7.1 Basic Ideas of Magnets

p. 262

Permanent MagnetsA magnet has two poles:

North-seeking pole points NorthSouth-seeking pole points South

Two poles interact with each other:Like poles repel each other. Unlike poles attract each other.

When a current is placed in a coil of insulated wire a strong magnetic field in generated creating an electromagnet. One major advantage of an electromagnet is the ability to turn the magnetic field on and off.

Alnico are strong permanent magnets made up as an alloy of cobalt, iron and aluminum. Ferromagnetic elements include iron, nickel and cobalt, and lesser known ones such as neodymium and gadolinium.

7.1 Basic Ideas of Magnets

Magnetic Fields and Lines of Force

p. 263

A bar magnet generates a magnetic field. The lines of magnetic force show the direction that an isolated north pole would take. The lines of magnetic force go from the north pole to the south pole.

The magnetic lines of force are packed closer together at the north and south poles of the magnet, showing these regions to have the greatest strength in the magnetic field.

Sprinkling iron fillings around a bar magnetic shows the magnetic field of a magnet.

7.1 Basic Ideas of Magnets

Electromagnets

p. 264

Hans Christian Oersted (1777 - 1851) noticed that a near-by compass was deflected whenever current was made to flow in a wire. Through further experimentation he showed that the magnetic field around the conductor was circular in nature. Andre Ampère showed that when a high current is made to flow through a conductor a circular magnetic field can be shown by sprinkling iron filings around the current carrying wire.

A solenoid is a coil of insulated wire. The coil of wire intensifies the magnetic field and generates a magnetic field very much like a bar magnet.

7.1 Basic Ideas of Magnets

Ampère’s Rule

p. 265 - 266

Also called right hand rule allows one to determine the direction of the magnetic field around a current carrying wire. Place your right hand right around a wire with the thumb pointing in the direction of the conventional current, your slightly open fingers will now point in the direction of the magnetic field.

When a piece of iron is placed inside a solenoid an electromagnet is formed. The iron core intensifies the magnetic field of the solenoid and creates a much stronger magnetic field that can be turned off and on.

7.1 Basic Ideas of Magnets

How Magnetic Fields Affect Moving Charged Particles

p. 268

A moving charged particle such as an electron or proton will be deflected by a magnetic field. An electric field can move a stationary charged particle, by a magnetic field only places a force on a moving charged particle.

Another Rule of Thumb: The Right Hand Motor RuleThis rule is used to predict the force acting on charged particles moving through magnetic fields.

1) Point your right thumb in the direction that a positively charged particles would move.

2) Point your fingers in the direction of the magnetic field.

3) Your palm now faces in the direction that the positive charges would be pushed due to the magnetic field.

A magnetic field altering the path of a beam of electrons in a CRT.

7.1 Basic Ideas of Magnets

Inv. 7.1.1 Deflection of a Beam of Electrons Using s Magnetic Field

p. 244

In Ch. 5 you were introduced to the CRT and deflection based on electric fields. The electron beam can also be deflected by a magnetic field as shown on the left.

If the accelerating voltage is kept constant then the deflection of the beam depends on the magnetic force created by the magnetic field.

y α F/v2

y = constant x F/v2

The amount of deflection, y, depends on the speed of the electrons and the strength of the magnetic field as shown by:

y =1 F x l 2

2 mv2

7.1 Basic Ideas of Magnets

Key Questions

In this section, you should understand how to solve the following key questions.

Page 267 – Quick Check #1 & 2

Page 272 – 273 – Review 7.1 #3,5,7,8,9, &10