EM 2 Electric and Magnetic Fields Electric Field

EM 2 Electric and Magnetic Fields

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

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

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

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)

Examples

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

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/

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

Electric field strength • E=kq/r 2 E = electric field strength (N/C) k = 9 x 109 (N m 2 / C 2) q = Charge (C) r = radius or distance (m)

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 1016 N/C

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

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

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)

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

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

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

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

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

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)

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

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

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

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

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)

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

“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”

“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)