Why batteries sag under load

Science · Energy · ages 17-18

Name ______________________ Date ____________

Why does terminal voltage fall as the current rises?
- Charges get used up
- Part of the emf drops across the internal resistance
- The wires cool down
What is electromotive force?
- A real force on charges
- Energy per charge from the source
- The current itself
A cell has emf 12 V and internal resistance 1 ohm. It delivers 2 A. What is the terminal voltage in volts?
Answer: ______________
A divider has Vin 12 V, R1 2 ohms, R2 4 ohms, with output across R2. What is Vout in volts?
Answer: ______________
Terminal voltage plotted against current gives a straight line. What is its intercept?
- The emf
- The internal resistance
- The short circuit current
In a divider with output across R2, R2 grows while R1 stays fixed. What happens to Vout?
- It falls
- It stays the same
- It rises
A divider output sits across R1 instead of R2. R1 grows while R2 stays fixed. What happens to the output?
- It shrinks
- It stays the same
- It grows
Two cells share the same emf, but one under heavy load reads lower at its terminals. Why?
- Its emf shrank
- The internal drop grew with current
- The divider changed

Answer key

For grown-ups. Fold this page away before handing over the rest.

Part of the emf drops across the internal resistance · V equals emf minus I r, so more current means a bigger internal drop.
Energy per charge from the source · emf is energy per charge, measured in volts, not a force.
10 · V equals 12 minus 2 times 1, which is 10 V.
8 · Vout equals 12 times 4 over 6, which is 8 V.
The emf · At zero current nothing drops inside, so the intercept shows the full emf.
It rises · R2 claims a bigger share of the total, so the output climbs.
It grows · The output resistor claims a bigger share, so the output grows.
The internal drop grew with current · Same emf with more current means more push spent inside.