What is a field?
•
A region of
space characterized by a physical property having a determinable value at every
point in the region. Examples: gravitational field, Electric field ,and
magnetic field
This means if we put anything appropriate in a field,
we can then calculate “something” out of that field. In the
gravitational field, this “something”
is called gravitational force.
Gravitational
Field
A = acceleration
G = gravitational constant G=
r = distance to the center of the big object
m = mass of big object
Example: Imagine the pink circle is the Earth and the lines
pointing inward are the gravitational field. If we put Bob in the gravitational
field, we can calculate the Force acting on him by the Earth. He will follow
the gravitational field lines and fall to the surface of the Earth.
(flash can be seen in the power point presentation)
Essentially, if we put anything that has a mass in the
Gravitational field, we can calculate that “something” I have mentioned before. That “something” is the “gravitational force” in this case. We can
determine different kinds of force
with different kinds of field.
Electric Field
E = Electric Field
F = Electric Force
q = Charge
Electric field is defined as the electric force per unit
Charge. If we put a +/- (positive or negative) charge in
a electric field, the +/- charge will experience a force. This force is called
“Electric Force”. (IMPORTANT!!) Recall from G-field. G-field
requires an object with a MASS to have a force. E-field requires
a +/- charge to have a force. It is the surrounding charges that create an
electric field
How do you draw
the E-field lines with different charges?
Electric field lines always comes out of the positive
charge into the negative charge. Negative charge always absorb the E-field
lines
What direction is
the electric force?
The direction of the Electric force is parallel or tangent
to the electric field.
Example: Look at the POINT P on the diagram above (far right).
The arrow in the diagram represents the direction of the electric force
Q: What
is the charge, positive or negative, if the direction of the force is pointing
outward like in the diagram above?
A: Positive
because it is point away from the stationary positive charge
Q: What
is the charge, if the direction of the force on the diagram switched?
A: Negative
charge
Magnetic Field
B = magnetic field
F = Magnetic
Force
Q = charge
V = velocity
of the moving charge
Magnetic field is a field that exerts a force on a moving
charge. A magnetic field can be caused either by another moving charge or by a
changing of electric field or magnetic dipoles of materials
How do you draw
the magnetic field lines (B-field lines)?
Magnetic field lines come out of N and go into S (see
diagram above)
What is the SI
units for magnetic field?
The unit of Tesla is a very SMALL unit
-
Earth magnetic field has around 10^(-5) Tesla
-
A very good Ferromagnet has around 0.5 to 1 Tesla
It is very hard to make material that has a very strong
magnetic field
Right Hand Rule
Charge moving in a magnetic field obeys the Right Hand
Rule. There are two types of RHR, RHR #1 and RHR #2
Right Hand Rule 1
The thumb represents the velocity of which the + charge
(i.e. current) is going. The remaining fingers tell you the direction of the
magnetic field
Right Hand Rule 2
I = direction of the charge
B = direction of the Magnetic Field
F = Force act on the charge
Palm Push Positive (*Remember!!)
Examples:
Path of electron moving perpendicular to the
magnetic field that is pointing out of the WEBPAGE
Path of electron moving perpendicular to the
magnetic field that is pointing into the WEBPAGE
Question to ponder
Put a Ferro magnet (N or S) close the turned on
CRT from the side.
Note that the electron beam will either go upward or
downward
Q:
What is the magnetic field (N or S) on the side facing the CRT?
A:
It depends on which side you put your magnet close to the CRT, but you can just
use RHR #2 to identify the N or S field on the Ferro magnet
Conclusion
The electromagnetic field is a physical
field that is produced by electrically charged objects and which affects the
behaviour of charged objects. The field can be viewed as the combination of an
electric field and a magnet field. The electric field is produced by stationary
charges, and the magnetic field by moving charges (currents), changing of the
electric field, and dipoles of the material.