Magnets and Magnetic FieldsPhysics 1-2 Chapter 21
How did magnets get their name?First discovered about 3000 years agoMagnesia, GreeceFirst naturally occurring magnetic rock, lodestoneMade up of iron-based material
Where do magnetic fields come from?All magnetic fields arise from electric currents.In the case of permanent magnets in ferromagnetic materials, the currents are from unpaired electrons orbiting the nucleus.
Magnetic PolesMagnets have a north pole and a south poleLike charges:Opposite poles attractLike poles repelCant isolate south pole from north poleIf magnet is cut, each piece will still have two poles
What are magnetic domains?Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.
Properties of MagnetsPermanent ALL the time, called permanent magnetsExample: lodestonesClassified as magnetically hard or softSoft magnets:Example: ironEasily magnetizedLoses its magnetic properties easily
Properties of MagnetsHard magnets:Example: cobalt, nickelMore difficult to magnetizeDont lose magnetism easily
Soft magnets:Example: Iron in a staple or in a nail.Easy to magnetizeCan be easily demagnetized by physical shock or heating
Magnets exert magnetic forces on each otherExample:When magnet is lowered into bucket of nails, it can pick up a chain of nailsEach nail is temporarily magnetized by nail above it (exert magnetic force on nail below it)Limit to how long chain of nails can beThe farther from the magnet smaller magnetic force
Magnets exert magnetic forces on each otherEventually, magnetic force not strong enough to overcome force of gravity bottom nails fall
Magnetic FieldsForce exerted by magnets acts at a distanceExample:Move south pole of magnet toward another south poleMagnet will move awayOther forces act at a distance:Gravitational forces, force between electric charges
Magnetic FieldsALL magnets produce a magnetic field Strength of magnetic field depends on:Material magnet made fromHow much object is magnetizedHow far from magnet.Magnetic field lines used to represent magnetic field Like electric field lines represent electric field
Magnetic Field LinesDirection is defined as the direction that the north pole of a compass will point at that location. ( go from N to S )Form closed loopsField lines that are closer together strong magnetic fieldField line that are farther apart weak magnetic fieldMagnetic field strongest near poles
Magnetic Field Lines
How do compasses work?Analyze magnetic fields directionCompass: magnet on top of pivot Aligns with Earths magnetic field Can be used to determine direction as Earth acts like a giant bar magnet
Earths Magnetic FieldEarths magnetic poles not same as geographic polesGeographic north pole (Canada) magnetic south poleGeographic south pole (Antarctica) magnetic north polePoles of magnet named for geographic pole they point toN: north-seeking poleS: south-seeking pole
Earths Magnetic Field
Earths Magnetic Field Source of magnetism is unknownEarths core made mostly of iron but too hot to have magnetic propertiesCirculation of ions or electrons in liquid layer of Earths core?Direction of Earths magnetism has changed20 reversals in last 5 million years We are due for a reversal in the next few thousand years!Aurora Borealis/Australis:Solar wind (charged particles emitted from sun) is deflected by Earths magnetic field
Aurora Borealis- Northern lights
Aurora AustralisAurora Australis
AurorasAuroras are only visible at night in extreme northern or southern latitudes.In cases of unusually high solar activity, the auroras may be visible further south.
Ch21.2 : ElectromagnetismThere is a magnetic field associated with any current (there is no magnetic field without a current!)The magnetic field lines are co-encentric circles around a straight wire. The field line direction is given by the right hand rule. Thumb points in the direction of the current and fingers gripping the straight wire point in the direction of the field.
Right hand rule (P770, hons P662):
SolenoidA long helically wound, insulated electric wire. The magnetic field is concentrated within the coil. It is further concentrated when a ferromagnetic material is placed inside the coil. Electromagnet: A magnet that consists of a solenoid and a ferromagnetic core. The magnetic field can be switched on and off with the flow of electric current.
21.3 Relationship between current and magnetic fieldA charge moving (a current) through a magnetic field experiences a force.F = B q v Sin F is force in NewtonsB is magnetic field strength in Tesla, Tq is charge in coulombs and,V is the velocity of the charge is the angle between mag field and motion direction.
Right hand rule shows direction of force (P774, P650 Hons) on a positevely charged particle USE LEFT HAND FOR ELECTRONvBF
Homework : Hons. P679 Q1,2,3 (use Voltage to calculate v first), 4, 5, 6. Draw picture!)Reg. P 775 Q1 to 5. Draw a picture!
Force on a current carrying conductorF = B I lWhere I is the current and l is the length of the conductorThere is therefore a force between any 2 current carrying conductors (note the demo)Do Q 1 5 page 778AND P779 1-5
The force on a current carrying conductor has uses in motors, moving coil meters (any meter with a needle) and in any device where electrical energy converts to kinetic energy where motion is produced.
An explanation of how a motor workshttp://youtu.be/fWyzPdyCAzU
Chapter 22 Induced currentIf a conductor is placed in a varying magnetic field, a voltage is induced in the conductor. (Faradays First Law)If the conductor can form a circuit, a ______ will flow.This induced voltage (emf) can happen in one of the following ways:1) Move the conductor into or out of the field.
Inducing voltage (contd.)2) Circuit is rotated in the field (angle between conductor and field changes)3) Change the intensity of the magnetic field.Before we go and do numerical problems based on this idea do some concept problems on the previous topics:P769 Q1-4P779 Q1-5
Practical applications of electromagnetic inductionMotor: This is more of an application of F = BIL . A current flowing through a loop of wire between two magnet poles experiences a force that causes rotation (See and understand demo). When the motor turns 180, a commutator (switch) changes the direction of the current so that the force is now changed 180, and rotation continues.
A moving coil meter (galvanometer) is like a motor without the commutator and it also has a spring to return it to zero.Generator: Identical to motor in construction BUT: the coil is forced to rotate with the magnetic field by an outside force (ex. A turbine) and the induced voltage causes a current to flow.
Speaker works due to F = BIL. Sound is just a pattern of changing pressure (vibration).A loudspeaker or headphone has a wire coil placed in a permanent magnetic field. Current passed through the coil causes the coil to experience a ______ . If the current changes at the same rate as the sound, the speaker coil and the permanent magnet interact to vibrate the coil at the same frequency as the desired sound.
Microphone identical to the speaker construction but the coil moves due to sound vibrations, causing a current to be induced in the coil. This current can than be recorded or increased (amplified).A microphone is to a speaker as a motor:generator is to a motor:generatorA motor is to a generator as a microphone:speaker is to a microphone:speakerA guitar pickup works the same way as a microphone.
TransformerTwo coils of wire that have a magnetic material between them. When an A.C. current flows in the primary coil, a changing magnetic field is produced in the magnetic material. This changing magnetic field induces a changing current in the secondary coil. Voltages may be changed:V2 = [N2 / N1] V1Where 1 means primary, 2 secondary, N = # of turns of wireDo Q1-6 page 818 Hons. P722 Q54-57 and 59
Pole transformer38,000V to 240VWhy are the 38kVwires (on top) thinnerthan the 240V wires?Pad transformer
Transformers (contd.)Transformers do not work with direct (constant) current as a changing magnetic field is necessary to induce a voltage in the secondary.Alternating current (changes direction 60 times per second in US, 50 /sec outside Americas) is necessary for a transformer to work.
Depending on the ratio of N2 /N1 the voltage may be stepped up or down by any amount. Efficiency can be as high as the high 90s % with some energy lost as heat in the coils and due to eddy currents producing heat in the magnetic core itself. Large transformers have cooling systems to remove this heat which can lead to failure.
Thomas Edison built a network of power plants in major cities producing DC current.They had to be very close to the power users and the voltage produced was the same as voltage consumed.Nicola Tesla emigrated to the US and was asked to solve the problem of supplying power to gold and silver mines in the West. Mines (where power was used) were often miles from fast-running rivers (where power was produced). Wires had to be very thick ($$$) if low voltage used (P=VI).