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Magnetic Forces and Magnetic Fields Chapter 21
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Chapter 21. Magnetic Forces and Magnetic Fields. 21.1 Magnetic Fields. Magnets, as you know, can exert forces on one another. In electricity, we talk about negative and positive dipoles or charges. In magnetism, we discuss north and south poles. - PowerPoint PPT Presentation

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Magnetic Forces and Magnetic Fields

Magnetic Forces and Magnetic FieldsChapter 21

21.1 Magnetic FieldsMagnets, as you know, can exert forces on one another. In electricity, we talk about negative and positive dipoles or charges. In magnetism, we discuss north and south poles.Like poles repel each other, and unlike poles attract.Electric charges vs. MagnetsElectric chargesCan be positive or negativePositive and negative charges can be separated so that a (+) or (-) charge is isolated.Produce an electric field that is a vector quantityElectric field points away from positive and toward negativeMagnetsHave a negative end and a positive end.ALL MAGNETS have a negative and positive or north and south end. Produce a magnetic field that is a vector quantityMagnetic field direction is determined by the direction of the north pole of a compass at a particular pointLines tend to originate at north and end at south without stopping in betweenSome VocabularyAngle of declination: angle that a compass needle deviates from the north geographic poleAngle of dip: the angle that the magnetic field makes with respect to the surface at any pointMagnetic north pole: true north pole as generated by the earth due, most likely, to currents of iron moving in the coreGeographic north pole: where the Earths axis of rotation crosses the surface in the Northern HemisphereClick here for an interactive description of the difference.21.2 The Force that a Magnetic Field Exerts on a Moving ChargeMagnetic force can be added to our bucket list of forces that can cause objects to accelerate and can be used in conjunction with Newtons 2nd Law of Motion.For a Charge to Experience a magnetic force when place in a field:The charge must be moving, for no magnetic force acts on a stationary charge.The velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field.Force on Moving ChargeIf the charge moves parallel or antiparallel to the field, the charge experiences no magentic force.If the charge moves perpendicular to the field, the charge experience the maximum possible magnetic force.If the charge moves at an angle, , only the velocity component (vsin), perpendicular to the field gives rise to a magnetic force.Right-Hand Rule #1Extend the right hand so the fingers point along the direction of the magnetic field (B) and the thumb points along the velocity of the charge. The palm of the hand, then, faces in the direction of the magnetic force that acts on a positive test charge.If the moving charge is negative, the direction of the magnetic force is opposite from described above.

Definition of Magnetic Field

Direction of field is determined by a small compass needle. SI Unit: Newton second/coulomb meter = 1 Tesla If magnetic field is much less than one Tesla, a gauss (G) is often used as a unit for magnetic field. 1 gauss = 10-4 tesla21.3 Motion of a Charged ParticleElectric FieldDirection of electric force is same as direction of electric fieldForce does work and increases KEMagnetic FieldDirection of magnetic force is always perpendicular to magnetic field and velocitySince displacement and force are perpendicular, no work is done by this forceForce changes direction but not magnitude of velocity

The Circular TrajectoryThe Force on a Current in a Magnetic FieldSince an electric current is a collection of moving charges, a current in the presence of a magnetic field can also experience a magnetic forceModify RHR-1 by replacing direction of velocity with direction of conventional current in order to determine direction of force.The magnetic force is maximum when the wire is oriented perpendicular to the magnetic field.Magnetic Force on current-carrying wireThe Torque on a Current-Carrying CoilIf a loop of wire is suspended properly in a magnetic field, the magnetic force produces a torque that can rotate the loop.This torque is responsible for the operation of an electric motor.When a current-carrying loop is placed in a magnetic field, the loop tends to rotate such that its normal becomes aligned with the magnetic field. Basically, the current loop behaves like a magnet suspended in a magnetic field.Physics of DC Electric Motorhttp://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html#c1http://www.learnapphysics.com/apphysicsb/magnetism.php

Magnetic Fields Produced by CurrentsA current-carrying wire will produce a magnetic field of its own.A compass needle will align itself with the net magnetic field produced by a current and the magnetic field of the earthThus, the beginning of the study of electromagnetism.

Long, Straight WiresLong, Straight Wires