrical energy equation

Dependence

Dependence the energy transferred depends on how long the appliance runs and on its power rating.

Equation

E = V × I × t.
What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
E is energy in joules, V is p.d. in volts, I is current in amps and t is time in seconds.

Time units

Time units always convert hours and minutes to seconds first.

Section 40 of 44Electrical power

Definition

Definition power is the rate at which an appliance transfers energy.

In terms of energy

P = E / t (or P = W / t), in watts; one watt is one joule per second.
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)
In terms of current and p.d. — P = I × V, so I = P / V.
What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Section 41 of 44Energy in kilowatt-hours

Why use it

Why use it joules are too small for household use; a 60 W lamp on for 6 h uses 1.296 × 10⁶ J.

The kW h

The kW h use power in kilowatts and time in hours: E = P × t gives energy in kilowatt-hours;
- the same lamp uses 0.36 kW h.

Conversion

1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J.
Multiply kW h by 3.6 × 10⁶ to get joules, and divide joules by 3.6 × 10⁶ to get kW h.

Cost

Cost cost = energy in kW h × price per kW h.

Section 42 of 44More from the specification

Electrostatic repulsion between identical charges

Electrostatic repulsion between identical charges like electric charges repel each other, requiring potential difference to drive electrical currents against Coulombic forces.

Electric current as rate of charge flow

Current is defined as I = Q / t, measured in amperes (A), representing the flow of free conduction electrons in metals.

Ohm's law and electrical resistance

Ohm's law states that current through an ohmic conductor is directly proportional to potential difference across it at constant temperature (V = IR).

Electrical power and Joule heating

Electrical power dissipated in circuit components is given by P = VI = I² * R = V² / R, converting electrical energy into thermal energy.

Potential difference across circuit resistance

Potential difference across circuit resistance measured potential difference between positive and negative terminals drives charge carriers through resistive circuit components.

Gold-leaf electroscope charge detection

Gold-leaf electroscope charge detection touching a rubbed polythene rod against the metal plate transfers static charge,
- causing electroscope leaf deflection.

Electrical energy transfer rate

Electrical energy transfer rate rate of electrical work done in circuits equals current multiplied by potential difference across components.

Terminal

Connection point of a cell, labelled + or −.

Insulator

Material that does not allow current to flow.

Electric field

Region of space in which a charge experiences a force.

Alternating current

Current that continually reverses direction.

Field lines

Lines showing the direction and strength of a magnetic field.

Direct current

Current that flows in one direction only.

Electromotive force

Energy transferred from other forms to electrical energy per unit charge.

Gold-leaf electroscope

A device in which a charged gold leaf falls when photoelectrons discharge the plate.

Resistance

Opposition to current, R = V / I, in ohms (Ω).

Electroscope

Instrument with a metal cap, stem and gold leaf used to detect charge.

Conductor

Material that allows current to flow.

Section 43 of 44Key terms

Term	Meaning
Electric charge	A property of matter that causes electric forces; positive or negative, measured in coulombs.
Electron	A negatively charged particle in atoms that can be transferred between objects.
Charging by friction	Giving insulators opposite charges by rubbing them together so electrons transfer.
Insulator	A material that does not let charge flow easily.
Coulomb (C)	The unit of electric charge.
Net charge	The overall charge left on an object after electrons are gained or lost.

Term	Meaning
Electroscope	A device with a metal plate, stem and leaf that detects electric charge.
Electric field	A region of space in which an electric charge experiences a force.
Uniform field	A field with parallel, equally spaced, straight lines, such as between charged plates.
Discharge	To remove charge from an object, for example by earthing it.
Conductor	A material that lets charge flow easily.
Delocalised electrons	Outer electrons in a metal that are free to move through the lattice.

Term	Meaning
Current	The rate of flow of electric charge; unit ampere (A).
Coulomb (C)	The unit of charge; one coulomb flows each second in a current of 1 A.
Ammeter	A meter connected in series to measure current.
Conventional current	Current taken as flowing from the positive terminal to the negative terminal.
Zero error	A reading shown by a meter when it should read zero.
Parallax error	An error caused by reading a scale from the wrong angle.

Term	Meaning
Direct current	Current that flows in one direction only.
Alternating current	Current that continually reverses direction.
Frequency	The number of complete a.c. cycles each second, in hertz (Hz).
E.m.f.	The work done by a source in moving unit charge round a complete circuit, in volts.
Potential difference	The work done by unit charge passing through a component, in volts.
Volt	The unit of p.d. and e.m.f.; one joule per coulomb.
Voltmeter	A meter connected in parallel to measure potential difference.

Term	Meaning
Resistance	Opposition to current, in ohms; R = V / I.
Ohm	The unit of resistance (Ω), one volt per amp.
Ohmic conductor	A component with constant resistance, giving a straight-line I-V graph through the origin.
Non-ohmic conductor	A component whose resistance changes, giving a curved I-V graph.
Forward bias	Connecting a diode so current flows through it.
Reverse bias	Connecting a diode so almost no current flows.
Variable resistor	A resistor whose resistance can be changed to control current.

Term	Meaning
Power	The rate at which energy is transferred; unit watt (W).
Watt	One joule of energy transferred per second.
Kilowatt-hour	The energy transferred by a 1 kW device in 1 hour: 3.6 × 10⁶ J.
Electrical energy	Energy transferred = V × I × t.
Motor	A device that transfers electrical energy to the kinetic store.
Heater	A device that transfers electrical energy to the thermal store.

Section 44 of 44Exam technique

Saying positive charge moves in a solid — only electrons move, so the positive object has lost electrons.

Thinking a neutral atom has no charges — it has equal numbers of positive protons and negative electrons.

Mixing up attract and repel — unlike charges attract, like charges repel.

Confusing electrostatic and magnetic forces — they are different phenomena.

Saying positive charge moves in the electroscope — the free electrons move.

Thinking the leaf rises because of attraction — it rises because the leaf and stem have the same charge and repel.

Drawing field lines from negative to positive — they run from positive to negative.

Drawing field lines meeting a conducting sphere at any angle — they meet at right angles.

Connecting an ammeter in parallel — it must be in series.

Forgetting to convert mA to A or minutes to seconds before using I = Q/t.

Saying conventional current is the direction of electron flow — it is the opposite way.

Ignoring a zero error on a meter — subtract it from every reading.

Connecting a voltmeter in series — it must be in parallel across the component.

Saying a.c. flows from positive to negative in one fixed direction — the terminals swap so the direction reverses.

Confusing the symbol V (quantity) with the unit V (volt) — 1 volt equals 1 joule per coulomb.

Treating e.m.f. and p.d. as identical — e.m.f. is the source and p.d. is across components.

Saying resistance is the same as p.d. — resistance is V divided by I.

Calling a filament lamp ohmic — its resistance changes with temperature.

Thinking a thicker wire has more resistance — a thicker wire has less resistance.

Believing a diode conducts in both directions — it conducts only in forward bias.

Using minutes or hours in E = VIt — the time must be in seconds.

Mixing kW with seconds or W with hours when using kW h — use kW with hours only.

Dividing instead of multiplying when converting kW h to J — multiply by 3.6 × 10⁶.

Forgetting that power is a rate — watts mean joules per second.

Last updated: 15 October 2026