Why the terminal voltage is less than the emf
"Why the terminal voltage is less than the emf" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the lesson "Cells, emf and internal resistance" in Current Electricity. The dashed box is the cell: an emf ε with a small resistance r inside it. The voltmeter reads the terminal voltage V. Change the load R and watch V and the current. The emf equals the terminal voltage only when no current flows. As the current rises, Ir is lost inside and V = ε − Ir falls in a straight line. The load gets the most power when R = r, and then only half the emf reaches it. JEE Main asked 65 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Cells, emf and internal resistance in Current Electricity. Change the values and watch what happens.
In JEE Main
42 shifts analysed
65 questions from Current Electricity across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT § "Cells, emf, Internal Resistance"
- The emf ε of a cell is the potential difference between its terminals when no current is drawn; despite the name, it is a potential difference, not a force.
- Charge moving through the electrolyte meets an internal resistance r.
- When the cell drives a current I through an external resistor R, I = ε/(R + r) and the terminal voltage is V = ε − Ir, which is less than ε.
- The terminal voltage equals ε only on open circuit (I = 0), and falls to zero when the terminals are shorted, where the maximum current ε/r flows.
- Two readings with different external resistors give two equations that can be solved for both ε and r.
- The internal resistance of a car battery is very small, so it can supply the large currents a starter motor needs; dry cells have larger internal resistance.
Questions
3 answers
- What does the "Why the terminal voltage is less than the emf" simulation show?
- The dashed box is the cell: an emf ε with a small resistance r inside it. The voltmeter reads the terminal voltage V. Change the load R and watch V and the current.
- Does JEE Main ask questions from Current Electricity?
- Yes. JEE Main asked 65 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Cells, emf and internal resistance" in Current Electricity (Class 12 Physics).
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