Solve the following problems. Answers are given at the end of the problem set.

  1. Electric Potential
  1. How much kinetic energy will an electron gain (in joules) if it falls through a potential difference of 275 V?
  1. An electron in the picture tube of an old television set is accelerated from rest through a potential difference Vba = +5000V.
  1. What is the change in potential energy of the electron?
  2. What is the speed of the electron as a result of this acceleration?
  3. What is the speed of a proton that is accelerated through a potential difference

Vba = -5000 V?

  1. Two parallel plates are charged to a voltage of 50 V. If the separation between plates is 5 cm, what is the electric field between them?
  1. The work done by an external force to move a -8.0 μC charge from point atob is 25.0 x 10-4 J. If the charge started from rest and has 5.2 x 10-4 J of kinetic energy when it reached point b, what is the potential difference between points a and b?
  1. The positive plate (A) and the negative plate (B) of a parallel plate capacitor connected to a 12 V source is separated by a distance d. Suppose a +6.24 µC charge is released from rest at the positive plate and that it reaches the negative plates with a speed of 3.4 m/s.
  1. What is the kinetic energy of the charge as it reaches the negative plate?
  2. What is the mass of the charge?
  1. What is the speed of a proton whose kinetic energy is 36.0 MeV?
  1. What is the electric potential 18.0 cm from a 4.00 μC point charge?
  1. Using the diagram below, calculate the electric potential at points A and B due to the charges shown.
1)-4.4 x 10-17 J / 2)a. -8.0 x 10-16 J
b. 4.2 x 107 m/s
c. 9.8 x 105 m/s / 3) 1000 V/m / 4) 250 V
5) a. 7.5 x 10 -5 J
b. 1.3 x 10-5 kg / 6) 8.31 x 107 m/s / 7) 2.00 x 105 V / 8) VA = 7.5 x 105 V
VB = 0 V
  1. Capacitance
  1. The potential difference between two parallel plates is 120 V. They have equal and opposite charge of magnitude 95 pC. What is the capacitance of the plates?
  1. A 7500 pF capacitor holds 16.6 x 10-8 C of charge. What is the voltage across the capacitor?
  1. How much charge flows from a 12.0 V battery when it is connected to a 9.00 µF capacitor?
  1. A 0.20 F capacitor is desired. What area must plates have if they are to be separated by a 2.2 mm air gap?
  1. A parallel plate capacitor is constructed with plates of area 0.0280 m2 and a separation of 0.550 mm. Find the magnitude of the charge on each plate of this capacitor when the potential difference between plates is 20.1 V.
  1. A parallel plate capacitor has plates area 3.45 x 10-4 m2. What plate separation is required if the capacitance is to be 1330 pF if the space between the plates is filled with (a) air or (b) paper k = 3.7?
  1. When a person’ heart undergoes ventricular fibrillation – a rapid, uncoordinated twitching of the heart muscles – it often takes a strong jolt of electrical energy to restore the heart’s regular beating and save the person’s life. The device that delivers this energy is known as a defibrillator and it uses a capacitor to store the needed energy. In a typical defibrillator, a 175 µF capacitor is charged until the potential difference between plates is 2240 V.
  1. What is the magnitude of charge on each plate of the fully charged capacitor?
  2. What is the energy stored in the “charged up” defibrillator?
  1. An electronic flash for a camera contains a capacitor with a capacitance of 850 µF. When fully charge the magnitude of charge on one plate of the capacitor is 0.28 C.
  1. What is the potential difference between the plates in the capacitor?
  2. How much energy is stored in the “charged up” capacitor?

1) 0.79 pF / 2) 22.0 V / 3) 1.08 x 10-4 C / 4) 5.0 x 107 m2
5) 9.06 x 10-9 C / 6) a. 2.30 µm
b. 8.5 µm / 7) a. 0.392 C
b. 439 J / 8) a. 330 V
b. 46 J