Electrical Energy and Capacitance Explain the conceptual meaning of capacitance, what causes the plates of a capacitor to become charged, the conceptual relationship between applied voltage and charge, and the conceptual relationship between energy, charge and voltage.

Find the total capacitance of the combination of capacitors in Figure 19.33. Figure 19.33 A combination of series and parallel connections of capacitors.

Nerve cells in the human body and in other animals are modeled as very long cylindrical capacitors. Portions of some nerves are covered with a layer of fat known as myelin, which functions as the dielectric ( = 7) between two plates in the cylindrical capacitor model. The potential difference between the inner and outer walls of myelin in resting nerve cells is roughly Vinner Vouter = 70 mV. Find the linear charge density on the inner (positive) plate. Hint: Use the result of Example 27.8.

Electronic flash units for cameras contain a capacitor for storing the energy used to produce the flash. In one such unit the flash lasts for 1/675 fraction of a second with an average light power output of 270 kW. (a) If the conversion of electrical energy to light is 95% efficient (because the rest of the energy goes to thermal energy), how much energy must be stored in the capacitor for one flash? (b) The capacitor has a potential difference between its plates of 125 V when the stored energy equals the value stored in part (a). What is the capacitance?

A parallel-plate capacitor is connected to a battery. What happens to the stored energy if the plate separation is doubled while the capacitor remains connected to the battery? (a) It remains the same, (b) It is doubled. (c) It decreases by a factor of 2. (d) It decreases by a factor of 4. (e) It increases by a factor of 4.

A parallel-plate capacitor with only air between its plates is charged by connecting the capacitor to a battery. The capacitor is then disconnected from the battery, without any of the charge leaving the plates, (a) A voltmeter reads 45.0 V when placed across the capacitor. When a dielectric is inserted between die plates, completely filling the space, the voltmeter reads 11.5 V. What is the dielectric constant of the material? (b) What will the voltmeter read if the dielectric is now pulled away out so it fills only one-third of the space between the plates?

You are working at an electronics fabrication shop. Your current project is on the team producing capacitors for the timer circuit that delays the closing of an elevator door. According to its design specification, the timer circuit is to have a capacitance of 32.0 F between two points A and B. As sour capacitors come off the assembly line, you find that they have a variation of 5.00% from this value. After a team meeting to evaluate this situation, the team decides that capacitances in the range 32.0 0.5 F are acceptable and do not need modification. For capacitances outside this range, the director does not wish to discard the capacitors, but rather to add extra capacitors in series or parallel with the main capacitor to bring the total equivalent capacitance to the exact design value of 32.0 F. You are put in charge of procuring the extra capacitors. What range of capacitances for these extra capacitors do you need to cover the entire range of variation of 5.00%? All capacitances can be measured to three significant figures.

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 1.00102 m2 and are 0.200 m apart? (b) What is the voltage between them it opposite charges of magnitude 2.00 nC are placed on them? (C) To show that this voltage poses a small hazard, calculate the energy stored.

An anxious physicist worries that the two metal shelves of a 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 1.00102m2 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 small hazard, calculate the energy stored, (d) The actual shelves have an area 100 times smaller than these hypothetical shelves. Are his fears justified?

A 40-pF capacitor is charged to a potential difference of 500 V. Its terminals are then connected to those of an uncharged 10-pF capacitor. Calculate: (a) the original charge on the 40-pF capacitor; (b) the charge on each capacitor after the connection is made; and (c) the potential difference across the plates of each capacitor after the connection.