sph4u university physics

11/26/2012

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SPH4UUNIVERSITY PHYSICS

GRAVITATIONAL, ELECTRIC, & ... FIELDS

L Magnets & Electromagnets

(P.378-385)

Auroras

As you know, the swirls of light that you sometimes see in the sky at nightare the aurora borealis (or the aurora australis if you are in the southernhemisphere). And when you see these lights did you know that you arereally watching the interplay of electricity and magnetism. But why do theymove and swirl?

November 26, 2012 4U3 - Magnets & Electromagnets 1

Permanent Magnets

Recall that the magnetic field lines of abar magnet extend from the northpole to the south pole outside themagnet and from south to north insidethe magnet, forming a closed loop.Magnetic field lines always formclosed loops – you can never haveonly a south pole or only a north pole.


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Permanent Magnets

PRACTICE

1. Where is the magnetic field thestrongest?

at the poles


Permanent Magnets

NOTE!

The magnetic field lines of a barmagnet are similar to the electric fieldlines of an electric dipole. While ironfilings will align with the magnetic field(B), the electric dipoles align with the

electric field (g).


Permanent Magnets

Just as opposite electric chargesattract, opposite magnetic polesattract. The effect is different,however, when like poles approacheach other – they repel each other.This is analogous to the way twopositive electric charges or twonegative electric charges repel eachother, except that the magneticpoles are electrically neutral.


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Permanent Magnets

PERMANENT MAGNETS

� all magnets have magnetic poles

� opposite poles attract and like poles repel

� a magnetic field surrounds all magnets andflows from north to south outside the magnet

and from south to north inside


Earth’s Magnetic Field

The largest magnet on Earth is Earthitself. If we were to place compassesat different spots in Earth’s magneticfield, each compass needle would bealigned parallel to the field.

NOTE!

Earth’s north geographic pole isactually a south magnetic pole.However, it is convention to refer tothis south magnetic pole as Earth’snorth magnetic pole.



As you already know, Earth’smagnetic field affects the motion ofcosmic rays. Charged particlesentering Earth’s magnetic field aredeflected and spiral along the fieldlines toward the magnetic poles.This motion results in a concentrationof charged particles at Earth’s northand south magnetic poles (i.e. theaurora borealis and australis).


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NOTE!

The circled x on the right indicatesthat the force is directed into thepage for that particle and vice versafor the particle on the left (recall theright-hand rule).



Our knowledge of what causes Earth’smagnetic field is incomplete, but severalclues point to an explanation. First,Earth’s magnetic poles move slowly fromday to day and year to year. Second,geological studies show that Earth’smagnetic field has completely reverseddirection many times during the planet’shistory.



NOTE!

The flipping of Earth’s magnetic field hasbeen used as a theory to help explain theextinction/evolution of some species.


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Electric currents in Earth’s core probablycause this behaviour of the magnetic field.Earth’s core is made of liquid metal. Thisliquid conducts electricity, and the spin ofEarth about its axis causes the liquid tocirculate much like the current in aconducting loop. The circulating currentcauses a magnetic field. Scientists believethat circulation within Earth’s core has acomplicated flow pattern that varies withtime. These variations cause changes inthe magnetic field, resulting in themovement of Earth’s magnetic poles.



EARTH’S MAGNETIC FIELD

� resembles that of a bar magnet

� changes orientation over time

� is able to redirect the motion ofcharged particles from space

(auroras)


Earth’s Magnetic Field – DYK?

Scientists have discovered many livingcreatures that use Earth’s magnetic fieldin different ways. For example,magnetotactic bacteria (see micrograph),honey bees, homing pigeons, anddolphins all rely on Earth’s magnetic fieldin some way.

NOTE!

The dark, round dots inside the cell aremagnetite crystals.


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Electromagnetism

In 1820, Danish physicist Hans ChristianOersted was demonstrating how a wirebecomes warmer when electric charge flowsthrough it. In the course of his demonstration,he noticed that the needle in a nearbycompass moved each time he switched on theelectricity. This strange event led Oersted toconclude that a magnetic field surroundsmoving electric charges. This is now known asthe principle of electromagnetism.


Electromagnetism

Moving charges, like those in an electric current, produce a magnetic field.Current in a straight wire or other long, straight conductor creates amagnetic field whose lines look like circles centred on the wire. You candetermine the direction of the magnetic field lines around a straight wire byusing the right-hand rule for a straight conductor.


Electromagnetism

If you make a circular loop from a straight wire and run a current throughthe wire, the magnetic field will circle around each segment of the loop.The field is strongest inside the coil because the field lines are closertogether. To determine the direction of the magnetic field in coiled wire,you must use the right-hand rule for a solenoid.


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Electromagnetism

NOTE!

Applying a current through a solenoid asdescribed earlier causes the solenoid tobecome an electromagnet. Strongerelectromagnets can be made by using asolenoid with a magnetic material, such asiron or nickel, within the coil. The effect ofthis core material is to increase the strengthof the magnetic field by aligning the electronswithin the core material in such away as toenhance the magnetic field.


Electromagnetism

ELECTROMAGNETISM

� moving electric charges produce a magnetic field

� straight conductor

• thumb points in direction of current

• curled fingers indicate direction of magnetic field lines

� coiled conductor

• fingers curl in direction of current

• thumb indicates direction of magnetic field lines through the centreof the coil (i.e. thumb points to the north pole)


U Check Your Learning

TEXTBOOK

P.385 Q.5


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