Chapter 26: Magnetic Field and Electromagnetism

Magnetic Fields
A force field in which a magnet, a wire carrying current or a moving charge experience a force is magnetic field. Magnetic fields are produced by current-carrying conductors or by permanent magnets. An electromagnet makes use of the magnetic field created by an electric current; which a coil is used to concentrate the magnetic field which is called solenoid.

Current Carrying Wire
When a current flows through a straight piece of wire it creates a circular magnetic field. The right hand grip rule shows us direction of magnetic field. If we use our right hand and do a thumps up then thumb is the direction of the conventional current and the fingers point the direction of the field line

Fleming’s Left Hand Rule

This rule links the direction of the force, magnetic field and conventional current which are all at right angles to each other. Your first finger points from north to south, middle finger points from positive to negative and thumb points in the direction of the force.

Magnetic flux density

The strength of the magnetic field is known as its magnetic flux density, B.

The magnetic flux density is greater at the closed poles and gets weaker as you move away from it.

From the gravitational field strength g at a point we get;

g = F/m

Electric field strength E is defined as;

E = F/Q

The magnetic flux density at a point is the force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point;

B = F/IL

Where F is the force experienced, I is current in conductor and L is the length of the conductor.

Current Crossing Fields

The magnetic fields circle around and in the space between the wires there is an extra-strong field, creating repulsive force on the two wires. In the space between the two wires, the magnetic fields cancel out hence the wires are pushed together.

F = (B sinθ) IL

F = BIL sinθ