Closed-circuit Ohm's law
A power source has resistance too, so the terminal voltage drops as the current rises.
A power source has an internal resistance r. Current through it produces an internal voltage drop Ir, so the terminal voltage U = E − Ir is always less than the emf. The smaller the external resistance, the larger the current and the lower the terminal voltage; when the external circuit is shorted, the current reaches E/r. The U–I graph is a straight line whose vertical intercept is the emf and whose slope has an absolute value equal to the internal resistance.
Terminal voltage vs. current (U–I graph)
The vertical intercept is the emf E, the horizontal intercept is the short-circuit current E/r, and the absolute value of the slope is the internal resistance r.
Think about it
What is special about the output power when the external resistance equals the internal resistance?
The output power is then at its maximum, E²/(4r), but the efficiency is only 50%. Set the external resistance equal to the internal resistance and watch the output power reading.
Model assumptionsConstant emf and internal resistance · Ideal ammeter and voltmeter
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.