Electromagnetism Problem Sheet
1. The Earth’s magnetic field strength is approximately 4 x 10-5 T vertically downwards. A
horizontal wire, 2m long is placed on a bench. What current would be necessary in the wire
to produce a force of 1N on it? If the current goes left to right as you face the wire,
describe the direction of the force in relation to your position.
(3 marks)
F
=
BIlsin
(
θ
)
1 mark
I
=
F
BIlsin
(
θ
)
=1
4
×
10−5
×
2
×
1
1mark
I
=1.25
×
104
A
1mark
2. In the diagram opposite, there is a current of 5.0A on
each of the conductors, OA, OB, OC and OD. They are
in a magnetic field, shown, of size 0.15T horizontally
to the right. What is the size and direction of force on
each conductor?
(8 marks)
F
=
BIlsin
(
θ
)
1 mark
OD
F
=
BIlsin
(
0
)
=0
N
1 mark
OC, l =
√
0.22+0.12=0.2236
…
θ
=tan−1
(
0.1
02
)
=26.6
°
F
=5
×
0.15
×
0.2236..
×
sin
(
26.6
)
=0.075
N
1 mark
1mark
1mark
OB, l = 0.141…
θ
=45
°
F
=5
×
0.15
×
0.141...
×
sin
(
45
)
=0.075
N
1 mark
1 mark
OA, l = 0.1,
θ
=0
°
F
=5
×
0.15
×
0.1
×
sin
(
90
)
=0.075
N
1 mark for final answer
3. A single turn circular coil has a radius of 2cm. A magnetic field of size, 0.1T passes through
the coil, perpendicular to its plane.
a) Find the magnetic flux passing through the coil.
b) What would be the flux linkage within the coil if there were 100 turns instead of 1?
c) What flux would pass through the single turn coil if its plane was at 45o to the magnetic
field?
(6 marks)
a)
φ
=
NBA
1 mark
φ
=1
×
0.1
×
(
π
0.022
)
=1.26
×
10−4
Wb
1 mark
b) N = 100 1 mark
φ
=
NBA
=100
×
0.1
×
(
π
0.022
)
=1.26
×
10−2
Wb
1 mark
c)
φ
=
NBAcos
(
θ
)=1.26
×
10−4cos
(45)
1 mark
φ
=8.91
×
10−5
Wb
1 mark
