Physics Charge and Current on a Circuit Help! Can someone show me how to do this

Question

Physics Charge and Current on a Circuit Help!

Can someone show me how to do this? I've been trying for over an hour and can't come up with the last two answers. The 4 I put into the table are correct. Please show work so I can understand.

Here's the question:

Consider the circuit in the figure below, in which V = 6 V, and the switch has been in position a for a long time. The switch is changed to position b at t = 0 s. What are the charge Q on the capacitor and the current I through the resistor at the times tabulated below?

time Q (µC) I (mA) (a) immediately after
the switch is closed                                12 120 (b) at t = 70 µs                      5.959 (c) at t = 200 µs              1.62

Explanation / Answer

a) immediately after the switch is closed,

Q = Qmax

= C*Vmax

= 2**10^-6*6

= 12 micro C

Imax = Vmax/R

= 6/50

= 120 mA

b) time constant of the ckt, T = R*C

= 50*2*10^-6

= 100 micro s

at t = 70 micro s
we know, while discharging

Q = Qmax*e^(-t/T)

= 12*10^-6*e^(-70/100)

= 9.959*10^-6 C

= 9.959 micr0 C

I = Imax*e^(-t/T)

= 120*10^-3*e^(-70/100)

= 59.59*10^-3 A

= 59.59 mA

c) at t = 200 micro s
we know, while discharging

Q = Qmax*e^(-t/T)

= 12*10^-6*e^(-200/100)

= 1.62*10^-6 C

= 1.62 micr0 C

I = Imax*e^(-t/T)

= 120*10^-3*e^(-200/100)

= 16.24*10^-3 A

= 16.24 mA

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

---