Recap
Lecture 13
© Matthias Liepe, 2012
Today:
• More on RC circuits
• Magnets and magnetic field
a
i

RC circuit: Charging
R
i
i
i
At time t  0
the switch is
C moved to
position a.
RC circuit: Discharging
a
R

b
i
i
C
• The switch has been at position a for a very long
time.
• At time t  0 move the switch to position b.
• Current i begins to flow to discharge the
capacitor.
RC circuit: Discharging
R
b
i
i
C
At time t  0 the
switch is moved to
position b.
120
Current i (mA)
What is the
100
approximate value of
80
the time constant  for
60
this decay of electric
40
current from a
20
discharging capacitor
0
in a simple RC circuit?
0
A.
B.
C.
D.
E.
~25 s
~35 s
~50 s
~100 s
~250 s
50
100
150
Time t (s)
200
250
120
A.
B.
C.
D.
E.
1.2 C
3.0 C
6.0 C
12 C
18 C
100
Current i (mA)
Approximately, what
was the discharging
capacitor’s initial
charge at time t = 0 ?
80
60
40
20
0
0
50
100
150
Time t (s)
200
250
At time t =2 , how much
charge is on the
capacitor?
A.
B.
C.
D.
E.
0.14 qf
0.37 qf
0.63 qf
0.79 qf
0.86 qf
Charge q
The graph shows the
electric charge on a
charging capacitor in a
simple RC circuit.
q6 f
5
4
3
2
1
0
0
50
Charging
100
Time150
t
200
250
Magnetic Fields and Forces
The Earth’s magnetic
field near the surface
can be approximated
by the field of a bar
magnet. In which
direction would the
magnetic north pole
of Earth’s magnet
point?
A.
B.
C.
D.
To the geographic north pole
To a point near the geographic north pole
To the geographic south pole
To a point near the geographic south pole
Magnetic Fields and Forces
How can we detect a magnetic field B?