Question

Two particles with charges +7e and -6e are initially very far apart (effectively an infinite distance apart). They are then fixed at positions that are 7.50 x 10^-12 m apart. What is EPEfinal - EPEinitial, which is the change in the electric potential energy?

Answer 1

`\Delta U=-1.2*^(-15)J`

Step-by-step explanation:

The formula for electric potential energy between 2 charges
`q_1` and
`q_2` separated a distance r is
`U=(kq_1q_2)/(r)`, where
`k=8.99*10^9Nm^2C^(-2)` is the Coulomb constant.

The initial electric potential energy
`U_i` is then 0J, since r is infinite.

The final electric potential energy is

`U_f=(k(7e)(-6e))/(r)=(-42ke^2)/(r)`

Where
`e=1.6*10^(-19)C` is the elementary charge.

Substituting values we have:

`U_f=-(42(8.99*10^9Nm^2C^(-2))(1.6*10^(-19)C)^2)/((7.5*10^(-12)m))=-1.2*^(-15)J`

So we finally have:

`\Delta U=U_f-U_i=-1.2*^(-15)J-0J=-1.2*^(-15)J`