Electromagnetism

Magnetic Field Lines Due to a Circular Loop and
Solenoid

Introduction:

The magnetic field lines around a straight current-carrying are in the form of concentric circles, the
center of which is located on the wire.

The Factors on Which the Magnetic Field Around a Straight Current-carrying Conductor Depends Are As Follows:

• The radii of the circular magnetic field lines increase, and the strength of the magnetic field decreases as the distance from the conductor increases.
• On increasing the current in the conductor, the magnetic field also increases.
• If the direction of the current flowing in the conductor is changed, the deflection of the magnetic needle will just be reversed.

If we bend the straight current-carrying wire into a circular current-carrying coil, the pattern of
magnetic field lines changes.

Explanation:

When we pass the vertical circular coil through two holes in horizontal cardboard and connect the
circular coil’s two free ends in series with a battery, a plug key, and a rheostat. On evenly
distributing iron fillings on the cardboard, we observe that:

• Close to the wire, the pattern of magnetic field lines is circular.
• As we move toward the center of the loop, the circles become larger and larger.
• Finally, when we reach the center of the circular loop, the magnetic field lines are straight.

If you look from one side where electric current is flowing in the clockwise direction and magnetic
field lines are entering the loop, this side of the loop represents the south pole of the bar
magnet.

Whereas while observing from the opposite side, the electric current is flowing anti-clockwise, and
magnetic field lines are leaving the loop, so this side represents the north pole of the bar
magnet.

This indicates that the current-carrying loop acts like a tiny bar magnet. Thus, by looking at the
magnetic field lines of the circular loop, the two poles, the North Pole and the South Pole of the circular
the loop can be marked.

The strength of the magnetic field (B) produced by a current-carrying coil can be increased by:

• Increasing the current (I) in the coil. (B 𝝰𝝰 I)
• Decreasing the radius (R) of the coil. (B 𝝰𝝰 1/R)
• Increasing the number of turns in the coil. (B 𝝰𝝰 N)

Let’s compare the magnetic field lines due to a straight current-carrying conductor and Magnetic
field lines due to a current-carrying circular coil. We find that the magnetic effect of electric current
increases if we convert a straight long current-carrying wire into a current-carrying coil.

We can further convert a coil into a long coil containing many close turns (solenoid).

Observations:

• When an electric current passes through the solenoid, it behaves like a bar magnet, with one side as the North Pole (N) and another as the South Pole (S).
• The magnetic field is the same at all points inside the solenoid, as the lines are parallel and equidistant. However, outside the solenoid, the magnetic field is nonuniform, as the magnetic field lines are not parallel.
• Inside the solenoid, magnetic field lines are directed from south to north, whereas outside the solenoid, they are directed from north to south.

The Strength of the Magnetic Field (B) Produced by a Current-Carrying Solenoid Increased by:

• Increasing the current in the solenoid. (B 𝝰𝝰 I)
• the number of turns in the solenoid. (B 𝝰𝝰 N)
• The nature of the material inside the solenoid. Inserting a magnetic material like a soft iron rod can increase the magnetic field several times.

A strong magnetic field produced inside a solenoid can be used to magnetize a piece of a magnetic
material like soft iron when placed inside the coil. The magnet formed is called an electromagnet.

Difference Between a Bar Magnet and an Electromagnet: