Physics · Electrostatic Potential And Capacitance · NEET
External means the field E (and potential V) is made by other source charges that are NOT the charge q you are studying. You are given V at each point; you do not care about the energy of the sources. You only ask: how much energy does q have sitting in this given V? Answer: U = qV. We assume q is so small (or the sources are held fixed) that q does not disturb the sources.
Both are correct but for different questions. kq1q2/r is the mutual PE of a PAIR of charges you place yourself (system PE, Section 2.7). U = qV is for ONE charge dropped into a field someone else already made — you are handed the potential V, not the source charges. In NEET, if the question gives you 'a field E' or 'a potential V', use U = qV.
Work by the external agent = change in PE = q(V_B - V_A) = qΔV. It depends only on the two endpoint potentials, never on the path taken. If A and B are on the same equipotential, ΔV = 0 and work = 0. This is why the electrostatic force is called conservative.
Put q = e = 1.6×10⁻¹⁹ C through a potential difference of 1 volt. Energy gained = qV = 1.6×10⁻¹⁹ × 1 = 1.6×10⁻¹⁹ J. This amount is defined as 1 electron-volt (1 eV). So 1 eV = 1.6×10⁻¹⁹ J, and 1 MeV = 1.6×10⁻¹³ J. It is just energy = charge × voltage with a small charge.
Yes. U = qV takes the sign of q and V. A positive charge at a positive-potential point has U > 0; a positive charge at a negative-potential point (near a negative source) has U < 0. A negative charge flips all signs. The zero of PE is at infinity where V = 0.
Diagrams (a),(b),(c),(d) show regions of equipotentials (10 V, 20 V, 30 V, 40 V) arranged differently. A positive charge q is moved from A to B in each diagram, where A lies on the 10 V line and B on the 40 V line. Which statement is correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
U = qV, where V is the external potential at the location of q. It equals the work done to bring q slowly from infinity (V = 0) to that point.
No. U = qV is for one charge placed in a field made by other sources. System PE (kq1q2/r summed over pairs) is the energy of the charges you assemble yourself. NEET questions that give you a potential or field want U = qV.
1 eV = 1.6×10⁻¹⁹ J. It is the energy an electron (charge e) gains when accelerated through a potential difference of 1 volt, from U = qV.
No. Work = qΔV = q(V_B − V_A) depends only on the start and end potentials. The electrostatic force is conservative, so the path does not matter.
When the charge moves between two points at the same potential (ΔV = 0), for example along an equipotential surface. Then U does not change and no net work is done by the external agent.
