Physics 08-08 Capacitors and Energy Stored in Capacitors
Name: _____________________________
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
𝐴
𝑑
𝜖0 = permittivity of free space = 8.85 × 10−12 F/m, A = area of plate, d = distance between plates
𝐶 = 𝜖0

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 = 𝜅𝜖0

o
o
𝐴
𝑑
𝜅 = dielectric constant
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
𝑄𝑉 𝐶𝑉 2 𝑄2
=
=
2
2
2𝐶
Capacitors can be used to __________________ deliver a __________________ like in a ______________________ or camera
__________________
Or it can be used to reduce electrical __________________ in __________________ /__________________
𝐸=


Physics 08-08 Capacitors and Energy Stored in Capacitors
Name: _____________________________
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 m 2 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