MAGNETIC EFFECTS OF ELECTRIC CURRENT
MAGNETIC
FIELD DUE TO A CURRENT-CARRYING CONDUCTOR
Experiment to find the direction of the field:
- Take a long straight copper wire, 2 or 3 cells of 1.5 V each, and a plug key. Connect them in series.
- Place copper wire parallel to and over a compass needle.
- If the current flows from north to south, the north pole of the compass needle moves towards the east.
- If the current flows from south to north, the needle moves in opposite direction (towards west).
- It means that the direction of magnetic field produced by the electric current is also reversed.
Magnetic Field due to a Current through a Straight Conductor
Insert
a straight thick copper wire through the centre, normal to the plane of a
rectangular cardboard.
Connect
the copper wire vertically between the points X & Y, in series with a
battery (12 V), a variable resistance (or rheostat), an ammeter (0–5 A) and a
plug key.
Sprinkle
some iron filings uniformly on the cardboard.
Close
the key. Gently tap the cardboard a few times. The iron filings align as a
pattern of concentric circles. They represent magnetic field lines around
the copper wire.
(a) A pattern of
concentric circles. The arrows show the direction of the field lines. (b) A
close up of the pattern.
Place
a compass at a point (say P) over a circle. The direction of the north pole of
the compass needle gives the direction of the field lines at P.
The direction of magnetic field lines is reversed if the
direction of current through the copper wire is reversed.
If we vary the current in the copper
wire, the deflection in the needle also changes. If the current is increased,
the deflection also increases. Thus, the magnitude of magnetic field
produced at a given point increases as the current through the
wire increases.
If
the compass is placed at a farther point (say Q) from the conducting wire, deflection
in the needle decreases. Thus, magnetic field produced by current in the
conductor decreases as the distance increases (inversely
proportional). The concentric circles representing magnetic field around a
current-carrying straight wire become larger as we move away from it.
Right-Hand Thumb Rule
It is an easy way to find the
direction of magnetic field associated with a current-carrying conductor.
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Right-hand thumb Rule |
Problem: A current through a horizontal
power line flows in east to west direction. What is the direction of magnetic
field at a point directly below it and at a point directly above it?
Solution
Applying the right-hand thumb rule, magnetic field
(at any point below or above the wire) turns clockwise in a plane perpendicular
to the wire, when viewed from east end, and anti-clockwise, when viewed from
the west end.
Magnetic Field due to a Current
through a Circular Loop
Suppose a straight wire is bent to form a circular loop and a
current is passed through it. Here, at every point of circular loop, the
concentric circles around it become larger as the distance from the wire
increases.
At the centre of
the circular loop, the arcs of these big circles
appear as straight lines. Every point on the wire give rise to the magnetic
field appearing as straight lines at the centre of loop. By applying the right-hand
rule, every section of the wire contributes to the magnetic field lines in same
direction within the loop.
The magnetic
field produced by a current-carrying wire at a given point depends directly on
the current passing through it. So, for a circular coil with n turns,
the field produced is n times as large as that produced by a
single turn. This is because the current in each circular turn has the same
direction, and the field due to each turn then just adds up.
Experiment:
- Take a rectangular cardboard
having two holes.
- Insert a circular coil having
large number of turns through them, normal to the plane of the cardboard.
- Connect the ends of the coil in
series with a battery, a key and a rheostat.
- Sprinkle iron filings uniformly
on the cardboard.
- Plug the key and tap the
cardboard gently a few times.
- Concentric circle patterns of the
iron filings emerge on the cardboard. At the centre, it appears as straight
line.
Magnetic Field due to a Current
in a Solenoid
A solenoid is a coil of
many circular turns of insulated copper wire wrapped closely in the shape of a
cylinder.
The pattern of the magnetic field
lines around a current-carrying solenoid looks similar to the pattern of the
field around a bar magnet.
One end of the solenoid behaves
as a magnetic north pole and the other as the south pole. The field lines
inside the solenoid are in the form of parallel straight lines. This indicates
that the magnetic field is same (uniform) at all points inside the solenoid.
If a magnetic material (e.g. soft iron) is placed inside a current-carrying solenoid, it becomes magnetised. The magnet so formed is called an electromagnet.
