Ch. 21: Magnetism

Magnetism• magnetic poles produce magnetic forces• poles always exist in pairs (N and S)• opposite poles attract, like poles repel• there are no magnetic “monopoles”

Magnetic Fieldsmagnetic field lines (B-field) always point from N to S

+ + + + ++ + + + ++ + + + +

• • • • •• • • • •• • • • •

Big and Little Magnets

• currents within the mantle produce the earth’s field• in atoms, orbiting and spinning electrons produce tiny magnetic fields • Fe, Ni, and Co are the most magnetic elements

Objectives

• Understand and apply the first magnetic “right hand” rule.

• Understand and apply the second magnetic “right hand” rule.

• Understand practical applications of electromagnets.

• Understand and explain the concept of magnetic domains.

Electric Current and B-Fields

• Hans Christian Oersted (1820) first noticed that an electric current will deflect a compass needle• first right hand rule

Electric Current and B-Fields

• a current in a coil (or solenoid) produces an electromagnet• second right hand rule

I

B

How a Speaker Works

Magnetic Domains• domains are clusters of billions of iron atoms with aligned fields• domains will align in a B-field• permanent magnets have been exposed to very strong fields• heat destroys magnets because domains become random

Objectives

• Understand how magnetic force is applied to moving charges.

• Apply the third “right hand” rule.• Understand some common applications of

magnetic force.• Solve magnetic force problems.

Magnetic Force

• a charged particle moving perpendicular to a B-field feels a force

• 1 Tesla (T) = 1 N/(C · m/s) = N/(A·m)

• third right hand rule:

F q v Bmagnetic

Auroras

Magnetic Force Problem

• A proton moving at 1200 km/s (in the solar wind) runs perpendicular into the earth’s magnetic field (B = 55 mT). How much force is applied to the proton? What is the acceleration of the proton (m = 1.67 x 10-27 kg)?

Particle Accelerators

Mass Spectrometer• mass spectrometer: an instrument that measures

the mass of charged particles• used to identify elements present in a sample

Magnetic Force on a Wire

• a current-carrying wire in a B-field will feel a force perpendicular to the wire

• How much force is applied to a 5-cm long wire carrying 12 A of current when it is placed in a 3 mT magnetic field?

F q v B

F B I Lmagnetic

m agnetic

Chapter 22: Induction and Alternating Current

Magnetic Fields and EMFs

• Michael Faraday (1831) and Joseph Henry:• electromagnetic induction: the production of a

current caused when a conductor is moved through a magnetic field (or the magnetic field is changed)

• emf: electromotive force; an increase in PE per charge (voltage) that pushes charges through a conductor; emf produces a current

• Use the 3rd right hand rule to determine direction of current.

Lenz’s Law

• Lenz’s law: the magnetic field of an induced current opposes the change in the applied magnetic field

• energy is conserved due to this “magnetic friction”

Faraday’s Law

• N = number of loops• A = area• B = magnetic field• t = time

• Use this law to calculate the voltage generated by a spinning coil.

em f N ABt

[ co s ]

Applying Faraday’s Law

Objectives

• Be able to explain how/why a generator works.

• Be able to explain how/why an electric motor works.

• Understand how different commutators are used to produce/use AC versus DC.

Generators and Motors

• generator: converts KE to electrical energy (current)

• spinning a coil in a B-field causes an AC to form

• commutator: determines if AC or DC

• armature: multiple-loop coil

Electric Motors• motor: a device that converts electric energy (AC or DC)

to KE

Transformers

• transformer: converts AC to higher or lower voltage (step up or step down)

• V2 = V1N2 / N1

• Electricity is transmitted at high V, low I (due to “I2R loss”) then stepped down

• 230kV to 20kV to 120V