I\'m in electricity and magnetism physics. I am having trouble understanding thi

Question

I'm in electricity and magnetism physics. I am having trouble understanding this problem. Can you please explain each step. And show how to check this problem. Thank you
uy iel the fesistors were connected in series. i) When is the voltage maximum across the resistor, and what is its value? ii) How long will it take for the capacitor to reach 80% of its ending voltage? ii) How much charge will the capacitor have when t-20sec? 30 10 Variation of 18.52: The resistance between points a and b in the figure drops to one-half its original value when switch S is closed. Determine the value of R. b 90 100 90

Explanation / Answer

Resistance when switch is open = R+ (30+10+90)||(90+10)

= R + 130*100/(130+100) = R + 56.5

Resistance when switch is closed = R +90*40/(90+40) + 90*10/(90+10)

= R +36.7

Now we have, R +56.5= 2*(R+36.7)

R + 56.5 = 2R + 73.4

R = - 16.9

Here it seems contradicting to get negative value of R, but with the data given in the question, thats tge only value possible. || is the symbol for parallel combination

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