Capacitors Tut 2

1. What is meant by capacitance?
Obviously a capacitor can store only so much charge. (It gets full!)
So capacitance is "how good is the capacitor!".
Or capacitance is a measure of the ability of a capacitor to store charge.
Audio 1

2. What is the symbol for capacitance?
a letter C
Don't confuse this C with the UNIT for charge called coulombs (C).
Audio 2

3. What is the unit for capacitance?
The unit for capacitance is the farad or F.
It is common to come across capacitance values that look like this:
C = 3 x 10-6F
Don't mix this F with the symbol for force!
Audio 3

4. What do the symbols in this equation mean?
C: capacitance
• measure of the capacitors ability
• final answer in units called farads

Q: charge
• measured in coulombs
• actual amount of charge

V: potential diffence
• this is just the voltage of the capacitor at full charge
Audio 4

5. What is the difference between "capacitor" and "capacitance"?
A capacitor is the actual device.
Capacitance is the measure of the device's ability to store charge.
Audio 5

6. A capacitor is rated 4V 50μF.
Calculate its charge Q when it is fully charged.
(remember to change 50μF)

Audio 6

7. The charge of a fully charged capacitor is 8nC. Its potential difference is 20V.
Calculate its capacitance.
(You must calculate the letter C.)

Audio 7

8. The specifications for a capacitor is 8nF and 5mC.
What would be its potential difference?
(You must calculate the letter V.)

Audio 8

9. The ratings of a capacitor is 5V 80μF.
What would most likely happen if you connect this capacitor to a 12V battery for it to be charged?
The capacitor would blow up. It is best to use a battery that is LOWER in voltage than the ratings of the capacitor.
This would be like placing too heavy a load on a small truck.
Audio 9

10. Why can a capacitor be used to perform "timings" in a circuit?
It takes time for a capacitor to become fully charged. As it is getting charged, the current is getting smaller until it becomes zero.
If there was a bulb in the circuit,the bulb would have gradually become dimmer too as the current decreases.
This feature can be used to gently dim the lights to eventually go off, rather than to go off immediately.
Such features are found in some new cars. Lights can remain on after you lock up and leave, providing you with light as you walk. The lights will gradually go off after a while, when the capacitor is fully charged.
Audio 10

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V. Gokal