Define electric potential at a point in words.
Two point charges A and B are $20\,\text{nm}$ apart in a vacuum, as shown in Fig. 3.1. Point P is located a distance $x$ from A along the line AB. Fig. 3.2 shows how the electric potential $V_A$ due to charge A alone changes with distance $x$. Fig. 3.2 also shows how the electric potential $V_B$ due to charge B alone changes with distance $x$.
State and explain whether charges A and B have the same sign or opposite sign.
Using Fig. 3.2, state how the combined electric potential from both charges can be obtained.
Without carrying out any calculations, use Fig. 3.2 to estimate the distance $x$ at which the combined electric potential of the two charges is minimum.
Point P is at a distance $x = 10\,\text{nm}$ from A. An $\alpha$-particle has kinetic energy $E_K$ at infinity. Use Fig. 3.2 to determine the minimum value of $E_K$ needed for the $\alpha$-particle to travel from infinity to point P.