Problems in Electric Potential and Capacitors to solve:
1. If 0.045 J of work is done to move a charge of 775 C from the negative to the
positive pole of a battery, (a) what is the electric potential difference of the battery?
(b) what is the strength of the electric field 32 cm from the positive pole?
(c) at infinity?
2. A camera battery is rated at 3.00 volts. (a) How much charge can two of these
batteries place on a 650.0 F (650 x 10-6 F) capacitor? (b) How much potential energy
is stored in the capacitor?
3. You have made a capacitor that has a surface area of 4 x 10-4 m2 and a distance of
1.5 x 10-4 m separates the plates. (a) what is the capacitance, (b) what charge will be on
the capacitor when it is connected to a 20.0 V power supply? (c) If a dielectric material (
= 7.9) what will be the new capacitance?
4. What is the total electric potential at point P given in this diagram?
+3.6 µC
P
.62 m
.41 m
-3.7 µC
5. A proton is moved from the negative plate to the positive plate of a parallel plate device. The
plates are 1.5 cm apart and the electric field is uniformly 1500 N/C or V/m. (a) What is the work
done in moving the proton? (b) What is the potential difference between the plates? (c) What is
the potential difference midway between the plates? (d) If the proton is released at the positive
terminal it will accelerate towards the negative terminal, what will its velocity be just before it
reaches the negative plate? (hint: the K.E of the proton = the work done to move it). m = 1.67 x
10-27 kg.
6. Two vertical plates are used to accelerate an electron (m = 9.1 x 10-31 kg) from rest to 1 %
the speed of light (c = 3 x 108 m/sec). The plates are 2.5 mm apart. (a) What voltage is required
between the plates? (b) Draw the electric field lines between the plates. (c) What is the electric
field in V/m?
7. A pair of conducting plates, 40 cm X 50 cm each are separated by 5 mm distance and carry
+/- 5 C of charge on each plate. Calculate (a) the electric field, (b) the potential difference.
8. An 8 F capacitor has a 1.1 mm thick dielectric material filling it. The area of the plates is
0.184 m2 each. What is the dielectric constant of this material?
9. A capacitor made of two parallel plates of sides 3 m x 0.04 m are separated by a polysterene
sheet that is 2 x 10-5 m thick. The dielectric constant is 2.56 for polysterene. (a) what is the
capacitance of the capacitor? (b) what is the potential difference between the plates if it stores
0.5 J of energy? (c) what is the charge on the capacitor?
10. A small drop of water is suspended motionless in air by a uniform electric field that is
directed downward and has a magnitude of 9000 N/C. The mass of the water drop is 3 x 10-9
kg. (a) Is the water drop positively or negatively charged. (b) How many excess protons or
electrons does it have? (Hint: The weight of the water drop must be equal to the
electrostatic force)
E = 9000 N/C