summary Capacitance Parallel plates, coaxial cables, Earth - PowerPoint Presentation

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summary

CapacitanceParallel plates, coaxial cables, EarthSeries and parallel combinationsEnergy in a capacitorDielectricsDielectric strength

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(25 - 12)

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Series combinations reduce the capacitance. Equal C reduce by the number involved.

In parallel the capacitance increases.

A basket of 4 capacitors, each of C = 6

nF

. How can you arrange them to get

1.5

nF

2

nF

g) 2.4

nF

3

nF

h) 3.6

nF

4

nF

i

) 4.5

nF

j) 6

nF

12

nF

k) 18

nF

24

nF

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clicker

All C’s are 8.00 nF. The battery is 12 V.What is the equivalent capacitance?

4

nF

6

nF

8

nF

10

nF

12

nF

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All C’s are 8.00 nF. The battery is 12 V.What is the equivalent capacitance?

C

12 = 4 nFC123 = 12 nFQ123 = C123 x V = 144 nCQ3 = C3 x V = 96 nCQ12 = C12 x V = 48 nC

U

123

=

½ C

123

V

2

= ½ x 12x10

-9

x12

2

= 864

nJ

U

1

= ½ C

1

V

1

2

= ½ x 8x10

-9

x6

2

= 144

nJ

= U

2

U

3

= ½ x 8x10

-9

x12

2

= 576

nF

C

3

stores most energy, also the highest electric field and most charge, the

most stressed

part of the circuit

.

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Circuits

All capacitors being the same, rank the equivalent capacitances of the four circuits.

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C

123 2.4 µF, q = 28.8 µCC2 C24 = 12 µFC1234 = 3 µF q =36 µCΔq = 7.2 µC

C

1

= C

3

= 8.00

μ

F,

C

2

= C

4

= 6.00

μ

F,

V = 12V

When the switch S is closed, how much charge flows through point P

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q

-q

q

'

q

q

-q

'

-q

-q

V

V

V

V

'

(25 - 15)

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q

-q

q

'

q

q

-q

'

-q

-q

V

V

V

V

'

(25 - 16)

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If the areas are A

1 and A-A1.

C

123 2.4 µF, q = 28.8 µCC2 C24 = 12 µFC1234 = 3 µF q =36 µC

Effect of a dielectric : C



κ

C

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The force on a filling dielectric as it is inserted between the parallel plates of a capacitor.

x

L

With the battery connected,

U

1

= ½CV

2

With the battery disconnected,

U

2

= Q

2

/2C

With the battery connected, since x is increasing downwards, a negative force is upwards, pushing the dielectric away.

With the battery disconnected, the force is positive and pointed downwards, pulling in the dielectric.

The force is proportional to (

κ

-1) and inversely to L.

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A question

What is the equivalent capacitance between the points A and B?

1 μF B. 2 μF 4 μF 10μF None of these

A

B

What would a 10V battery do, i.e. how much charge will it provide,

when it is connected across A and B? 40

μ

C

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Question

A parallel-plate capacitor has a plate area of 0.3m2 and a plate separation of 0.1mm. If the charge on each plate has a magnitude of 5x10-6 C then the force exerted by one plate on the other has a magnitude of about:0 5N 0. 9N 1 x104 N 9 x 105 N

The electric field =

σ

/2

ε

o

why?

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Question

A parallel-plate capacitor has a plate area of 0.3m

2

and a plate separation of 0.1mm. If the charge on each plate has a magnitude of 5x10

-6

C then the force exerted by one plate on the other has a magnitude of about:

A. 0 B. 5N C. 9N D. 1 x10

4

N E. 9 x 10

5

N

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A question

Each of the four capacitors shown is 500

μ

F. The voltmeter reads 1000V. The magnitude of the charge, in coulombs, on each capacitor plate is:

A. 0.2 B. 0.5 C. 20 D. 50 E. none of these

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HITT

A parallel-plate capacitor has a plate area of 0.2m

2

and a plate separation of 0.1 mm. To obtain an electric field of 2.0 x 10

6

V/m between the plates, the magnitude of the charge on each plate should be:

A. 8.9 x 10

-7

C B. 1.8 x 10

-6

C C. 3.5 x 10

-6

C D. 7.1 x 10

-6

C E. 1.4 x 10

-5

C