Patterns of Magnetic Fields
A magnet field is produced when a current flows through a wire i.e. a current-carrying conductor produces a magnetic field.
Straight Wire:
Using a plotting compass the magnetic field pattern can be drawn. Place a wire upwards through a small hole in a horizontal cardboard and placing the compass on the board. The magnetic field lines are circles around the wire. The right-hand grip rule is used to predict the direction of the direction of the magnetic field around the wire. The strength of the magnetic field decreases further down the wire. The strength also depends on the magnitude of the current. The larger the current the greater the strength.
A magnetic field will be generated when a current flows through a wire. The magnetic field forms by straight wire are concentric circles around the wire as shown in the figure above. If the direction of the current is inversed, the direction of the magnetic field line is also inversed.
The plan view, a dot in the wire shows the current coming out from the plane whereas a cross in the wire shows the current moving into the plane.
Grip the wire with the right hand, with the thumb pointing along the direction of the current. The other fingers give the direction of the magnetic field around the wire.
The strength of the magnetic field form by a current carrying conductor depends on:
the magnitude of the current; a stronger current will produce a stronger magnetic field around the wire.
the distance from the wire; the strength of the field decreases as you move further out.
Flat coil:
Flat coil magnetic field pattern: the strength of the magnetic field is stronger along the inside of the coil than the outside. More magnetic field lines per unit area lie on the inside region of the coil and the field lines at the centre are straight and perpendicular to the plane of the coil.
The direction of the field can be determined by the Right Hand Grip Rule. Grip the wire at one side of the coil with your right hand, with thumb pointing along the direction of the current. Your other fingers will be pointing in the direction of the field.
To increase the magnetic field strength at the centre of the coil:
increase the current;
increase the number of turns if the flat coil.
Solenoids:
A solenoid is said to have poles because its’ magnetic field pattern resembles that of a bar magnet. The right-hand grip rule is used to predict the direction of the field and another way is: when viewing one end of the coil, it will be the N-pole is the current is flowing in the aNticlockwise direction, and a S-pole is the current is flowing an a clockwiSe direction.
A solenoid is a long coil made up of a numbers of turns of wire. The magnetic field of a solenoid resembles that of the long bar magnet, and it behaves as if it has a North Pole at one end and a South Pole at the other.
Imagine your right-hand gripping the coil of the solenoid such that your fingers point the same way as the current. Your thumb then points in the direction of the field. Since the magnetic field line is always coming out from the North Pole, therefore the thumb points towards the North Pole.
The magnetic field is stronger inside the solenoid. It can be increased by:
increasing the current;
increasing the number of turns per unit length of the solenoid;
using a soft iron core within the solenoid.
Uses
A simple magnetic relay:
A switching device which uses an electromagnetic, and has two circuits, one dependent on the other without direct electrical contact. The input circuit supplies current to the electromagnet, and when a certain current level is reached, the electromagnet attracts one end of the iron armature which is pivoted to make the other end act as a lever. The lever opens or closes the contacts in the second circuit. Therefore, the low current/ voltage circuit controls the higher current/ voltage circuit.
The reed switch:
Is a pair of soft iron strips inside a glass tube which contains an inert gas to prevent contamination and oxidation. There is a small gap between the reeds. Movement of the reed switch will make or break contact between the two terminals. When a magnetic field of a permanent/ electro-magnet is brought near the reed switch, the reeds become magnetised and attract each other closing the contact and activating the external circuit.
The circuit breaker:
Switches off the current when it becomes too large. The current flows along the springy copper strip through the iron armature and solenoid. The electromagnet attracts the armature if the current is too large, breaking the circuit.