Physics 2B, Electricity and Magnetim -- Winter 2009 Instructor: B. Grinstein
Quiz # 5
INSTRUCTIONS: Fill, tear and return the bottom strip of the front page with your scantron. Keep the top portion of the
front page and the rest of the quiz. Use a pencil #2 to fill your scantron. Write your code number and bubble it in under
“EXAM NUMBER”. Bubble in the quiz form (see letter A--D at bottom of page) in your scantron under “TEST FORM”
Useful numbers: K = 9.0 109 Nm2 /C2 , o = 8.85 10-12 C2 /Nm2 , e = 1.60 10-19 C, me = 9.11 10-31 kg
µo = 4 10-7 T m/A
1) A solid cylindrical wire of length L and radius a has a resistance R. What is the resistance of a hollow
cylinder of length L, inner radius a and outer radius 2a, made of the same material as the solid cylinder?
C) 3R
D) R/3
E) R
A) 2R
B) R/2
2) The density of free electrons in gold is 5.90 x 1028 m-3. The resistivity of gold is 2.44 x 10-8 · m. A gold
wire, 0.8 mm in diameter and 20 cm long, carries a current of 300 mA. The number of electrons per second
passing a given cross section of the wire, is closest to:
C) 2 x 1017
D) 6 x 1016
E) 6 x 1018
A) 6 x 1017
B) 2 x 1018
3) A 1.0 m length of gold wire is connected to a 1.5 V battery, and a current of 4.0 mA flows through it. What is
the diameter of the wire? The resistivity of gold is 2.24 10 -8 · m.)
A) 4.4 µm
B) 8.7 µm
C) 2.2 µm
D) 12 µm
4) The figure below shows a steady electric current passing through a wire with a narrow region. What
happens to the drift velocity of the moving charges as they go from region A to region B and then to region
C?
A) The velocity decreases all the time.
B) The velocity decreases from A to B and increases from B to C.
C) The velocity remains constant.
D) The velocity increases from A to B and decreases from B to C.
E) The velocity increases all the time.
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Name________________________________________
Quiz ID_________________________
quiz version D-1
5) A copper wire of length L and radius b is attached to another copper wire of length L and radius 2b, forming
one long wire of length 2L. This long wire is attached to a battery, and a current is flowing through it.
Relative to the electric field within the wire of radius b, the magnitude of the electric field within the wire of
radius 2b is
A) four times stronger.
B) two times weaker.
C) four times weaker.
D) two times stronger.
E) equal.
6) Electrons in an electric circuit pass through a source of emf. The wire has the same diameter and are made of the same
material on each side of the source of emf. Compared to the drift speed of the electrons before entering the source of
emf, the drift speed of the electrons after leaving the source of emf is
A) the same
B) slower
C) faster
D) not enough information to decide
7) Consider two wires made of different materials. Wire 1 carries a current I and has a cross-sectional area A.
Wire 2 carries a current 2I and has a cross-sectional area A/2. Under what condition would the electrons in
the two wires have equal drift velocities?
A) If the electron concentration in wire 2 is twice the electron concentration in wire 1
B) If the electron concentrations in the two materials are the same
C) If the electron concentration in wire 2 is four times the electron concentration in wire 1
D) If the electron concentration in wire 1 is twice the electron concentration in wire 2
E) If the electron concentration in wire 1 is four times the electron concentration in wire 2
8) The figure below shows the motion of positive and negative charges within a semiconductor. If the density
of the negative charges is twice that of the positive charges, what is the conventional direction of the current?
A) from right to left in the region with a higher concentration of negative charges and left to right in the
other regions.
B) from right to left
C) from left to right
D) from left to right in the region with a higher concentration of negative charges and right to left in the
other regions.
E) The electric current is equal to zero.
quiz version D-2
Answer Key
Testname: QZ5
1) D
2) B
3) B
4) D
5) C
6) A
7) C
8) C
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Name________________________________________
Quiz ID_________________________
quiz version D-3