Which one among the following statements related to electric cells is correct?
- A.A primary cell is rechargeable.
- B.A secondary cell can be used only once.
- C.The emf of a battery is the power provided by the battery per unit current.
- D.The unit of emf is Joule.
Correct answer
Explanation
The correct answer is C. The emf of a battery equals the power it supplies divided by the current it drives, because power P = EI.
Electromotive force (emf) is the energy a cell gives to each coulomb of charge that passes through it, so its unit is joule per coulomb, that is the volt. Since power is energy per second and current is charge per second, power divided by current also gives energy per unit charge, which is the emf. Primary cells such as the dry cell are used once and thrown away; secondary cells such as lead-acid and lithium-ion batteries can be recharged many times.
A is wrong: a primary cell cannot be recharged.
B is wrong: a secondary cell is the rechargeable kind, used again and again.
D is wrong: the unit of emf is the volt (joule per coulomb), not the joule.
Exam tip: primary cell one-time (dry cell); secondary cell rechargeable (lead-acid, lithium-ion); emf measured in volts.
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