# Thread: Finding values from two equations

1. ## Finding values from two equations

A transmission line is designed so it has self inductance, L = 7.0x10^-7 Hm^-1 & signal speed 70% of c 9speed of light)
Find characteristic impedance

Now
Z = √L/C (where C is capacitance, but shouldn't really matter just for the maths bit)
also
v = 1/ √LC

remembering
L = 7.0x10^-7 Hm^-1
v = 0.7c
(c = 3x10^8 ms^-1)

7(3x10^8)/10 = 1/√7.0x10^-7 * C
so
C = √((10(7.0x10^-7))/(7*3x10^8))
C = √ 3.33x10^-15
C = 5.77x10^-8 F

Z = √L/C
Z = √7.0x10^-7/5.77x10^-8
Z = 3.4 Ω

Which isn't in the park as far as expected result goes, so AGAIN, please could you show me where I went wrong.
__________________
Just trying to keep up.

2. ## Re: Finding values from two equations

Originally Posted by froodles01
A transmission line is designed so it has self inductance, L = 7.0x10^-7 Hm^-1 & signal speed 70% of c 9speed of light)
Find characteristic impedance

Now
Z = √(L/C) (where C is capacitance, but shouldn't really matter just for the maths bit)
also
v = 1/ √LC

remembering
L = 7.0x10^-7 Hm^-1
v = 0.7c
(c = 3x10^8 ms^-1)

7(3x10^8)/10 = 1/√(7.0x10^-7 * C )
so
C = √((10(7.0x10^-7))/(7*3x10^8))
C = √ 3.33x10^-15
C = 5.77x10^-8 F

Z = √L/C
Z = √7.0x10^-7/5.77x10^-8
Z = 3.4 Ω

Which isn't in the park as far as expected result goes, so AGAIN, please could you show me where I went wrong.
__________________
Just trying to keep up.
As far as I understand your calculations:

1. You've forgotten to use parantheses (added in red)

2. To get the value of C you have to square both sides of the equation first.

3. ## Re: Finding values from two equations

A fresh pair of eyes has it every time.
Thanks. X