Magnetism: IntroPhysics 12

Clip of the day:

•Minutephysics…..▫How far is a second?▫http://www.youtube.com/watch?v

=Wp20Sc8qPeo&list=PLED25F943F8D6081C

Review Question:1. A soccer ball is kicked horizontally off a

22.0-meter high hill and lands a distance of 35.0 meters from the edge of the hill. Determine the initial horizontal velocity of the soccer ball.

• ANSWER: 16.5 m/s

Magnet Brainstorm….and play time!

•Are all magnets alike?•What kind of things do magnets attract?•Which part of the magnets are strongest?•How can you tell where the poles are?

▫How can you tell which is which?•Do magnets work through other

materials?▫What does it depend on?

•Can you make a magnet out of a nail?

Shall we start with Bill Nye on magnets?

•http://www.youtube.com/watch?v=ak8Bh9Zka50

Magnetism:•Magnets have 2

poles (north and south)

•Like poles repel•Unlike poles attract•Magnets create a

MAGNETIC FIELD around them

Magnetic Force:•Magnetic forces have properties that

are similar to electric and gravitational force in that:▫The force varies following an inverse

square relationship due to the separation of poles

▫The force varies directly with the strength of the poles

•However, since we don’t have magnetic monopoles there is no equation for magnetic force

Magnetic Domains:•Within a magnetic

compound, there are many different magnetic domains

•If these are randomly arranged, the material is not magnetic

•If they are aligned, the material will be magnetic

Temporary vs Permanent Magnets:• Some materials will easily align the domains in the

presence of a magnetic field; however they will usually return to a random arrangement after the field is removed; these are called temporary magnets (like iron)

• Other materials will not easily align the domains, however once aligned they will remain aligned; these are called permanent magnets (like steel)

Magnetic Field Lines:•It is possible to map a magnetic field in a

manner similar to that of electric or gravitational fields

•Convention says field lines go from north to south

Magnetic Field:

•Symbol =B • SI units =tesla (T)

T = N__ A• m

Electric Charges and Magnetic Poles:•In 1819, Oersted

noticed that a current carrying wire would create a magnetic field

•This was the first step into the realm of electromagnetism

•Oersted’s experiment led to Right Hand Rule #1

Oersted’s experiment • When there is no

current the needle of the compass points north

• When the circuit is complete, electricity flows and the compass needles deflected

• There must be some forces causing the deflection…magnetic force!

Why does the needle move?THE WIRE ITSELF MUST BE MAGNETIC!!! In other words the wire has its own INTERNAL MAGNETIC FIELD that is attracted or repulsed by the EXTERNAL FIELD.

As it turns out, the wire’s OWN internal magnetic field makes concentric circles round the wire.

Electromagnetism:•Moving electrons produced a magnetics

field •A changing magnetic field causes

electrons to move

Right Hand Rule 1:

• The direction of magnetic field lines around a current carrying wire can be determined using a right hand rule

• Thumb = direction of current

• Fingers curl around in direction of magnetic field

Ampere’s experiment:• Ampere discovered

that when compasses were placed close to a current carrying wire the compass would be deflected based on the direction of the current

• A the needles are moving counter clock wise

• B the needles are moving clock wise

• Ampere theorized that the magnetic fields between two charge carrying wires would either result in attractive or repulsive forces

• Based on this, Ampere was able to develop an equation to describe the magnetic field

IL

FB

Magnetic Field:

Length of wire (m)

Current (A)

Force (N)Magnetic field (T)

For a solenoid:

Number of turns

Length of each turn (m)

Sample problem:

Try it:

•Page 767 ▫1 a, b, c and 2

•Page 778▫1-4