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Electromagnetism
Electromagnetism is a process where a magnetic field is created by introducing the current in the conductor. When a conductor is electrically charged it generates magnetic lines. For example, if current i.e., positive charges move in a wire, it produces the magnetic field along the wire, and the direction of magnetic lines and force can be determined using Right-hand Rule. Refer to the image for a detailed explanation.
The electromagnetic force is a type of physical interaction that occurs between electrically charged particles. It acts between charged particles and is the combination of all magnetic and electrical forces. The electromagnetic force can be attractive or repulsive.
Electromagnets
An electromagnet is a soft metal core made into a magnet by passing an electric current through a coil surrounding it .
They only maintain their magnetism if current continues to flow, if switched off they lose their magnetism.
Factors affecting the strength of an electromagnet.
- Increasing current through the coil.
- Increasing the number of turns of the coil.
- Using iron of C- core shape which brings both magnetic poles together.
Some applications of electromagnets.
- a) Electric bell
When the switch is closed the current passing through the solenoids magnetizes them and they pull the soft iron armature which makes the hammer hit the gong therefore producing sound. When the hammer hits the gong the contact between the spring and the screw is broken and then stops the current from flowing.
The soft iron core loses its magnetism and releases the armature which is then pulled back by the screw.
The contact between the spring and the screw is regained and the process repeats itself again and again therefore the gong is struck continuously.
- b) Telephone receiver.
It consists of a u-magnet made by attaching two soft-iron bars to the end of a short permanent magnet.
The solenoids are wound in opposite directions around the bars. When the phone is lifted the current flows through the solenoids depending on the microphone on the other end of the line.
These varying current spasms induce magnetism of varying strengths in the iron bars which in turn causes the magnetic alloy diaphragm to vibrate differently producing sound.
Force on a current-carrying conductor in a magnetic field.
When a conductor carries a current in a magnetic field a force acts on it.
The direction of the force depends on the directions of the field and current.
The factors affecting the magnitude of the force are;
- a) The current flowing in the conductor
- b) The strength of the magnet
- c) The length of the conductor in the magnetic field.
The relationship between the directions of the current, field and force are mutually perpendicular.
They are summarized in a law called Fleming’s right-hand rule or the motor rule.
This rule states that “if you hold the first finger, the second finger and the thumb of your left hand mutually perpendicular to each other, so that the first finger points in the direction of the magnetic field and the second finger points in the direction of the current in the conductor, then the thumb points in the direction of the force acting on the conductor”.
Applications of the force on a conductor.
Simple D.C motor.
Consists of a rectangular coil of wire mounted on an axle which can rotate between the poles of a magnet.
For the rotation to be continuous the ends of the coil is connected to half -rings called the split-ring commutators.
The battery terminals are attached to brushes which slide on these half-rings. D.C motors are useful as car starter motors, hand drills, machine motors, fans etc.
