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Unformatted text preview: 120 The Exponential Function
f(t) = et/W Current i (mA) 100 W is called the time f(t)
constant and is
the time that it
takes for e t W to
reach 1 e, because
f (W ) e W W e 1. 1.2 1
1 0.8 )
t
1 e
(
f
0W
0
0.2
0 0 1 2 3 4 t 5 0.8
0.6
0.4 0 0 0 f
0 1 2 3 4 t 5 W (e f W e 0 W ) W . 50 100
150
Time t (s) 200 250 What is the approximate value of the
time constant W for this decay of electric
current from a discharging capacitor in a
simple RC circuit?
A. 25 s.
B. 35 s.
C. 50 s.
D. 100 s. 0.2 W e t W 20
0 1 e t W dt 40 0.4 1.2 ³ 60 0 0.6 e 1  0.37  3 8.
f( t)
df
1 t W
1
e , so
dt
W
the slope of f (t ) at )
t
df
1(
. f
t 0 is
0W
W
dt t 0
0
f 80 E. 250 s. Approximately, what was the discharging
capacitor’s initial charge at time t 0?
A. 1.2 C.
B. 3.0 C.
C. 6.0 C.
D. 12 C. E. 18 C. Charge q q
6f
5
4
3 C 2 R 1
00 0 0
Charging50 100
Time150
t 200 250 When the charge on the capacitor
drops to 1 2 of its initial value, the
electric energy stored in the
capacitor drops to what fraction of
its initial value?
B. 1 2 .
A. 3 4. The graph shows the electric charge
on a charging capacitor in a simple
RC circuit.
At time t 2W , how much charge is on
the capacitor?
B. 0.37qf .
C. 0.63qf .
A. 0.14qf . C. 12. D. 0.79qf . E. None of the above. E. 0.86qf . D. 1 4. ...
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This note was uploaded on 12/27/2011 for the course PHYSICS 2208 at Cornell.
 '09
 FULBRIGHT
 Current

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