# elementary questions on capacitance

Discussion in 'Homework Help' started by electronicstech07, Sep 23, 2008.

1. ### electronicstech07 Thread Starter Member

Nov 16, 2007
18
0
I am having some problems understanding what I am reading on the subject of capacitance. The first is temperature coefficient. The question in my textbook is asking a 2 uF capacitor has a positive temperature coefficent of 50 ppm/C, what is the capacitance when the temperature increases by 100 degree C.

I understand that positive temp. coefficent means the capacitance will increase per degree C increase, but can't seem to calculate the right answer. How exactly do I calculate this?

The next question is to obtain a reactance of 2 kil ohm from a 0.1 uF capacitor, what frequency is required? How can I calculate this?

Help is greatly appreciated!

2. ### scubasteve_911 AAC Fanatic!

Dec 27, 2007
1,202
1
Hey,

So, if temperature increases by a degree celcius, the capacitor will increase its capacitance by 50 parts per million. So,

C(100degrees) = Cnominal(1 + 100 * 50 * 10^-6)
C(100degrees) = 2.01uF

Xc = 1/(2*pi*f*C)

Solving for f

Xc(2*pi*f*C) = 1
f = 1/(2*pi*C*Xc)
f = 795.77 Hz

Steve

3. ### electronicstech07 Thread Starter Member

Nov 16, 2007
18
0
That cleared up the confusion. Thanks again!

4. ### scubasteve_911 AAC Fanatic!

Dec 27, 2007
1,202
1
You're welcome sir,

I'm just glad you didn't mean 'elementary' as in calculating capacitance from electromagnetics principals

Steve