Question

The positive terminal of $12V$ battery is connected to the ground. Then the negative terminal will be at:
(A) $ - 6V$
(B) $ + 12V$
(C) $zero$
(D) $ - 12V$

Answer 1

Hint: The battery is a voltage source that maintains the voltage difference in it. The voltage rating of a battery is the potential difference between the positive and negative terminal of that battery.

Complete step by step answer:
Grounding or also called earthing is usually the reference point in an electric circuit from which voltages are measured. Most of the time the voltages of terminals are measured with respect to the ground. That is calculating the potential difference between that terminal and the ground.
This is usually done for measurement purposes. The ground serves as a constant potential reference of zero-voltage against which the other potentials are measured and calculated. Let us just say that the ground is a sink of neutralizing charges that could help you neutralize and hence calculate the other potentials using their neutralizing nature.
So, when you ground a battery or connect it to the earth, there will be flow of charges between the connected battery and the ground. Thus, causing the battery terminal to lose all the charges thus making that terminal to be $0V$ which is again said with respect to the earth. However, to maintain the exact potential difference in the battery the other terminal must gain the exact charge lost but with the opposite sign.
Also we know that a $12V$ battery is nothing but a battery with potential difference $12V$ .$\Delta V = {V_2} - {V_1} = 12V$. Now when the positive terminal of the battery is grounded, the flow of charges will the cause the battery to drain out and cause the positive terminal to be almost $0V$, but to maintain the potential difference between the terminals of the battery, the negative terminal gains opposite charges, such that the voltage of the negative terminal is $ - 12V$.
Thus when a battery of $12V$ has its positive terminal grounded the negative terminal becomes $ - 12V$.

Hence the correct answer will be option D.

Note:
Sometimes when the battery of $12V$ is mistaken to be a battery with $6V$ in each terminal which is not the right way because the voltage mentioned is the potential difference of the battery, which causes the flow of charges when the battery is connected to the circuit. Unless the battery is connected to a circuit (or grounded) the charges will not flow.