# Practice problems: Magnetic field

```REVISION 2 : Magnetic Field
Question 1
a) How can the motion of a charged particle be used to
distinguish between a magnetic and an electric field?
b) Inside a motor, 30.0 A in anticlockwise direction
passes through a 250-turn circular loop that is 10.0
cm in radius. What is the magnetic field strength
created at its center and in what direction?
c) How strong is the magnetic field inside a solenoid
with 10,000 turns per meter that carries 20.0 A?
[Ans:b) 0.047 T, c) 0.1257 T]
a) A charged particle in
magnetic field moves in
circular motion, where as it
will move in linear motion
when it is in electric field.
Question 2
The wire carrying 400 A to
the motor of a commuter
train feels an attractive force
of 4.00×10−3 N/m
due to a parallel wire
carrying 5.00 A to a
(a) How far apart are the
wires?
(b) (b) Are the currents in
the same direction?
Question 3
The force on the rectangular loop of wire in the magnetic
field as in Figure below can be used to measure field
strength. The field is uniform, and the plane of the loop is
perpendicular to the field.
(a) What is the direction of the magnetic force on the loop?
(assume the loop is fully in the field)
(b) If a current of 5.00 A is used, what is the force per tesla
on the 20.0-cm-wide loop? (1 N T-1)
Question 4
A mass spectrometer is being used to separate
common oxygen-16 from the much rarer oxygen-18,
taken from a sample of old glacial ice. The ratio of the
masses of these two ions is 16 to 18, the mass of
oxygen-16 is 2.66×10−26 kg, and travel at 5.00×106 m/s
in a 1.20-T magnetic field. What is the separation
between their paths when they hit a target after
traversing a semicircle?
Question 5
A 200-turn circular loop of radius 50.0 cm is vertical,
with its axis on an east-west line. A current of 100 A
circulates clockwise in the loop when viewed from the
east. The Earth’s field here is due north, parallel to the
ground, with a strength of 3.00×10−5 T . What are the
direction and magnitude of the torque on the loop?
Q4
Q5
REVISION 2 : Electromagnetic Induction
Question 1
a) Explain how magnetic flux can be zero when the
magnetic field is not zero.
b) Suppose a 50-turn coil lies in the plane of the page
in a uniform magnetic field that is directed into the
page. The coil originally has an area of 0.250 m2 .
It is stretched to have no area in 0.100 s. What is
the direction and magnitude of the induced emf if
the uniform magnetic field has a strength of 1.50 T?
Question 2
In the August 1992 space shuttle flight, only 250 m of the
conducting catether could be let out. A 40.0 V motional
emf was generated in the Earth’s 5.00×10−5 T field, while
moving at 7.80×103 m/s. What was the angle between
the shuttle’s velocity and the Earth’s field, assuming the
conductor was perpendicular to the field. (24.22 º )
Question 3
The heating coils in a hair dryer are 0.800 cm in diameter,
have a combined length of 1.00 m, and a total of 400 turns.
(a) What is their total self-inductance assuming they act like
a single solenoid?
(b) How much energy is stored in them when 6.00 A flows?
(c) What average emf opposes shutting them off if this is
done in 5.00 ms?
Q1-b
Q2
```