Let’s learn about magnetic fields and magnetic field lines. The substance that has the ability to pull or attract certain things towards itself is referred to as a magnet. A magnet only attracts the substances which have the metals iron, nickel or cobalt in them.
Magnets are of two different kinds:
- Natural Magnets
- Artificial Magnets
The natural magnets possess the ore magnetite in them. They are weak and often irregular in shape.
When a natural magnet is rubbed against an iron or steel bar, the same property of attraction is communicated to these bars. These are then called artificial magnets. Around every magnet, there is a space in which the force of attraction or repulsion due to the magnet can be detected. This space is called the magnetic field.
Let us now discuss the various properties of magnetic field lines.
- A magnetic field line has both direction and magnitude and is thus a vector quantity. Outside a magnet, magnetic field lines are directed from the north pole to the south pole.
- They are closed and continuous curves.
- The magnetic field lines are crowded near the poles and far apart near the center, thus imparting a stronger magnetic field at the poles as compared to the center where the magnetic field is weak.
- The field lines never intersect each other because it would imply two directions of the magnetic field at that point which is impossible.
- Parallel and equidistant field lines represent a uniform magnetic field.
Magnetic field due to a current through a circular loop
After Oersted’s discovery of the magnetic effect of current-carrying wire, Ampere found that a loop of wire also had a magnetic field. In order to find the field of magnitism due to a current-carrying circular coil, the coil is held in a vertical plane and is made to pass through smooth cardboard at points A and B in such a way that the center of the coil lies above the cardboard.
A current is passed through the coil and iron filings are sprinkled on the cardboard. These iron filings arrange themselves in a specific pattern in circular and concentric rings. The direction of the field lines can be found by applying the right-hand thumb rule.
At the center of the circular loop, the arcs of the big circles appear as straight lines. The magnetic field at the center of the coil can be taken to be uniform and also maximum, as the two semi-circular segments of the coil through A and B assist each other.
Magnetic field due to a current carrying solenoid
An insulated copper wire wrapped on a non-conducting cylindrical tube (for e.g.plastic) such that its length much greater than its diameter is called a solenoid. This is the pattern of the magnetic lines around a current-carrying solenoid. If you compare the pattern of the field so formed with the magnetic field around a bar magnet, you will find them quite similar.
In fact, one end of the solenoid behaves as a magnetic north pole while the other behaves as a south pole. The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the field is the same at all points inside the solenoid i.e. the field is uniform inside the solenoid.
Magnetic field due to a Straight Current carrying Conductor
When a current is passed through a straight conductor the magnetic needle placed parallel to this shows deflection. This is because a magnetic field is produced around a current-carrying conductor
This field can be traced with the help of iron filings and a small compass needle.
Take a thick conductor (copper wire) XY and pass it through the centre O of a thick card board. Connect the ends of the conductor to the terminals of a battery through a rheostat; a key and ammeter A so that the current flow from Y to X. Sprinkle some iron fillings uniformly on the cardboard. Close the key so that the current flows through the wire. Gently tap the cardboard a few times. We observe that the iron filings arrange themselves in concentric circles around O.
These concentric circles represent the magnetic field lines. Now place the compass needle at any point over the circles and observe the direction of the needle. The direction of the north pole of the compass needle would give the direction of the field lines produced by the electric current through the straight wire. When we change the direction of the current in the copper wire the direction in the compass needle placed at a given point also changes
Now increase the current in the copper wire, the deflection in the compass needle also increases. This indicates that the magnitude of the magnetic field produced at a given point increases as the current through the wire increases. But as we move the compass needle away from the conductor, the magnitude of the field decreases
To find the direction of the magnetic field due to a straight current-carrying conductor. We use the right-hand thumb rule.
The rule states that- If a conductor carrying a current is held in the right hand such that the thumb is pointed in the direction of the current then the direction in which your fingers encircle the wire gives the direction of the magnetic lines of force around the wire. The rule is also called Maxwell’s corkscrew rule
Suppose current through a horizontal power line flows in east to west, the direction of the field at a point directly above the wire is from south to north but the direction below the wire is from north to south.