# PHYS-222 Worksheet 7 for Section 25 &amp; 36 Problem 7-1 ```PHYS-222 Worksheet 7 for Section 25 &amp; 36
TA: Yang Li, leeyoung@iastate.edu
October 2, 2012
Problem 7-1
In the circuit of Figure 1, when the switch is at A, capacitor C is charged. After a long time, the switch is
moved to B (assume the switching takes neglectable time). The capactior is discharged.
(a) . What is the time constant τ when charging? A.
RC
(b) . What is the time constant τ 0 when discharging? A.
B.
RC
2RC
B.
2RC
C.
1
2 RC
C.
D.
1
2 RC
1
RC
D.
1
RC
Problem 7-2
A charged-particle velocity selector consists a orthogonal magnetic field and an electric field. Assume the
magnetic field is perpendicular to the page and going in, with magnitude B. The charge of the particle is q.
Suppose we want to select particles with velocity v. (See Fig.(a) )
(a) What is the direction of E, if q is positive? A. Up B. Down What if q is negative? A. Up B. Down
(b) What is the magnetic of the elctric field E? A. E = vB
(a) a velocity selector for charged particles;
B. E = qvB
C. E = vB/q
(b) a velocity selector for neutral particles;
1
D. E = qB
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