Potential difference and energy transfer

By the end of this lesson

By the end you can use W = VQ = VIt to calculate energy transferred in electrical circuits, and explain the energy transformations in components and sources.

Energy per coulomb (voltage) across battery and load. W = VQ = VIt; emf = ΣV around loop.

Key to the symbols

The main symbols used in this lesson:

Symbol	Meaning
$W$	energy transferred (work done)
$V$	potential difference
$Q$	charge
$I$	current
$t$	time

From definition to energy calculations

We know V = W/Q. Rearranging:

$W = V Q$

And since current I = Q/t (so Q = I t), we can substitute:

$W = V (I t) = V I t$

Symbol	Meaning	Unit
W	Energy transferred (work done)	joule (J)
V	Potential difference	volt (V)
I	Current	ampere (A)
t	Time	second (s)

Key point: Use W = VQ when you know total charge. Use W = VIt when you know current and time.

Where does the energy go?

In a circuit, energy is conserved. The battery (source) supplies energy; components receive it.

Resistor / bulb: Electrical energy → thermal energy (heating). Charge carriers collide with lattice ions, transferring kinetic energy.
Motor: Electrical energy → mechanical energy (kinetic) + some thermal.
Battery charging: Electrical energy → chemical energy (stored).
LED: Electrical energy → light + some thermal.

The voltage across a component tells you how much energy each coulomb gives up there.

Quick check — energy formulas

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Apply

Multiple choiceQuestion 3 · Apply

Answer the 3 questions above to continue.