Welcome Back Minions!!!

Warm Up What are the properties of magnets? How do magnetic poles interact? What is the shape of a magnetic field? How can an atom behave like a magnet? How are Magnetic Domains arranged in a magnetic material? How can magnets be changed? How is the Earth like a bar magnet? What are the effects of the Earth’s Magnetic Field?

Whatayathink… What is electromagnetism? What are the 3 characteristics found in a magnetic field produced by a current? What is an electromagnet?

Discovery! 1820 Hans Christian Ørsted Experiment Danish Physicist & Chemist Experiment Current through a wire Brought a compass near The needle moved!

He did what?

What is this? Electric current produces a magnetic field. Electromagnetism Relationship between electricity and magnetism.

Three Characteristics… Magnetic field produced by a current has 3 characteristics… 1) Field can be turned on or off. 2) Field can have its direction reversed. 3) Field can have its strength changed.

Turn it On/Off… Example of this is an electromagnet at a scrap yard. Turn current on. Magnet picks up metal. Turn current off. Magnet drops metal.

Reversed! What do you notice about the current? What do you notice about the field? What happens when you switch the current?

Stronger! When a wire is looped it bunches up the magnetic field, increasing its strength. The more loops, the greater the strength! Solenoid A coil of wire with a current

Stronger! 4 Ways… Increase the current! Add more loops of wire! Wind the coils closer together! Stronger ferromagnetic material for the core!

Make your own magnet! Battery Wire Metal rod or nail

Look familiar?

Electromagnet What is it? Produced by the current and the core! Solenoid with a ferromagnetic core. Produced by the current and the core! Overall magnetic field is hundreds or thousands of times stronger than the field produced by a current alone! It can be turned on and off!

Electromagnet Remember the crane with the electromagnet?

Right Hand “Rule of Thumb” The thumb points in the direction of the magnetic NORTH POLE. The fingers curl in the direction of the MAGNETIC FIELD.

Lenz’s Law In 1835 Heinrich Lenz stated the law that now bears his name.  An electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it.  These opposing fields occupying the same space at the same time result in a pair of forces.   These forces are felt when you turn a generator and generate electricity.  The more current you generate, the greater the force opposing you. 

Lenz’s Law (continued) This force can also be felt if you try to drag a conductive, non-magnetic plate between the poles of a horseshoe magnet.  The plate sees a changing magnetic field which creates a current in the plate, which creates its own magnetic field opposing the one that created it.  

Lenz’s Law Another great example of Lenz's law is to take a copper tube (it's conductive but non-magnetic) and drop a piece of metal down through the tube.   The piece of steel will fall through, as you might expect.  It accelerates very close to the acceleration due to gravity.   (Only air friction and some possible rubbing against the inside of the tube prevent it from reaching the acceleration due to gravity.)

Lenz’s Law Now take the same copper tube and drop a magnet through it (hopefully a strong one, Neodymium or other rare earth magnets work the best)    You will notice that the magnet falls very slowly.  This is because the copper tube "sees" a changing magnetic field from the falling magnet. This changing magnetic field induces a current in the copper tube.

Lenz’s Law The induced current in the copper tube creates its own magnetic field that opposes the magnetic field that created it.

Review… Discovered the electromagnet. Discovered magnets had an electrical field. Discovered currents had a magnetic field. Discovered compasses are not affected by currents.

Review… Electromagnetism is… 1) Sticky for metals. 2) Relationship between electricity and magnetism. 3) Useless in space. 4) Relationship between magnets and wire coils.

Review… What are the 3 characteristics found in a magnetic field produced by a current? 1) Turned on/off, repels plastic, strength changed. 2) Turned on/off, direction reversed, series circuit. 3) Fireproof, direction reversed, strength changed. 4) Turned on/off, direction reversed, strength changed.

Review… What is an electromagnet? 1) Solenoid with a ferromagnetic core. 2) Series circuit with an iron core. 3) Parallel circuit with a reversed current. 4) Comic book villain.