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EM2 Electric and Magnetic Fields

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Electric Field Electric Field (E)- A region where a positive charge experiences a force

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Vectors An electric field has magnitude and direction (vector quantity)

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Drawing Electrical Fields When drawing an electrical field, you show the direction a small POSITIVE test charge would move if put in the field – Test charge-Charge measuring an electric field

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Rules for Drawing Electrical Fields (Similar to magnetic field lines) 1. Field lines are perpendicular to the surface of the charged objects 2. Field lines never cross each other 3. Electric field lines point from positive (out) to negative (in)

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Examples

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Electric Field Strength The strength of a magnetic field is determined by the amount of force acting on a charge in the field – The force is strongest near the surface of the charged object (close to a charge) Represented by lines that are close together

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Faraday’s Cage a hollow, conducting shell that does not possess any electric field, even when it is placed in a very strong external electric field. The charges on the conducting surface rearrange themselves in such a manner that the electric field within the shell becomes zero http://www.faradaycage.org/

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Electric Field Strength E=F/q E=electric field strength (N/C) F=force (N) q=charge (C)

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Electric field strength E=kq/r 2 E = electric field strength (N/C) k = 9 x 10 9 (N m 2 / C 2 ) q = Charge (C) r = radius or distance (m)

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Example #1 An electron (1.6 X 10 -19 C) experiences a force of 2.3 X 10 -3 N. Calculate the electric field strength. 1.4 x 10 16 N/C

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Example #2 A charge (1.5 X 10 -15 C) creates an electric field with a strength of 3.2 X 10 -6 N/C at point P. How far away is point P? 2.0 m

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Magnetic Fields Magnetic field-Region where a pole (north) experiences a force

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Magnets There is no such thing as a north or south all by themselves If you break a magnet in ½, each piece will have a N and S pole (due to the arrangement of the atoms throughout the magnet)

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Magnetic Fields Like poles repel each other – South pole and South pole repel – North and North repel Unlike poles attract each other – South pole and North pole attract

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Magnetic Field A magnetic field has both magnitude (strength) and direction (vector quantity) Can be represented by vectors (arrows)

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Rules for Drawing Magnetic Fields 1. Magnetic field lines (flux lines) are perpendicular to the surface where they touch the magnet 2. Magnetic field lines never cross each other 3. Magnetic field lines point from North to South

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Compass If you put a compass in a magnetic field, the compass will line up parallel to the magnetic field lines

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Magnetic Field Strength Magnetic field strength is strongest close to the poles of the magnet – Gets weaker as you get farther from the magnet

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Current and Magnetic Fields Current (moving charge)-Rate a flow of charge moves through a wire In Physics, the flow of positive charges (from positive to negative)

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Current Current is NOT how fast charge moves through a wire, but how much charge moves through a wire

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Math: Current I=q/t I=current (C/s or amps) q=charge (C) t=time (sec)

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Current When current passes through a wire, a magnetic field is created which circles the wire (moves around it)

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Current The strength of the magnetic field is influenced by the amount of current in the wire and the distance from the wire

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Mathematically: Strength B=KI/r B=magnetic field strength (N/(a)(m) I=current in wire (amps) R=distance from wire (m) K=magnetic constant (2 X 10 -7 N/a 2 )

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Magnetic Strength If bend wire into a loop, the magnetic field lines bunch up inside the loop The magnetic field is strongest at the center of the loop

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“Right Hand Rule” B is a vector quantity (has direction) To determine the direction of the magnetic field around a straight, current carrying wire, use the “right hand rule”

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“Right Hand Rule” The thumb of your right hand points in the direction of the positive current (I) Your fingers curl in the direction of the magnetic field (B)

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