Basic Physics II Assignments

Chapter 16: Electric Fields and Charges − Due Friday, September 18

1. How many protons does it take to make up 2mC of charge?

2. Charges of 1mC and 3mC each exert a force of 0.9N on each other. How far apart are they?

3. Three charges of 1mC, 2mC and −3mC are in a row and are 0.3m apart.

a) What is the net force on the right charge (−3mC)? (Include the direction.)

b) What is the net force on the middle charge (2mC)? (Include the direction.)

4.** A charge of Q is at the origin, x=0, and a 3Q charge is at x = d. If I place a third charge q at x=b, the net force on each of the first two charges is zero. Find q in terms of Q and find b in terms of d.

5. Three charges are located at the corners of a square of side a. Find the magnitude and direction of the net force on the charge –Q due to the other two charges. Let the horizontal direction be x and the vertical direction be y.
6. Two charges are located at the opposite corners of a rectangle as shown at the right. a) Find the components of the electric field at the corner A. Let the x-axis be horizontal and the y axis be vertical, as shown. b) Find the magnitude and direction of that Electric Field.
7. Two charges are located as shown at the right, −3.6mC at (−3m,0m) and +2.5mC at (0m,4m). a) Find the x & y components of the electric field at point P (x=3m, y=0). b) Find the magnitude and direction of the electric field.

8. The electric field just outside a conductor is 400N/C. What is the surface charge density on the conductor?

9.** The electric field due to a point charge has a magnitude of 200N/C at a distance x from the charge. If I move 2m farther away from the charge the field is 50N/C. Find x.

Chapter 17: Electric Potential − Due Friday, September 25

1. +1J of work is required to move 0.2C of charge from a potential of 2V to another potential of V_final. What is that final voltage?

2. The electric field between two parallel plates is 400N/C. It takes 0.5J to move 0.1C from one plate to the other. What is the distance between the plates?

3. Two charges are located at the corners of a rectangle as shown. a) Find the electric potentials at A and B. b) How much work would I have to do to move a charge of – 2mC from A to B? (Note that it is the work I have to do.)

4. An electron is 0.053nm from a proton.

a) What is its electric potential energy there? Give it in Joules and in eV. (On average the electron in hydrogen is about 0.053nm from the proton.)

b) What would be the electrons potential energy if it were moved very far away, i.e. to infinity?

5. Three charges are located at the corners of an equilateral triangle of side d. The charges are +Q, +2Q and -2Q.

a) What is the electric potential energy of this configuration?

b) How much energy would I have to add to separate these charges, i.e. make d →∞?

6. Two parallel conducting plates are 0.7mm apart. The area of each plate is 7.9cm². What is the capacitance of this configuration?

7. The potential difference between the plates in the capacitor above is 9V.

a) How much charge is on the plates?

b) What is the electric field between the plates?

c) How much energy is stored in the capacitor.

8. I have two parallel plates arranged so that their capacitance is 4nF (n=nano = 10^-9) when there is no material between the plates. I insert a dielectric between the plates (filling the volume between the plates) and measure a capacitance of 200nF. What is the dielectric constant of that dielectric?

9.** Two charges are separated by a distance d. The electric potential at A between the two charges, Q and 3Q is 800V. What will the potential be half way between the two charges, i.e. at the midpoint between them?
10. I want to take an electron (M = 9x10^-31kg, q= -1.6x10^-19C) at rest and give it a speed of 10^7m/s. I will do this by letting it accelerate from a plate at a potential of 0 volts to a plate at a potential of V1 volts. What must the potential V1 be to do this?