"The sum of emf's and potential differences around a closed loop equals zero" is a consequence of
- A.Ohm's law.
- B.Conservation of charge.
- C.Conservation of momentum.
- D.Conservation of energy.
Correct answer
Explanation
The correct answer is D, conservation of energy. The statement is Kirchhoff's voltage (loop) law, and it follows from the fact that energy is neither created nor destroyed.
Potential difference is the energy given to or taken from each unit of charge. A charge that goes once around a closed loop returns to its starting point with the same potential it began with. So the energy it gains from cells (the emfs) must equal the energy it loses across resistors and other elements, and the algebraic sum of all emfs and potential differences around the loop is zero. The electrostatic field is conservative, so the work done around a closed path is zero.
A is wrong because Ohm's law only relates current and voltage across a single conductor.
B is wrong because conservation of charge is the basis of Kirchhoff's current (junction) law.
C is wrong because momentum has no role in circuit laws.
Exam tip: junction law means conservation of charge; loop law means conservation of energy.
Practice Questions
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- A.1 only
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