Magnetism

Magnet Properties All magnets have polarity - a North Pole and a South Pole If you cut magnet in half……. Can you make a monopole? N S It makes two magnets! N S N S No – a chopped magnet will turn itself into two N/S magnets

Magnetism and Planet Earth Like poles repel, Opposites attract (N and S) North Pole of suspended magnet (compass) seeks geographic South So why do compasses point north? Because the magnetic pole in the N. Hemisphere is in actuality the SOUTH MAGNETIC pole of the Earths magnetic field!!! Geographic North & Magnetic South Geographic South & Magnetic North

Magnetism and Planet Earth Remember: The north geographic pole is near the south magnetic pole of the earth (and vice versa)

Magnet Make-up Only certain materials can be magnetized Iron (Fe) Gadolinium (Gd) Cobalt (Co) Nickel (Ni) These materials are ferromagnetic

Where does magnetism come from? Oersted discovered that moving electric charges create magnetic fields All magnetism arises from MOVING ELECTRIC CHARGES Spin of electrons in atom creates a magnetic domain If magnetic domains are aligned, then material is magnetic

How to Ruin a Magnet To ruin a magnet, cause the magnetic domains to become randomized Drop it Heat it The Curie temperature is the temperature above which a material loses all magnetic properties Expose it to another magnetic field VHS and cassette tapes record sound/video by aligning magnetic material on the tape.

How to Create a (Temporary) Magnet Place an object made of a ferromagnetic substance in a larger magnetic field Rub the object over and over again in the same direction (aligns magnetic domains) Run current through a coil of wire (electromagnet)

Drawing Magnetic Field Lines Magnetic Field lines are drawn as loops Point from NORTH to SOUTH How to draw magnetic field lines for the Earth? Point up from southern hemisphere (magnetic North) toward northern hemisphere (magnetic south) If the dark part of my compass is “north,” which way should it point when put to the right of the magnet? I I I I I I I I I I I I I I I I I I I I I I I I

Electromagnets A solenoid is a current-running coil of wire Produces a magnetic field like the magnetic field of a bar magnet. Found in doorbells, car starters Electromagnet – A solenoid that is wrapped around a ferromagnetic core To make an electromagnet, all you need is: Wire (the more loops the better) Battery (the more voltage the better) Ferromagnetic core

From here on out we will refer to a magnetic field as a B-field Magnetic Force From here on out we will refer to a magnetic field as a B-field A charge will feel the greatest force when it is moving PERPENDICULAR to the B-Field Charge feels no net force if it is moving PARALLEL to the B-Field

Magnetic Fields Symbols for drawing B-fields: Arrows – going left/right or up/down into the page (like seeing an arrow on a target) out of the page (like seeing an arrow coming toward you) Point charges move PARALLEL to electric fields and PERPENDICULAR to magnetic fields

B-Field Around a Wire Right-hand rule Thumb = direction of current flow Wrap fingers as if grabbing wire Fingers show you direction of B-Field into the page out of the page

Magnetic Force – Point Charge Magnetic fields are described using Tesla (T). A particle carrying a charge of 1 coulomb and passing through a magnetic field of 1 tesla at a speed of 1 meter per second perpendicular to said field experiences a force with magnitude 1 newton Two magnets will apply a Force on each other Therefore, moving electric charge (which has a B-field around it) can be forced to move by another B-field Fm = Force (Newtons) q = Charge (Coulombs) v = velocity (m/s) B = Magnetic Field (Tesla)

Magnetic Force on Current-Carrying Wire F: Force in Newtons I: current in Amps L: length in meters (direction of current) B: magnetic field in Tesla

B-Field around a Wire in a Loop More loops = stronger B-Field Direction of the B-field depends on current flow through the wire A coil of wire is called a solenoid You can magnetize a solenoid by running electrical current through it I

Right-Hand Rule This technique helps you to figure out which way the magnetic force is directed. Thumb: force (F) Forefinger: velocity or current (v or I) Middle finger: magnetic field (B) Negative Q: F is same as the thumb’s direction Negative Q: F is opposite the thumb’s direction Force x v or I field

Physics Gang Symbols To determine the direction of the variables in our two equations, we need to know the “Physics gang symbols” These directions are for ELECTRONS Thumb = direction of B-field (thumb tip points like arrow) Remember – “ThumB” has a “B” in it First finger = direction of the velocity Remember - ONE finger – ONE object has ONE velocity All other fingers = force FingerS – NewtonS (both end in ‘s’) Fingertips point toward force felt by electron Knuckles point toward force felt by proton

Physics Gang Symbols Determine the direction of the velocity of the electron for each scenario. - B F - B F - B is out of the page V = (out of the page) F V = (into the page) V = - B is into the page F V =

Sample Problem #1 An electron in a beam experiences a downward force of 2 E -14 N while traveling in a magnetic field of 8.3 E -3 Tesla directed to the West. Find the DIRECTION and MAGNITUDE of the velocity of the electron. - B F

Sample Problem #2 A PROTON traveling in a magnetic field experiences a downward force of 2 x 10-14 N of 8.3 x 10-3 Tesla directed to the West. Find the DIRECTION and MAGNITUDE of the proton’s velocity. + B F

Electricity Magnetism + and – charges N and S poles like charges repel like poles repel unlike charges attract unlike poles attract electric monopole exists no magnetic monopole electric field lines flow from + to - magnetic field lines flow from N to S density of lines equals strength of E strength of B SI unit: ampere, 1 A = 1 C/sec SI unit: Tesla, 1 T = 1 N/Amp meter E exerts force on a charge, or E = F/q Field exerts force on a moving charge, or B = F/(qvsin)