Read Ch. 19 First, a little background on Magnetism… As you know, magnets exert forces on one another… Just as electric charges produce electric forces, magnetic _____ produce magnetic forces. These poles are termed _____ & _____ Like poles _____ & opposite poles ______ poles northsouth repelattract

These poles always exist in pairs… When you break a magnet, you get two smaller magnets!!

Magnetic Fields …the “force field” that surrounds a magnet The magnetic field lines can be seen by placing iron filings (tiny magnets themselves) into a field. The filings “line up” along the direction of the field lines. ( __________ would do the same!) Compasses

The direction of the field lines is DEFINED to be from NORTH TO SOUTH, which is the direction that a compass would point when placed into the field.. Demo with compass and bar magnet Demo with iron filings/cow magnets AND large magnet NS NN

The Earth’s Magnetic Field More than 2000 years ago, in a region of Greece called Magnesia, rocks were discovered with magnetic properties. They contained ____ ____ and were eventually used as the first compasses…. The earth is a GIANT _______! iron ore magnet

If you were to suspend a bar magnet from a string (tied at its center) and let it “settle”, you would find that the NORTH pole of the magnet would be pointing North! So, the earth’s North Pole is really a _____ _________ pole! Note: Earth’s magnetic poles are NOT in the same location as the earth’s geographic poles (on earth’s rotational axis) … See any globe!! south magnetic

A Little History… Electricity and Magnetism were regarded as two unrelated phenomena until the early 1800’s. In 1819, Hans Christian Oersted discovered (quite by accident!) that a compass ______ when placed near a current- carrying wire. moved

The following drawings summarize our observations: The magnetic field around a wire is a _______ pattern. When the current direction is changed, the compasses _________ I circular flip 180°

A “Right-Hand Rule” … So, the direction of the magnetic field depends on the direction of the current, I. The direction of the field can be found using what’s called a “Right-Hand Rule”: Point thumb in direction of current ( + to - ) Fingers will “curl” in direction of magnetic field I + -

Right hand rule Continued: When looking for the force, place you index finger in the direction of current and middle finger in the direction of the magnetic field. Then, you will get the direction of the deflecting force.

What would happen if an insulated, current-carrying wire is bent into a loop? I The magnetic field lines inside the loop become more _____________, meaning that the strength of the magnetic field is ________ inside the loop. If MORE loops are coiled, concentration ___ even more, ____ the magnetic field strength. concentrated increased ↑ ↑

A coil of wire is called a ________. The following diagram shows how the right- hand rule can give us the magnetic field orientation around a solenoid. solenoid

The graphic below was created with three programs: The magnetic field was calculated using Mathematica, while the coloring was achieved with Adobe Photoshop. The solenoid creation, itself, and final rendering were done with a program called TrueSpace 4.3.

Hey…. That’s the same magnetic field that we observed around a bar magnet!!

So now we know that the motion of electric charge (a current) produces a magnetic field, but how, then, can a bar magnet be explained? Every “spinning” e - is a tiny magnet. 2 e - s “spinning in the same direction  ______ the field 2 e - s “spinning” in opposite directions ______, which is why most materials are NOT magnetic. The field is NOT canceled in Iron (Fe), Cobalt (Co), and Nickel (Ni) double cancel

Magnetic Domains The magnetic north-south axes of groups of iron atoms line up in the same direction. These groupings are called magnetic domains. In unmagnetized iron, the domains are _________ oriented, as shown at right. When the unmagnetized iron is placed into a magnetic field, the domains begin to _____. The more the domains are aligned, the stronger the resulting magnetic field. randomly align Demo: Paper Clip magnet

An Application: The Electromagnet What is it? A coil of insulated wire wrapped around an iron core that is magnetized only when current flows through the wire. The magnetic field created by the current induces the magnetic domains in the iron to align, increasing the strength of the field. How can an electromagnet be made stronger? Increase the _______ and/or the __________. current # of coils

“Real World” EXAMPLES: 1. Junk Yard electromagnets 2. “Fire” Door electromagnet

3. Circuit Breakers (From HowStuffWorks.com) When the switch is flipped to the on position, electricity can flow from the bottom terminal, through the electromagnet, up to the moving contact, across to the stationary contact and out to the upper terminal.

The electricity magnetizes the electromagnet. Increasing current boosts the strength of the electromagnet, and decreasing current lowers the strength. When the current jumps up to unsafe levels, the electromagnet is strong enough to pull down a metal lever connected to the switch linkage. The entire linkage shifts, tilting the moving contact away from the stationary contact to break the circuit. The electricity shuts off. howstuffworks.co m/circuit- breaker2.htm