B.5 · IB Physics
EMF and internal resistance
Change the emf, the internal resistance and the load to see how current, terminal pd, lost volts and load power respond. Drag along the graph to move the operating point.
ε = I(R + r)
V = ε − Ir = IR
P = I²R = ε²R ⁄ (R + r)²
P is maximum when R = r
- current I
- terminal pd V
- lost volts Ir
- load power P
- total power εI
- efficiency R⁄(R+r)
- max load power
Try this
- Press “Set R = r”, note the load power, then move R a little either way: does P go up or down?
- Double r while keeping ε fixed. Which intercept of the V–I line changes, and what happens to the gradient?
- Slide R to 0 Ω. Check that V = 0 and that the short-circuit current equals ε ⁄ r.
Exam tip: when you plot terminal pd against current from experimental data, read ε from the V-intercept and r from the magnitude of the gradient of the best-fit line, not from a single data point.