Physics 08-08 Capacitors and Energy Stored in Capacitors Name: _____________________________

Created by Richard Wright – Andrews Academy To be used with OpenStax College Physics

Capacitor

Two __________________ conductor __________________ separated by a __________________

Stores __________________

One plate __________________, one plate __________________

__________________ (Q) is the __________________ plate

Amount of charge stored depends on

Electric __________________

__________________ __________________ of capacitor (like distance)

𝑄 = 𝐶𝑉

o Q = charge, C = capacitance, V = electric potential

Capacitance describes the __________________ __________________ of the capacitor

o Unit: __________________ (C/V)

Capacitance of Parallel Plate Capacitor

𝐶 = 𝜖0𝐴

𝑑

𝜖0 = permittivity of free space = 8.85 × 10−12 F/m, A = area of plate, d = distance between plates

Dielectrics

Some times the __________________ can become so __________________

that the __________________ between the plates __________________ down

and starts __________________

To have ________________ capacitance, use some __________________ in

place of the air between the plates that doesn’t break down as

easily

This is called __________________

Capacitor with a dielectric

𝐶 = 𝜅𝜖0𝐴

𝑑

o 𝜅 = dielectric constant

o In the table, dielectric strength is the E-field at which the

material starts conducting

Find the capacitance of a parallel plate capacitor with an area of 1 𝑚2 and

a separation of 1.0 mm if it is filled with paper.

Energy stored in capacitor

𝐸 =𝑄𝑉

2=𝐶𝑉2

2=𝑄2

2𝐶

Capacitors can be used to __________________ deliver a __________________ like in a ______________________ or camera

__________________

Or it can be used to reduce electrical __________________ in __________________ /__________________

Physics 08-08 Capacitors and Energy Stored in Capacitors Name: _____________________________

Created by Richard Wright – Andrews Academy To be used with OpenStax College Physics

What voltage is there across the 100 𝜇F capacitor of a defibrillator if it stores 900 J of energy?

Homework

1. Does the capacitance of a device depend on the applied voltage? What about the charge stored in it?

2. Use the characteristics of the Coulomb force to explain why capacitance should be proportional to the plate area of a capacitor. Similarly, explain why capacitance should be inversely proportional to the separation between plates.

3. Give the reason why a dielectric material increases capacitance compared with what it would be with air between the plates of a capacitor. What is the independent reason that a dielectric material also allows a greater voltage to be applied to a capacitor? (The dielectric thus increases C and permits a greater V.)

4. Water has a large dielectric constant, but it is rarely used in capacitors. Explain why.

5. A parallel plate capacitor is charge up by a battery. The battery is then disconnected, but the charge remains on the plates. The plates are then pulled apart. Explain whether each of the following quantities increases, decreases, or remains the same as the distance between the plates increases: (a) the capacitance of the capacitor, (b) the potential difference between the plates, (c) the electric field between the plates, (d) the electric potential energy stored by the capacitor. Explain.

6. What charge is stored in a 180 μF capacitor when 120 V is applied to it? (OpenStax 19.46) 21.6 mC

7. Find the charge stored when 5.50 V is applied to an 8.00 pF capacitor. (OpenStax 19.47) 44.0 pC

8. Calculate the voltage applied to a 2.00 μF capacitor when it holds 3.10 μC of charge. (OpenStax 19.49) 1.55 V

9. What voltage must be applied to an 8.00 nF capacitor to store 0.160 mC of charge? (OpenStax 19.50) 20.0 kV

10. What capacitance is needed to store 3.00 μC of charge at a voltage of 120 V? (OpenStax 19.51) 25.0 nF

11. What is the capacitance of a large Van de Graff generator’s terminal, given that it stores 8.00 mC of charge at a voltage of 12.0 MV? (OpenStax 19.52) 667 pF

12. Find the capacitance of a parallel plate capacitor having plates of area 5.00 m2 that are separated by 0.100 mm of Teflon. (OpenStax 19.53) 0.93 μF

13. (a) What is the capacitance of a parallel plate capacitor having plates of area 1.50 m2 that are separated by 0.0200 mm of neoprene rubber? (b) What charge does it hold when 9.00 V is applied to it? (OpenStax 19.54) 4.4 μF, 𝟒. 𝟎 × 𝟏𝟎−𝟓 C

14. (a) What is the energy stored in the 10.0 μF capacitor of a heart defibrillator charged to 9.00 × 103 V ? (b) Find the amount of stored charge. (OpenStax 19.63) 405 J, 90.0 mC

15. In open heart surgery, a much smaller amount of energy will defibrillate the heart. (a) What voltage is applied to the 8.00 μF capacitor of a heart defibrillator that stores 40.0 J of energy? (b) Find the amount of stored charge. (OpenStax 19.64) 3.16 kV, 25.3 mC

16. A 165 μF capacitor is used in conjunction with a motor. How much energy is stored in it when 119 V is applied? (OpenStax 19.65) 1.15 J