Magnetism Chapter 17 Magnets and Magnetic Fields Section

Magnetism Chapter 17

Magnets and Magnetic Fields Section 17. 1

Introduction l Magnets l Lodestones- 1 st naturally occurring magnetic rocks l Contain Magnetite- an iron based material l From Greece

Magnets l Permanent magnets- substances that are magnetic all the time (lodestones) l Can weaken (can’t eliminate) magnetism: l Heat l “soft”- or hammer it easy to magnetize, but lose magnetism easily or quickly l “hard”- difficult to magnetize, but stays magnetized for awhile

Magnets (LISTEN!!) l Exert a magnetic force on each other l EX: use a magnet to pick up a paper clip, that paper clip will pick up other paper clips l Chain of paper clips has a limit, (move away from magnet weakens the force) l Paper clip will not be permanently magnetized

Magnets l Magnetic pole- one of two points that have opposing magnetic qualities l Like poles repel l Opposite poles attract l North and South poles l CAN’T isolate one pole l Cut a magnetic in ½ you will still have a N and S pole on each piece

Magnetic Fields l Don’t write!! Listen! l When two North poles are facing each other the force that causes that magnet to move away, or the force you feel pushing magnets apart when you try to force them together are a result of magnetic fields

Magnetic Fields l Magnetic fields- a region where a magnetic force can be detected l Produced by all magnets l Depends on: l Material of magnet l Degree of magnetism

Magnetic Fields l Magnetic field lines l Field lines form closed loops l Gets weaker with distance from magnet l Magnet’s field is strongest at poles

Magnetic Fields l Compasses track magnetic fields l Earth’s magnetic field is like a bar magnet l Has 2 poles l Background l Earth’s Info core is Iron (too hot 4 magnetism) l Believed that the liquid outer core’s electrons cause the magnetism l Field has reversed at least 20 times in 5 mil yr

Magnetic Field l Earth’s magnetic poles are NOT the SAME as the geographic poles l Field points from geographic South Pole to geographic North Pole l The magnetic North pole is in Antarctica l The magnetic South pole is in Canada

Magnetic Field l Magnets named for the geographic pole that they seek l. N end of magnet is a “north seeking” pole l S end of magnet is a “south seeking” pole

Magnetism from Electric Currents Section 17. 2

Magnetism from Electric Currents l Hans Christian Oersted l Discovered that moving electric charge produced magnetism l Electric currents produce magnetic fields l Free moving iron fillings will align with the magnetic field (fig 6) l No current=no magnetic field

Magnetism from Electric Currents l Right Hand Rule (how to predict direction of a field) l Imagine holding the wire in your right hand w/your thumb pointing in the direction of the (+) current, the direction your fingers would curl is in the direction of the magnetic field l Never touch an uninsulated wire you may be electrocuted

Magnetism from Electric Current l Magnetic field of a coil of wire resembles that of a bar magnet l Coiling a wire=stronger magnetic field l Solenoid- a coil of wire with an electric current in it l Has a north and south pole l Strength depends on # of loops in wire and amount of current in wire

Solenoid Cont’d l Increase solenoid strength by inserting an iron rod in the center of coils l Electromagnet- a coil that has a soft iron core and that acts as a magnet when an electric current is in the coil

Magnetism from Electric Currents l Magnetism caused from moving charges l Some atoms have own magnetic field l Magnetic atoms rotate to align w/the magnetic fields of other atoms forming domains l Magnetic fields of atoms in a domain point in the same direction

Electromagnetic Devices l Galvanometers- instrument that detects, measures, and determines the direction of a small electric current l Electric motor- a device that converts electrical energy into mechanical energy l Stereo speakers use magnetic force to produce sound

Electric Currents from Magnetism Section 17. 3

Introduction l Electromagnetic induction- the process of creating a current in a circuit by changing a magnetic field l Created by: l Pushing a magnet through a coil

Faraday’s Law l An electric current can be produced in a circuit by a changing magnetic field l Induced l Loop Current of wire passing between 2 magnet poles in a direction that is NOT parallel to the field l Rotate the circuit l Change the strength of the field

Faraday’s Law & Electromagnetic Induction l Moving electric charges experience a magnetic force when in a magnetic field l Magnetic force is 0 when the charge moves along or opposite the direction of the magnetic field lines l Magnetic force is at max when charge moves perpendicular to field

Faraday’s Law & Electromagnetic Induction l Max current is produced when wire moves perpendicular to magnetic field l Zero current (no induced current) when wire moves parallel to magnetic field

Faraday’s Law & Electromagnetic Induction l Generators convert mechanical energy to electrical energy l Generator- a machine that converts mechanical energy into electrical energy l Alternating current- an electric current that changes direction at regular intervals l AC generator l Most outlets

Generators l Pg 579 Table 1 l When metal loop is parallel to magnetic field you have 0 current l Current increases as loop rotates l Metal loop is perpendicular to magnetic field you have max current l Current decreases as loop rotates

Generators Produce electricity used in homes l Mechanical energy comes from several sources l Dams (water on turbines) l Coal Power Plants (heat from burning coal makes steam) l Wind l Geysers (geothermal) l Solar power l Nuclear fission l

Generators l Electricity and magnetism are 2 parts of a single electromagnetic force l Resulting energy formed is called electromagnetic energy l Light is an example (EM waves) l Made of oscillating electric and magnetic fields that are perpendicular to each other l Both fields are perpendicular to direction the wave travels

Transformers l Transformer- a device that increases or decreases the voltage of alternating current l 2 wires on opposite sides of iron loop Secondary current (an appliance) l Primary current (electric socket) l l Direction of the current in secondary coil changes when the direction of the primary coil changes

Transformers l Increase l More of decrease voltage coils = more voltage l Equal # of coils = voltage is the same on both sides l Adding more secondary devices as long as coil #’s match then each device will have same amount of voltage

Transformers l Step-up transformer- voltage across the secondary coil is greater than the voltage on primary coil l Used near power plants l Step-down transformer- secondary coil has fewer loops than primary coil l Used in your home