### Electric Fields, Potential, and Capacitors Sample Problems

1. A +40 mC test charge is placed in an electric field produced by a +5 C charge. It experiences a 6 N force. What is the strength of the electric field at this point?
2. A +3 mC test charge is placed in a 20,000 N/C field. What force does it experience?
3. A test charge experiences an electric field of 400,000 N/C at a point 1 cm away. What is the magnitude of the charge producing the field?
4. 30 J of work is done to move a +5 C charge from one point to another in an electric field. What is the difference in potential between the two points?
5. How much work is done to move an electron across a potential of 3 V?
6. Two parallel plates are separated by 0.5 m. An electric field of 6000 N/C exists between the plates. What is the potential difference between the plates?
7. A potential difference of 60 V exists between two parallel plates separated by 3 cm. What electric field exists between the plates?
8. An oil drop carrying a charge of -8 x 10-19 C is suspended between two plates separated by 8 mm. A 1200 V potential difference exists between the plates. What is the weight of the suspended drop?
9. An oil drop weighs 5.8 x 10-14 N. It is suspended in a 3000 N/C field. What is the charge of the drop? How many electrons does it carry?
10. What is the capacitance of a capacitor carrying a charge of 0.00012 C and having a 6 V potential difference?
11. What charge exists on a 30 mF capacitor with a 120 V potential difference between its plates?
12. Lab-type problem: A 330 mF capacitor and a 100 W resistor are used. The signal generator is set for 4 V.
beginning time: 1.2471 sec
time to reach 2 V: 1.2671 sec
maximum voltage: 3.95 V
area under curve: 0.0015303 A sec
• Calculate the theoretical value for the charging half-life.
• Calculate the experimental value for the half-life.
• Calculate an experimental value for the capacitance using your charging half-life.
• Use the area under the curve and the maximum voltage to calculate the energy stored.
• Use the maximum voltage and your experimental value for the capacitance to calculate the energy stored.