Batteries in Series: EMFs Add
1 · Predict
Two 6 V batteries are connected in series (same orientation) driving a single resistor. Does the circuit behave like one 6 V source or like a bigger combined source?
- The EMFs add — the circuit behaves like a single 12 V source.
- The circuit behaves like the average, 6 V.
- The batteries cancel out, driving no current.
2 · Set Up
- Open the series-batteries preset and press Reset. b1 = 6 V is fixed; R1 = 100 Ω is fixed.
- Enable the current readout on R1.
- Set b2's EMF for each trial and record the circuit current.
3 · Collect Data
| Battery b2 EMF (V) | Total EMF (V) | Current I (A) |
|---|---|---|
| 4 | ||
| 6 | ||
| 9 |
Plot current I (y-axis) against total EMF (x-axis) for your three trials.
4 · Analyze
- For one trial, compute total EMF = b1 + b2 using b1 = 6 V, then I = EMF_total/R1 using R1 = 100 Ω. Compare to the table.
- Explain why two batteries wired in the same orientation (positive terminal to negative terminal) add their EMFs, the same way series resistances add.
5 · Extend
- If b2 were flipped (wired backward), its EMF would subtract instead of add: total EMF = b1 − b2. Explain why reversing one battery in a series pair works against the other.
- A flashlight using 2 AA batteries (1.5 V each) in series runs its bulb at 3 V. Using this experiment's relationship, explain why adding a third AA battery in series would make the bulb burn out faster.
The Physics Behind This Experiment
Series EMF Sources
Batteries wired in series with matching polarity add their EMFs directly: EMF_total = b1 + b2. The single resulting loop current is then I = EMF_total/R, by Ohm's law applied to the combined source.