W06D1
Current, Current Density,
Resistance and Ohm’s Law,
Magnetic Field, Magnetic Force
Concept Questions with Answers
8.02
W06D1
2
Concept Question: Current Density
A current I = 200 mA flows in the wire below. What is
the magnitude of the current density J?
5 cm
10 cm
1.
2.
3.
4.
5.
6.
J = 40 mA/cm
J = 20 mA/cm
J = 10 mA/cm
J = 1 mA/cm2
J = 2 mA/cm2
J = 4 mA/cm2
20 cm
Concept Q. Answer: Current Density
Answer: 6. J = 4 mA/cm2
5
cm
10
cm
20 cm
The area that matters is the cross-sectional area
that the current is punching through – the 50 cm2
area shaded grey.
So:
J = I/A = 200 mA/50 cm2 = 4 mA/cm2
P18- 3
Concept Question: Resistance
P18- 4
Concept Question Answer: Resistance
When a current flows in a wire of length L and cross
sectional area A, the resistance of the wire is
3. Proportional to L; inversely proportional to A.
The longer the wire the higher the resistance.
The bigger the cross-sectional area of the wire,
the more ways that current can flow through it,
so the lower the resistance.
1.  Proportional to A; inversely proportional to L.
2.  Proportional to both A and L.
3.  Proportional to L; inversely proportional to A.
4.  Inversely proportional to both L and A
So, if resistivity is ρr , then
P18- 5
R=
ρr L
A
P18- 6
1
Concept Question: Units Magnetic Field
What are the correct SI units for the magnetic
field?
1.
2.
3.
4.
5.
Concept Q. Answer: B Field Units

 
Fmag = q v × B
Since
C/N-m-s
N-m-s/C
N/C
N-s/C-m
C-m/N-s
B Units =
(
newton
N
N
=1
=1
C ⋅m s
A ⋅m
coulomb meter / second
)(
)
This is called 1 Tesla (T)
1 T = 10 4 Gauss (G)
P18- 7
Concept Question: Cross Product
Concept Question Answer: Cross
Product and Magnetic Force
 
Answer: 5. v × B points out of the page but
An electron is traveling to the right with speed v in a
magnetic field that points up. What is the direction of

the force on the electron
v

1.  up
B
2.
3.
4.
5.
6.
8
the charge of the electron is negative

 
so FB = q v × B points into the page.
down
left
right
into page
out of page
P18- 9
Concept Question: Force Direction
P18- 10
Concept Q. Answer: Force Direction
Is this picture (deflection direction) correct?
Answer: 1. Yes
1.  Yes
2.  No
3.  I don’t know
Field from N to S, beam velocity right to left, cross
product is up. But charges are negative so force is
down, as pictured.
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12
2