2 A little history… Electricity and Magnetism are related! Until the early 19th century, scientists thought electricity and magnetism were unrelatedIn 1820, Danish science professor Hans Christian Oersted was demonstrating electric currents in front of a class of studentsWhen electric current was passed in a wire near a magnetic compass, the compass needle movedElectricity and Magnetism are related!
3 Comparing E&M Magnets exert forces on one another Similar to electric charges:Can attract and repel without touchingStrength of interaction depends on the distance of separation of the two magnetsDifferent from electric charges:Electric charges produce electrical forcesRegions called magnetic poles produce magnetic forces
4 Magnetic Poles All magnets have both a north and a south pole Like poles repelOpposite poles attractMagnetic poles always exist in pairs
5 Magnetic FieldsMagnetic Field (B): The space around a magnet in which a magnetic force is exerted [measured in Tesla – T].Magnetic Field is a VECTOR (has magnitude and direction).The direction of the magnetic field outside a magnet is from the north to the south poleB FieldNS
6 Magnetic DomainsPermanent magnets are made by placing pieces of iron or certain iron alloys in strong magnetic fieldsMagnetic Domain = large clusters of atoms lined up with each otherDomains start out randomly oriented in this piece of ironDomains align in the direction of the magnetic field as they are brought closer to a magnet
7 How are E&M related? Moving electric charges create magnetic fields Charges in motion have both E (electric) fields and B (magnetic) fields associated with themIn a bar magnet, electrons inside are constantly movingMoving charge = current magnetic fieldElectrons also spinSpinning charge =motion magnetic field
8 ElectromagnetsIf a current carrying wire is bent into a loop, the magnetic field lines become bunched up inside the loopA current-carrying coil of wire with many loops is an electromagnetIron filings sprinkled on paper reveal the magnetic field configurations about a.a current-carrying wire,b. a current-carrying loop, andc. a coil of loops
9 The Right Hand RuleOut of the pageInto the pageTo find the direction of the magnetic field in a wire, point the thumb/fingers of your right hand in the direction of current flow.Your fingers/thumb point in the direction of the magnetic field.
10 The Right Hand RuleWhat is the direction of the magnetic field in this wire?What is the direction of the magnetic field in this coil or wire?
11 ANSWERS What is the direction of the magnetic field in this wire? What is the direction of the magnetic field in this coil or wire?B FieldB Field
12 When MOVING electric charges are placed in magnetic fields, they feel a FORCE. Force is greatest when the particle moves perpendicular to the magnetic fieldForce becomes less at angles less than 90 and zero when the particle moves parallel to the field lines
13 Magnetic ForceThe force that acts on a moving charged particle depends on the particle’s charge, its velocity, and the strength of the magnetic field.B = magnetic field [T]v = charge velocity [m/s]F = force [N]q = charge [C]F = qvB
14 The Right Hand RuleTo find the direction of the magnetic force on a chargeTake two pensHold them perpendicular to each other as in the pictureTake your RIGHT handPlace your RIGHT hand at the point where the two pens meetPush v towards BThe direction your thumb points is the direction of FOut of the pageInto the page
16 Magnetic Force cont’d F = ILB Now we know: So… F = force (N) a charged particle moving through a magnetic field experiences a deflecting forceSo…a current of charged particles moving through a magnetic field also experiences a deflecting forceF = force (N)I = current (A)L = length of wire (m)B = magnetic field (T)F = ILB
17 Magnetic Force F = qvB F = ILB When an electric charge moves in a magnetic field, it feels a force.Single charge: Many charges (current):F = qvBF = ILBF = force (N)q = charge (C)v = velocity (m/s)B = magnetic field (T)F = force (N)I = current (A)L = length of wire (m)B = magnetic field (T)
18 Earth’s B FieldA compass points northward because Earth itself is a huge magnetThe compass aligns with the magnetic field of the earthMost geologists think that moving charges looping around within Earth create its magnetic fieldThe magnetic field of Earth is not stableIt has flip- flopped throughout geologic timeStudies of deep-sea sediments indicate that the field was virtually switched off for 10,000 to 20,000 years just over 1 million years agoThe magnetic poles of Earth, however, do not coincide with the geographic poles—in fact, they aren’t even close to the geographic poles. Figure illustrates the discrepancy. The magnetic pole in the Northern Hemisphere, for example, is located some 800 kilometers from the geo- graphic North Pole, northwest of Sverdrup Island in northern Canada. The other magnetic pole is located just off the coast of Antarctica. This means that compasses do not generally point to true north.
19 Electromagnetic Induction Physicsmagnetism can produce electricity & electricity can produce magnetism
20 Electromagnetic Induction Electric current can be produced in a wire by simply moving a magnet into or out of a wireMovement of the magnet induces a voltage, which causes current flowVoltage is induced whether the magnet is moved through the wire or the wire is moved through the magnet
21 FluxMagnetic Flux = the number of magnetic field lines passing through a given areaMeasured in Webers (Wb)If a loop of wire lies perpendicular to a magnetic field, the maximum possible number of lines of flux will pass through the loop.If the loop of wire lies parallel to the field, the flux through the loop will be zero.Φ = flux (Wb)A = area of loop (m2)B = magnetic field (T)Φ = AB
22 ExampleEleanor is undergoing an MRI procedure and is placed inside a chamber housing the coil of a large electromagnet that has a radius of 25.0 cm. A flux of Wb passes through the coil opening. What is the magnetic field inside the coil?
23 greater number of loops of wire = greater induced voltage = greater current Faraday’s Law: Induced voltage in a coil is proportional to:the product of the number of loopsthe cross-sectional area of each loopand the rate at which the magnetic field changes within those loops
24 ExampleΦ = ABThe hood ornament on Abe’s sedan is shaped like a ring cm in diameter. Abe is driving toward the west so that Earth’s 5.00*10-5 T field provides no flux through the hood ornament. What is the induced voltage in the metal ring as Abe turns from this street onto one where he is traveling north, if he takes 3.0 s to make the turn?
25 Lenz’s LawAn induced voltage always produces a magnetic field that opposes the field that originally produced itIn other words:If the original magnetic field, and thus the flux, is going toward the north, the induced voltage will produce an opposing field and flux that goes toward the south
26 Generators & MotorsGenerator = A machine that produces electric current by rotating a coil within a stationary magnetic fieldA motor converts electrical energy into mechanical energy.A generator converts mechanical energy into electrical energy.
28 TransformersA transformer works by inducing a changing magnetic field in one coil, which induces an alternating current in a nearby second coilVoltages may be stepped up or stepped down with a transformer
29 Power TransmissionPower is transmitted great distances at high voltages and correspondingly low currents, a process that otherwise would result in large energy losses owing to the heating of the wires.Power may be carried from power plants to cities at about 120,000 volts or more, stepped down to about 2400 volts in the city, and finally stepped down again by a transformer to provide the 120 volts in our houses
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