# Day 109 (problems)

### Try the following Electric Potential Problems:

1) Two metal plates are attached to the two terminals of a 1.50 V battery. How much work is required to carry a +5.0 μC charge

(A) from the negative to the positive plate and

(B) from the positive to the negative plate?

2) The plates described in Question #1 are in a vacuum. An electron is released at the negative plate and falls freely to the positive plate. How fast is it going just before it strikes the plate?

3) A proton is accelerated from rest through a potential difference of 1 MV. What is its final speed?

4) An electron gun shoots electrons at a metal plate that is 4 mm away in vacuum. The plate is 5.0 V lower in potential than the gun. How fast must the electrons be moving as they leave the gun if they are to reach the plate?

5) The potential difference between two large parallel metal plates is 120 V. The plate separation is 3.0 mm. Find the electric field intensity between the plates.

Answer (40 kV/M to the negative plate)

6) An electron is shot with speed 5 x 106 m/s parallel to a uniform electric field of intensity 3 kV/m. How far will the electron go before it stops?

7) A potential difference of 25 kV maintains a downward-directed electric field between two horizontal parallel plates separated by 1.8 cm. Find the charge on an oil droplet of mass 2.2 x 10-13 kg that remains stationary in the field between the plates.

8) Determine the absolute potential in air at a distance of 3.0 cm from a point charge of 500. μC.

9) Compute the electric field intensity and absolute potential at a distance of 1 nm from a helium nucleus of charge +2e. What is the potential energy (relative to infinity) of a proton at this position?

Answer (2.9 x 10^9 N/C, 2.9 V, 4.6 x 10^-19 J)

10) A charge of 0.20 m C is 30. cm from a point charge of 3.0 m C in vacuum. What work is required to bring the 0.20 m C charge 18 cm closer to the 3.0 m C charge?

11) A point charge of +2 μC is placed at the origin of coordinates. A second, of -3 μC, is placed on the x-axis at x = 100. cm. At what point (or points) on the x-axis will the absolute potential be zero?

12) In Question #11, what is the difference in potential between the following two points on the x-axis: point A at x = 10. cm and point B at x =90.cm? Which point is at the higher potential?

Answer (4 x 10^5 V, point A)

13) An electron is moving in the +x direction with a speed of 5 x  m/s. There is an electric field of 3 kV/m in the +x direction. What will be the electron’s speed after it has moved 1.00 cm?