Capacitors Tut 3

(Click the question to get the answer.)

1. What is meant by each of the variables in this equation?
C: capacitance
εo: permittivity of free space (value: 8,85 x 10-12 F.m-1)
A: area of the plates
d: distance between the plates
Audio 1

2. The area of each plate of a capacitor is 0,04m2. The air gap between the plates is 5mm.
Calculate the capacitance C of this capacitor.
(Since Area is given, we will use this new equation to calculate C. Change 5mm to 5m.)

The answer of 7,08 x 10-11F can be changed to pF.
    7,08 x 10-11F
= 70,8 x 10-12 F
= 70,8 pF
(Remember that p = 10-12)

Audio 2

3. A capacitor is rated 8nF and the plates are 2mm apart. Calculate the area of each plate.

Audio 3

4. How far apart should the plates of a 200pF parallel plate capacitor be placed if the area of each plate is 0,4cm2?
The Area is 0,4cm2.
To convert a cm2 to m2 you must:
divide by 100, and divide by 100 again

Hence 0,4cm2 = 4 x 10-5m2
Now substitute:

To convert the answer to mm, multiply by 1000.
    0,0177 x 1000
= 17,7mm

Audio 4

5. Which part of a capacitor is considerd as "physical designs"?
The Area A of the plates.
The distance d between the plates.
Audio 5

6. To maximise the capacitance C of a capacitor, what must be done to each of the following?
(answer the following questions)

6.1. area of the plates
The Area must be maximised.

C is proportional to A, so an increase in A will result in an increase in C.

Audio 6.1

6.2 distance between the plates
C is inversley proportional to d.
Hence the distance between the plates must be minimised.

Audio 6.2

6.3 dielectric permittivity
C is proportional to the dielectric constant

Use a substance that has the highest possible dielectric constant.

Audio 6.3

7. A certain parallel plate capacitor consists of two plates, each having dimensions of 3cm by 12cm. The plates are 0,2mm apart and are held at a potential difference of 30 V. The space between the plates is filled with air.
Calculate the capacitance C of this capacitor.
Area = 3 x 12
      = 36cm2
Then 36 ÷ 100 ÷ 100
      = 3,6 x 10-3 m2

Audio 7

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