Magnetism

and its applications

Laws of Magnetism - 1) Like magnetic poles repel, and 2) unlike poles attract.

Magnetic Direction and Strength

• Law 3 - Magnetic force, either attractive or repelling varies inversely as the square of the distance between the poles. For example, if the distance between two magnets with like poles is increased to twice the distance, the repulsive force reduces to one-quarter of its former value.

• The direction of any magnetic field is defined as the direction the north pole of a magnet would point if placed in the field.

• The magnetic field of a magnet points from the north pole of the magnet to the south pole.

Earth’s Magnetic Field

The magnetic north pole of Earth corresponds to the geographic South Pole, and the magnetic south pole corresponds to the geographic North Pole.

The Magnetic Field

• A magnetic field exists around any current-carrying wire; the direction of the magnetic field follows a circular path around the wire.

• The Danish physicist, Hans Christian Oersted, discovered that a magnetic field existed around a conductor carrying an electric current.

Magnetic Force

• The direction of the force on a positive charge moving through a magnetic field can be found using the right-hand rule.

Magnetic Force

Example - A proton moving east experiences a force of 8.8 x 10-19 N upward due to the Earth’s magnetic field. At this location, the field has a magnitude of 5.5 x 10-5 T to the north. Find the speed of the particle.

You Try!

• A proton moves perpendicularly to a magnetic field that has a magnitude of 4.20 x 10-2 T. What is the speed of the particle if the magnitude of the magnetic force on it is 2.40 x 10-14 N?

(1q = 1.6 x 10-19C)• A proton traveling to the right along the x-axis

enters a region where there is a magnetic field of magnitude 2.5 T directed upward along the y-axis. If the proton experiences a force of 3.2 x 10-12 N, find the speed of the proton.

Answers

Attraction and Repulsion

• Two parallel current-carrying wires exert on one another forces that are equal in magnitude and opposite in direction. If the currents are in the same direction, the two wires attract one another. If the currents are in opposite directions, the wires repel one another.

Electromagnetic Induction• EMF – Voltage from induction (electromotive force)• The magnitude of the induced emf depends on the velocity with

which the wire is moved through the magnetic field, the length of the wire, and the strength of the magnetic field.

• Every time the bar crosses a line of force a current is induced. increases in strength. The induced current in the coil must be in a direction that produces a magnetic field that opposes the increasing strength of the approaching field. The induced magnetic field is therefore in the direction opposite that of the approaching magnetic field.

• Lenz’s Law - The magnetic field of the induced current opposes the change in the applied magnetic field.

• Faraday’s Law - If a circuit contains a number, N, of tightly wound loops, the average induced emf is simply N times the induced emf for a single loop.

InductionExample: A coil with 25 turns of wire is wrapped

around a hollow tube with an area of 1.8 m2. Each turn has the same area as the tube. A uniform magnetic field is applied at a right angle to the plane of the coil. If the field increases uniformly from 0.00 T to 0.55 Tin 0.85 s, find the magnitude of the induced emf in the coil. If the resistance in the coil is 2.5 ohms, find the magnitude of the induced current in the coil.

Practice – You Try!

• A single circular loop with a radius of 22 cm is placed in a uniform external magnetic field with a strength of 0.50 T so that the plane of the coil is perpendicular to the field. The coil is pulled steadily out of the field in 0.25 s. Find the average induced emf during this interval.

• A coil with 205 turns of wire, a total resistance of 23 ohms, and a cross- sectional area of 0.25 m2 is positioned with its plane perpendicular to the field of a powerful electromagnet. What average current is induced in the coil during the 0.25 s that the magnetic field drops from 1.6 T to 0.0 T?

Answers

Solenoid and Electromagnetic Coils

• The magnetic field created by a solenoid or coil is similar to the magnetic field of a permanent magnet.

Magnetic Domains

• A domain is a group of atoms whose magnetic fields are aligned.

• Hard disks and magnetic tape lay down domains as a way to store data. Un-magnetized material has domains that are randomly oriented.

Applications Electromagnet.

THE RELAYThe relay is a device used to control a large

flow of current by means of a low voltage low current circuit. It is a magnetic switch. When

the coil is magnetized, its attractive force pulls the lever arm, called an ARMATURE, toward the coil. The contact points are touched and

the large current flows through.

Electric Motor

Electric Generator•In 1831 another great scientist, Michael Faraday, pondered this question: If electricity would produce magnetism; can magnetism produce electricity? Based on the research and discoveries of Mr. Faraday, the electric dynamo was developed. He is known as the “father of the dynamo.”

•In order to produce an electric current, there must be a magnetic field, a conductor and RELATIVE MOTION BETWEEN THE FIELD AND THE CONDUCTOR. A GENERATOR is a device which converts mechanical energy into electrical energy.

•A MOTOR is a device which converts mechanical energy into electrical energy.

•EMF – Electromotive Force