MAGNETIC FIELD OF A CURRENT THROUGH A COIL
OBJECTIVES
-To determine the direction and magnitude of the magnetic field due to the current flowing
through the wire of a coil.
-To estimate the magnetic field of earth at college location.
Note: The magnetic field of EARTH (BE) is ALWAYS present. We cannot eliminate it.
If
and
B I = Magnetic field of current (I) in coil
B E = Magnetic field of the EARTH — then we will always observe.
B
BI
BE
(1)
And hence in order to find BI we must first "know" BE and work with their resultant B which is aligned
along the direction shown by the compass.
(We can "know" BE by simply calling it a unit "BE". We don't need an actual value in Tesla for this.)
The measurement procedure will be simple:
a) Find direction of BE without any I in coil.
b) Set up the coil so that its plane (vertical) contain
the direction of BE. This way BI will always be
perpendicular to BE and therefore
BI = BE*tan θ
(2)
PART A. MAGNETIC FIELD DUE TO A COIL
EXPERIMENT _1
a) Verify the direction of magnetic field due to the coil.
For this make sure initially that the plan of the coil is perpendicular to the direction of magnetic field of
earth (by using the compass). Apply the right hand rule to align the expected direction of magnetic field
of the coil in the same direction as BE. Pass the current through the coil and verify: Does the compass
needle move? Explain the observation.
b) Verify the relation between the magnitude of magnetic field and the magnitude of the current in coil.
Find direction of BE without any I in coil. Set up the coil so that its plane (vertical) contain the direction
of BE. This way BC will always be perpendicular to BE and therefore
BC = BE*tan θ
(2)
Find the direction "θ" of the total "B" for a set of values of current I through the coil. For each value of
"I", measure the angle θ, calculate tanθ and BI in BE units. With 4 turns of wire in the coil obtain
5 readings of I (say 0.3, 0.6, 1.2, 2, 4) and BI , record the data in table1 and plot the graph BI vs. I.
What kind of relation do you find? Does it fit to what is expected?
EXPERIMENT _2
Verify the relation between the magnetic field and the number of turns in the coil.
With a fixed I value, change the number of turns N (1,2,3,4,5) and obtain 5 different readings of BI.
Record the data in table 2 and plot a graph of BI vs. N (the number of turns) with I constant.
What kind of relation do you find? Does it fit to what is expected?
Observion. Wrap 2 loops one way and 2 loops the opposite way. Charge the current through coil
and verify BI. What do you find out? Does it fit to what you expected?
PART B CALCULATING THE MAGNITUDE OF BE
At this part, you will use the results obtained above to estimate the magnitude ( in Tesla) of the earth’s
magnetic field BE. At first, you will need the geometry of your coil (a and b in figure) and then, together
with the values N, I for a given measure (in table 1 and 2) substitute them to the expression given below
for the magnetic field of a rectangular coil at its centre (derivation as homework). Once you have a few
BI values, use the corresponding expression in BE “unit” to get the magnitude of BE in Tesla. With a
collection of a set of 5 such values you may get n final estimation of BE here at Vanier College location.
A
BI
N
2
o
I
1
*
a
2
b
2
a
b
b
a
b
TabNo1 N= 4loops BC=tanθ BE
I(A)
0.3
0.7
1.5
2
4
TabNo2 I= 1.5A
N
1
2
3
4
5
θ
tan θ
BC [ BE units]
tan θ
BC [ BE units]
BC= anθ BE
θ