19.7 Energy Stored in Capacitors

Consider a parallel plate capacitor, with no dielectric material, attached to a battery with a fixed voltage. What happens when a dielectric is inserted into the capacitor?

1. Nothing changes, except now there is a dielectric in the capacitor.
2. The energy in the system decreases, making it very easy to move the dielectric in.
3. You have to do work to move the dielectric, increasing the energy in the sy…

What happens to the energy stored in a circuit as you increase the number of capacitors connected in parallel? Series?

1. increases, increases
2. increases, decreases
3. decreases, increases
4. decreases, decreases

Consider a parallel plate capacitor with metal plates, each of square shape of 1.00 m on a side, separated by 1.00 mm. What is the energy of this capacitor with 3.00×10³ V applied to it?

1. 3.98×10^-2 J
2. 5.08×10¹⁴ J
3. 1.33×10^-5 J
4. 1.69×10¹¹ J

Consider a parallel plate capacitor with metal plates, each of square shape of 1.00 m on a side, separated by 1.00 mm. If the plates grow in area while the voltage is held fixed, the capacitance ___ and the stored energy ___.

1. decreases, decreases
2. decreases, increases
3. increases, decreases
4. increases, increases

(a) What is the energy stored in the $\text{10}.\text{0 }\mu\text{F}$ capacitor of a heart defibrillator charged to $9.00 \times \text{10}^{\text{3}} \text{V}$? (b) Find the amount of stored charge.

In open heart surgery, a much smaller amount of energy will defibrillate the heart. (a) What voltage is applied to the $\text{8}.\text{00 }\mu\text{F}$ capacitor of a heart defibrillator that stores 40.0 J of energy? (b) Find the amount of stored charge.

Suppose you have a 9.00 V battery, a $\text{2}.\text{00 }\mu\text{F}$ capacitor, and a $\text{7}.\text{40 }\mu\text{F}$ capacitor. (a) Find the charge and energy stored if the capacitors are connected to the battery in series. (b) Do the same for a parallel connection.

A nervous physicist worries that the two metal shelves of his wood frame bookcase might obtain a high voltage if charged by static electricity, perhaps produced by friction. (a) What is the capacitance of the empty shelves if they have area $1.00 \times \text{10}^{\text{2}} \text{m}^{\text{2}}$ and are 0.200 m apart? (b) What is the voltage between them if opposite charges of magnitude 2.00 nC are placed on them? (c) To show that this voltage poses…

(a) On a particular day, it takes $9.60 \times \text{10}^{\text{3}} \text{J}$ of electric energy to start a truck’s engine. Calculate the capacitance of a capacitor that could store that amount of energy at 12.0 V. (b) What is unreasonable about this result? (c) Which assumptions are responsible?