Classroom Assignment 4-1

7/28/2019 Classroom Assignment 4-1

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Spring 2013

Physics 2 (PHYN002)

Classroom Assignment 4

2. How much charge flows from a 12.0- V battery when it is connected to a 12.6- F capacitor?

Ans( Q=1.51x10

-4

C)

5. A 7.7-F capacitor is charged by a 125-V battery (Fig. 24-20a) and then is disconnected from the batteryWhen this capacitor (C1) is then connected (Fig. 24-20b) to a second (initially uncharged) capacitor, C2 , the

final voltage on each capacitor is 15 V. What is the value of C2 ? [Hint: Charge is conserved.]

Ans. ( C2=56 F)

6. An isolated capacitor C1 carries a charge Qo. Its wires are then connected to those of a second capacitor C 2previously uncharged. What charge will each carry now? What will be the potential difference across each?Ans. ( V=Qo/(C1+C2))

8. A 2.70- F capacitor is charged to 475 V and a 4.00- F capacitor is charged to 525 V. (a) These capacitors

are then disconnected from their batteries, and the positive plates are now connected to each other and the

negative plates are connected to each other. What will be the potential difference across each capacitor and thecharge on each? (b) What is the voltage and charge for each capacitor if plates of opposite sign are connected?

Ans. ( a) Q1=1.36x10-3

C, Q2= 2.02x10-3

C , b) Q1=3.3x10-4

C, Q2= 4.9x10-4

C )

12. What is the capacitance per unit length (F/m) of a coaxial cable whose inner conductor has a 1.0-mmdiameter and the outer cylindrical sheath has a 5.0-mm diameter? Assume the space between is filled with air.

Ans. (3.5x10-11

F/m)

13. Determine the capacitance of the Earth, assuming it to be a spherical conductor.Ans. (7.1x10

-4F)

17. How strong is the electric field between the plates of a 0.80- F air-gap capacitor if they are 2.0 mm apart

and each has a charge of 92 C?Ans. (5.8x10

4V/m)


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18. A large metal sheet of thickness L is placed between, and parallel to, the plates of the parallel-platecapacitor of Fig. 24-4. It does not touch the plates, and extends beyond their edges. (a) What is now the net

capacitance in terms of A, d, and L? (b) If L = 0.4 d, by what factor does the capacitance change when the sheet

is inserted?

Ans. ( a) C=oA/(d-L)), b)1.7 )

21. The capacitance of a portion of a circuit is to be reduced from 2900 pF to 1600 pF. What capacitance can be

added to the circuit to produce this effect without removing existing circuit elements? Must any existing

connections be broken to accomplish this?

Ans. ( Yes, the circuit must be disconnected in order to connect additional capacitor C=3570 pF in series)

30. Suppose that C1 = C 2 = C 3 = 16.0 F and C 4 = 28.5 F. If the charge on C2 is Q2 = 12.4 C,

determine the charge on each of the other capacitors, the voltage across each capacitor, and the voltage Vab

across the entire combination.

Ans. (1 2 3 4

1 2 3 4 ab

12.4 C ; 24.8 C ; 37.2 C

0.775V ; 1.55V ; 1.31V ; 2.86V

Q Q Q Q

V V V V V

)


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60. Two different dielectrics fill the space between the plates of a parallel-plate capacitor as shown in Fig. 24-31. Determine a formula for the capacitance in terms of K1 , K2 , the area A, of the plates, and the separation d

= d2 = d /2. [Can you consider this capacitor as two capacitors in series or in parallel?]

Ans. ( C=2oA K1K2/(d(K1+K2)) )

63. A slab of width d and dielectric constant K is inserted a distance x into the space between the square parallelplates (of side L) of a capacitor as shown in Fig. 24-32. Determine, as a function of x, (a) the capacitance, (b)

the energy stored if the potential difference is Vo , and (c) the magnitude and direction of the force exerted on

the slab (assume Vo is constant).

Ans. (C = 2

01 1

xK

d

l

l

, U= 2

2

0 01 1

2

xK V

d

l

l

, 2

0 0 1 , left .2

VK

dF

l

)


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Classroom Assignment 4-1