EM2 Electric and Magnetic Fields
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)