# 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?

Answer (7.5 uJ, -7.5 uJ)

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?

Answer (7.3 x 10^5 m/s)

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

Answer (1.4 x 10^7 m/s)

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?

Answer (1.33 x 10^6 m/s)

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?

Answer (2.4 cm)

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.

Answer (1.6 x 10^-18 C)

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

Answer (1.5 x 10^8 V)

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?

Answer (27,000 J)

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?

Answer (40 cm, -200 cm)

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?

Answer (3.8 x 10^6 m/s)

14) An electron has a speed of 6 x 105 m/s as it passes point A on its way to point B. Its speed at B is 12 x 105m/s. What is the potential difference between A and B and which is at the higher potential?

Answer (3.7 V, B)

(source)