Transcript PHY2054-S09-REV2

Chapter 20-21-23
Review
The behavior of bar magnets
Our Earth itself has a magnetic field
Charges moving with respect to a field
Charges moving with respect to a field
Charges moving with respect to a field
The Right Hand Rule
Using the right hand rule, one may determine the direction of the
field produced by a moving positive charge.
Magnetism and circular motion
F = |q|vB
If the motion is
Circular
F = mv2/R
R = mv/ |q|B
ω = v/R = |q|B/m
Force on a conductor with current
F = ILB
Applications of force on a conductor
Magnetic field of long straight conductor
Magnetic field of a long, straight wire:
B = μ0I/(2πr)
r is the distance from the wire
μ0 is called the permeability of vacuum
μ0 = 4π x 10-7 T.m/A
Fields in two conductors side-by-side
Fields in two conductors side-by-side
2 wires with currents flowing in the same direction
attract each other
2 wires with currents flowing in opposite directions
repel each other
F = μ0 L(I1 I2)/(2πr)
Force per unit length
F/L = μ0 (I1 I2)/(2πr)
Currents in a loop
Magnetic field at the center of a circular loop
B = μoI /(2R)
For N loops:
B = μo NI /(2R)
Electromagnetic Induction
Does the field induce a current or not?
Magnetic flux at various orientations
Magnetic flux at various orientations
Magnetic flux at various orientations
FRADAY’s LAW
• When the magnetic flux ΦB changes in time,
there is a an induced emf directly proportional
to the time rate of change of the magnetic flux :
ɛ = |Δ ΦB /Δt |
If we have a coil with N identical turns, then
ɛ = N |Δ ΦB /Δt |
Vab = vBL
a
b
Lenz’s Law
Lenz’s Law
Transformers
TRANSFORMERS
V2 / V1 = N2 / N1
If energy completely transformed
V1I1 = V2I2
Energy associated with an induced current.
•energy is stored in an electronic device.
The R-L circuit
The L-C circuit
•
Copyright © 2007 Pearson Education, Inc. publishing as Addison-Wesley
In the case of an inductor with a capacitor,
the energy is transferred from the electric field
(capacitor) to magnetic field (inductor) and vice versa.
The total energy is however conserved:
The back and forth of the energy constitutes an oscillatory
behavior with a frequency ω:
•A metal loop moves at constant velocity toward a long wire
carrying a steady current , as shown in the figure . The
current induced in the loop is directed
•A) Clockwise B) counterclockwise C) zero
• A metal loop moves at constant velocity toward a long
wire carrying a steady current , as shown in the figure .
The current induced in the loop is directed
• A) Clockwise B) counterclockwise C) zero
B out of page increasing
ΔΦ out of page
Bi into page
•The slide wire of the variable resistor in the figure is moved
steadily to the right, increasing the resistance in the circuit.
While this is being done, the current induced in the small circuit
A is directed :
•A) clockwise
B) counterclockwise
C) zero
• A) clockwise
B) counterclockwise
C) zero
I=V/R I decreases when R increases
B due to I decreases as I decreases
B out of page and decreases hence ΔΦ into page
Bi out of page
I