AS 91526 · 2024 paper · DC and capacitors

Question 1(a) Achieved

Stefan investigates new and used car batteries. His car has a new battery with an EMF of 12.0 V and an internal resistance of 0.0500 Ω, connected to a 7.00 Ω load.

ε	12.0 V
r	0.0500 Ω
R	7.00 Ω

Explain the meaning of the term EMF.

The 2024 paper, page 2 ↗ Marking schedule, page 1 ↗

Work it out

Try it on paper first. Stuck? Take one hint at a time.

1 Where does the energy that pushes charge round a circuit come from?
From the battery. As charge passes through it, stored chemical energy is turned into electrical energy.
2 How does “energy per charge” become a number of volts?
One volt is one joule per coulomb. The EMF is the energy the battery gives each coulomb of charge that passes through it, so 12.0 V means 12.0 J per coulomb.
3 How could you measure it with a voltmeter?
Draw no current. With nothing flowing, no voltage is lost across the internal resistance, so the voltmeter across the terminals reads the EMF itself.

Answer The EMF is the energy the battery supplies to each coulomb of charge that passes through it (12.0 J per coulomb here). It is what a voltmeter reads across the terminals when no current flows.

What the marker wanted

Achieved A correct meaning: the energy given to each coulomb of charge, or the voltage across the terminals when no current is drawn.

In our words, from NZQA’s 2024 marking schedule.

Common mistake

Answering about the voltage a changing magnetic field sets up in a coil. This EMF belongs to a battery. NZQA’s 2024 report says many candidates could only discuss EMF as part of electromagnetic induction.

How did it go?

Got it moves it further out. Review brings it back tomorrow.