Physics 12 Assignment
Chapter 16 – Electromagnetism
KEY
1.
• Magnetic dipole - another name for a magnet that always has two poles such as a bar
magnet
• Domain - small region under a particular magnetic influence so that all its magnets point a
particular direction
• Electromagnetism - phenomena associated with moving electrons producing a magnetic
field and a changing magnetic field causing electrons to move
• Solenoid - a closely wound helix of wire that acts as a magnet when current runs through
the wire
• Right-Hand Rules #1, #2, & #3 -the rules which help to visualize the directions of vectors
by using the fingers and thumb of your right hand.
#1. used to determine the direction of the magnetic lines of force from a currentcarrying conductor: imagine grasping a current-carrying conductor so that your
thumb lies along the wire in the direction of the current and your fingers curl around
it; then the magnetic lines of force encircle the wire in the direction of your fingers
#2. used to indicate the direction of the magnetic lines of force in a solenoid:
imagine grasping a solenoid so that your fingers point in the direction of the current,
then your thumb points in the direction of the magnetic lines of force inside the coil
#3. used to find the direction of the force exerted on a conductor by a magnet: with
your hand flat, point your thumb along a conductor in the direction of the current
and your fingers in the direction of the magnetic field (from the magnet), then your
palm faces in the direction of the force that the magnet’s field exerts on the
conductor
• Rotor - the rotating part of an electric motor, which consists of a coil with an iron core, also
called armature
• Armature – see rotor
• Split ring commutator - a device which allows continuous connection of the rotating rotor
(or armature) to the rest of the circuit in a motor or generator; used in DC motors and
generators to reverse the current direction
• Slip ring commutator - a device that allows the continuous connection of the rotating rotor
(or armature) to the rest of the circuit in a generator; used in an AC generator to transmit
alternating current
• AC generator - a device which converts mechanical energy into electrical energy. Here, the
induced current alternates or changes its direction every half-cycle; the current from an AC
generator has a sinusoidal form
• DC generator - a device which converts mechanical energy into electrical energy. Here, the
current always leaves in the same direction
• AC motor - a device which converts electrical energy into mechanical energy. The AC
motor runs on alternating current through the use of a slips-ring commutator
•
•
•
2.
DC motor – a device which converts electrical energy into mechanical energy. The DC
motor runs on direct current through the use of a split-ring commutator
Electromagnetic induction - the generation of a current in a wire (circuit) due to the relative
motion of the wire and a magnetic field
Lenz’s Law – a law which states that when a conductor interacts with a magnetic field,
there must be an induced current that opposes the interaction, because of the law of
conservation of energy
The force acting on a straight wire at right angles to a 0.80-T magnetic field is 3.6 N. The
current flowing through the wire is 7.5 A. How long is the wire?
Fmax = BIL
L = Fmax/(BI) = 3.6 N/(0.8T••7.5A) = 0.60 m
3.
A beta particle (or high-speed electron) is ejected from an atomic nucleus. It then travels at
right angles to a 0.60-T magnetic field. It has a speed of 2.5 x 107 m/s. What is the size of
the magnetic force that acts on the particle?
Fmax = qvBsinθ
θ = (1.6 x 10-19 C)( 2.5 x 107 m/s)(0.60 T)(sin 90o) = 2.4 x 10-12 N
4.
A force of 5.78 x 10-16 N acts on an unknown particle traveling at a 90.0o angle through a
magnetic field. If the velocity of the particle is 5.65 x 104 m/s and the field is 3.20 x 10-2 T, how
many elementary charges does the particle carry?
Fmax = qvBsinθ
θ
Therefore, q = Fmax/(vBsinθ
θ) = (5.78 x 10-16 N)/( 5.65 x 104 m/s••3.20 x 10-2 T•• sin 90o)=
3.20 x 10-19 C
3.20 x 10-19 C * (1 e/1.6 x 10-19) = 2 electrons (or 2 protons)
5.
Draw a Venn diagram illustrating the similarities and differences between an electric motor and
an electric generator.
6. A bar magnet was dropped South pole first through a small coil. A current sensor was connected
to the coil. On a current vs. time graph, sketch a representation of the current from the time the
South pole of the magnet enters the coil to the time the North pole of the magnet exits the coil.
(Remember…the magnet is dropped vertically!)
7. Explain how Lenz’s Law is really the Law of Conservation of Energy.
An electric current that is induced by a changing magnetic field will in turn induce its
own magnetic field. According to Lenz's law, the induced electric current must be in
such a direction that the magnetic field induced by the current opposes the original
cause of the induced current. The change is the original magnetic field, or magnetic
flux, induces the electric current. For example, the original field is decreasing, then the
induced magnetic field must be in the same direction as the original field to oppose the
decrease.
Lenz's law is a consequence of the law of conservation of energy. According to the law
of conservation of energy the total amount of energy in the universe must remain
constant. Energy can be neither created nor destroyed. Hence it is impossible to get
free energy from nothing. Think about this experiment similar to Faraday's original
experiment. Push a bar magnet through a coil of wire. The moving magnet induces an
electric current in the wire, which in turn induces its own magnetic field. According to
Lenz's law, the induced magnetic field opposes the cause, which is the moving magnet.
Hence the induced magnetic field is in a direction to try to stop the moving magnet. If
this were not the case, the induced magnetic field would increase the magnet's velocity
and thereby increase its kinetic energy. There is no source for this energy. So if the
induced magnetic field helped rather than opposed its cause, conservation of energy
would be violated.
8. The primary coil of a transformer has 150 turns. It is connected to a 120-V source. Calculate the
number of turns on the secondary coil needed to supply the following voltages:
a. 625 V
Ns = Np•Vs/Vp = 150 turns••625 V/120 V = 782 turns
b. 35 V
Ns = 44 turns
c. 6.0 V
Ns = 8 turns
9. A transformer has input voltage and current of 12 V and 3.0 A respectively, and an output current
of 0.75 A. If there are 1200 turns on the secondary coil of the transformer, how many turns are on the
primary coil?
Np = Is * Ns / Ip
Np = (0.75 A * 1200 turns)/3.0 A = 300 turns
10. An electron travelling at 0.5c enters a magnetic field (B = 5.0 T) at right angles, it is confined to a
circular trajectory. What is the radius of this circular trajectory? Important: circular motion is caused
by a centripetal or center-seeking, force which in turn depends on an object’s mass, velocity and
radius of curvature. The formula for centripetal force is Fc = mv2/r.
Fc = Fm
mv2/r = qvB
r = mv/qB
r = (9.109 x 10-31 kg)(0.5)(3.0 x 108 m/s)/(1.6 x 10-19)(5.0 T) = 1.4 x 10-4 m