EM2 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/r2

E = electric field strength (N/C)k = 9 x 109 (N m2 / C2)

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/a2)

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)