MAGNETIC EFFECTS OF CURRENT
In 1820, Hans Christian
Oersted while preparing for his lecture in physics noticed that electric
current passing through a wire deflects the nearby magnetic compass. By proper
investigation, he observed that the deflection of magnetic compass is due to
the change in magnetic field produced around current carrying conductor (Figure
3.32). When the direction of current is reversed, the magnetic compass deflects
in opposite direction. This lead to the development of the theory
‘electromagnetism’ which unifies the two branches in physics, namely
electricity and magnetism.
Suppose we keep a
magnetic compass near a current carrying straight conductor, then the needle of
the magnetic compass experiences a torque and deflects to align in the
direction of the magnetic field at that point. Tracing out the direction shown
by magnetic compass, we can draw the magnetic field lines at a distance. For a
straight current carrying conductor, the nature of magnetic field is like
concentric circles having their center at the axis of the conductor as shown in
Figure 3.33 (a).
The direction of
circular magnetic field lines will be clockwise or anticlockwise depending on
the direction of current in the conductor. If the strength (or magnitude) of
the current is increased then the density of the magnetic field will also
increase. The strength of the magnetic field (B) decreases as the distance (r)
from the conductor increases are shown in Figure 3.33 (b).
Suppose we keep a
magnetic compass near a current carrying circular conductor, then the needle of
the magnetic compass experiences a torque and deflects to align in the
direction of the magnetic field at that point. We can notice that at the points
A and B in the vicinity of the coil, the magnetic field lines are circular. The
magnetic field lines are nearly parallel to each other near the center of the
loop, indicating that the field present near the center of the coil is almost
uniform (Figure 3.34).
The strength of the
magnetic field is increased if either the current in the coil or the number of
turns or both are increased. The polarity (north pole or south pole) depends on
the direction of current in the loop.
The right hand rule is a
mnemonic to find the direction of magnetic field when the direction of current
in a conductor is known.
If we hold the current
carrying conductor in our right hand such that the thumb points in the
direction of current flow, then the fingers encircling the wire points in the
direction of the magnetic field lines produced.
The Figure 3.35 shows
the right hand rule for current carrying straight conductor and circular coil.
This rule is used to
determine the direction of the magnetic field. If we rotate a right-handed
screw using a screw driver, then the direction of current is same as the
direction in which screw advances and the direction of rotation of the screw
gives the direction of the magnetic field. (Figure 3.36)
The magnetic field shown
in the figure is due to the current carrying wire. In which direction does the
current flow in the wire?.
Solution
Using right hand rule,
current flows upwards.
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