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)