INTRODUCTION

Magnetic effect of current was first discovered by Oersted in 1820. He noticed that a compass needle besides a current-carrying conductor is always deflected. If a current-carrying conductor is placed in a magnetic Held due to some source other than itself, the two fields will interact to produce a mechanical force ; a principle that is used in. The electric motor.

MAGNETIC FIELD FOR A CURRENT-CARRYING CONDUCTOR

The figure below shows the experimental set-up that can be used to show that a magnetic field is created around a current-carrying conductor.

When current is passed through the conductor wire, the iron filings are seen to line up in circles centred on the wire.

• This shows that a magnetic field has been created around the conductor and the field lines are centred on the wire.
• The compass needle shows the direction of the field at any point on the cardboard.
• The current should be about 20 A so that the field created should be strong enough to displace the irons filings.

MAXWELL 'S CORKSCREW RULE

The direction of the magnetic field around a current-carrying conductor can be determined using Maxwell's corkscrew rule and this rule states: "Imagine a right-handed screw being driven into the direction of the conventional current, and then the direction of rotation of the screw gives the direction of the magnetic field lines."

Figure below shows the field lines about two conductors whose directions are determined using Maxwell's screw rule.

FIELD PATTERNS FOR CURRENT-CARRYING CONDUCTORS

For a single current-carrying conductor, the field pattern is as shown in figure above.

However when two or more current-carrying conductors are placed side by side, the various fields interact to produce a resultant magnetic field.

Two current-carrying conductors placed side by side

If two current-carrying conductors are placed side by side, the two fields interact as shown in the figure below.

In (a) Current direction the same (b) current flowing in opposite directions e. g. a loop

ln (a), the lines concentrate more at the sides than the centre the wires are pushed towards each other (attraction). At the neutral point, X, the two fields cancel out.

Meanwhile in (b), the lines concentrate more between the wires than the sides; so the wires are pushed apart (repulsion). In this case, there is no neutral point.

Field pattern due to current in a solenoid

A solenoid is a long coil of wire having so many turns. When current flows through a solenoid, a magnetic field pattern is created around it which is similar to that of a bar magnet. This is shown in figure the figure below.

The direction of the magnetic field can be determined using the right-hand grip rule which can be stated as follows:

"Imagine gripping the coil using your right hand in such a way that your fingers are in the same direction as the conventional current arrows on the coil. Then your thump points towards the North Pole."

Inside the solenoid, the field is uniform.

Note that the right-hand grip rule can also be used to determine the field direction around a current-carrying straight conductor. In this case, imagine gripping the wire in such a way that your thumb points towards the direction of the conventional current, then the other four fingers indicate the direction of the field.

Factors affecting the strength of the field around a solenoid

The strength of the magnetic field around a solenoid is determined by

1. The magnitude of the current; The greater the current, the stronger the field created
2. The number of turns per unit length; the larger the number of turns per unit length, the stronger the magnetic field created.
3. The medium of the core; the magnetic field of a current-carrying solenoid is stronger if a soft magnetic material like iron. Is inserted into its middle. The field becomes stronger because the soft magnetic material concentrates the field lines passing through it.