electromagnetism Exam coverage.pptx

Electromagnetism
Revision
Lesson 1: INDUCING CURRENTS

INDUCING CURRENTS
Electromagnetic induction is the process of generating current through
a wire in a circuit in a changing magnetic field
Electromotive force is the potential difference across the battery’s
terminals
𝐸𝑀𝐹 = 𝐵 𝐿 𝑣 sin 𝜃 , θ is the angle between v and B

Textbook page 155
a) EMF = B L v
6 = 5 x 10-5 x 25 x B
B = 6 / (5 x 10-5 x 25 )
B = 0.1 T
b) I = EMF/R
I = 6/5
I = 1.2 A

INDUCING CURRENTS
Average power and Effective current
PAC = ½Pmax
PAC = I eff
2R
Ieff
=
2
2
æ
è
ç
ö
ø
÷Imax
= 0.707Imax
Effective Current
Effective potential difference
Veff
=
2
2
æ
è
ç
ö
ø
÷Vmax
= 0.707Vmax
Effective Voltage
Effective voltage is also commonly referred to as RMS (root mean square) voltage.

• When a wire moves perpendicular to a magnetic field, there is a
force on the charges in the wire.
• The force causes negative charges to move to one end of the wire,
leaving positive charges at the other end.
• This separation of charge produces an electric field and therefore a
potential difference across the length of the wire which called
Induced electromotive force (EMF)
Electromotive force and Induced
electromotive force

date 19
Induced EMF in microphones
Microphones convert sound to electrical energy by electromagnetic
induction
• A microphone has a thin aluminum diaphragm attached to a
coil in a magnetic field, as shown in the Figure.
• Sound waves cause the diaphragm to vibrate. This moves the
attached coil.
• The motion of the coil in the magnetic field induces an EMF
across the coil's ends that generates a current.
• The induced EMF and current vary as the frequency of the
sound varies. In this way, the Sound waves are converted to
electrical signals.
• The potential differences generated are small, typically 10³ V,
but they can be increased, or amplified, by electronic devices

Electric Generators
20
• An electric generator converts mechanical energy to electrical
energy.
• An electric generator consists of a number of wire loops placed in
a strong magnetic field.
• The wire may be wound around an iron core to increase the
strength of the magnetic field.

Electric Generators
21
• The iron and the wires are called the armature, which is
similar to that of an electric motor.
• The armature is mounted so that it can rotate freely in the
magnetic field.
• As the armature turns, the wire loops cut through the
magnetic field lines and induce an EMF. When a generator
is connected in a closed circuit, the induced EMF produces
an electrical circuit.

Electric Generators
22
• The EMF developed by the generator depends on the length of
the wire rotating in the field.
• Increasing the number of loops in the armature increases the wire
length, which increases the induced EMF.
• The direction of the induced current can be found from the third
right-hand rule.

Electric Generators
1- The maximum current is generated when the
rotating loop is Horizontal to the magnetic field
2- The current is zero when the rotating loop is
Vertical to the magnetic field

INDUCING CURRENTS
Generator Types:
1- Direct current (DC) generators
Charges move in a single direction, as they do in batteries
Current is in one direction because the wires of the armature
connected to a circuit by means of a commutator

INDUCING CURRENTS
Generator Types:
2- Alternating current (AC) generators

A slip-ring device is connected wires to a circuit.
One ring is connected to one end of the
armature, the other ring is connected to the
other end of the armature
As the armature rotates through 180, the
induced EMF reverses direction
This means the current also reverses direction.
The current alternates from positive to negative
2- Alternating current (AC) generators
Generator Types:

INDUCING CURRENTS
War of currents
1- Edison’s system used Direct current
2- Tesla system used Alternative current
3- Alternative current won

electric motor
D
electric generator
C
battery
B
armature
A
A device that converts mechanical energy to
electrical energy is called a(n) __________.
CORRECT

Current is zero when the wire segments
move perpendicular to the field.
C
Current is zero when the wire segments
move parallel to the field.
D
Current is zero during the first 180° of the
wire’s rotation.
B
It is never zero.
A
When is current zero in an electric generator?
4
.
CORRECT

Electromagnetism
Revision
Lesson 2: APPLICATIONS OF
INDUCED CURRENTS

APPLICATIONS OF INDUCED CURRENTS

If the south end of a magnet is moving towards a conducting coil that is wound perpendicular to the movement
of the magnet, a current will flow in the coil, generating a magnetic field within the coil such that the open end of
the coil nearest the magnet will be ___South pole____
If the south end of a magnet is moving away from a conducting coil that is wound perpendicular to the movement of
the magnet, a current will flow in the coil, generating a magnetic field within the coil such that the open end of the
coil nearest the magnet will be ___North pole___
If the north end of a magnet is moving towards a conducting coil that is wound perpendicular to the movement of
the magnet, a current will flow in the coil, generating a magnetic field within the coil such that the open end of the
coil nearest the magnet will be ___North pole___
If the north end of a magnet is moving away from a conducting coil that is wound perpendicular to the movement of
the magnet, a current will flow in the coil, generating a magnetic field within the coil such that the open end of the coil
nearest the magnet will be ___South pole___

in a perpendicular direction to that of the
original field
D
in the opposite direction as that of the
original field
C
in the same direction as that of the original field
B
twice as strong as the original field
A
According to Lenz’s law, the magnetic field produced by the
induced current is ________ .
1.
CORRECT

mutual inductance
D
self-inductance
C
conductivity
B
resistivity
A
Which is the term for the property of a wire to create
an induced EMF that opposes the change in the
potential difference across the wire?
2.
CORRECT

18 Lenz’s Law You hang a coil of wire with its ends joined so that it can swing easily. If you
now plunge a magnet into the coil, the coil will start to swing. Which way will it swing
relative to the magnet and why?
19.Motors If you unplugged a running vacuum cleaner from a wall outlet, you would be much
more likely to see a spark than you would if you unplugged a lighted lamp from the wall.
Why?
20.Transformers and Current Explain why a transformer may be operated only on AC.

18 Lenz’s Law You hang a coil of wire with its ends joined so that it can swing easily. If you
now plunge a magnet into the coil, the coil will start to swing. Which way will it swing
relative to the magnet and why?
Away from the magnet; the changing magnetic field induces a current in the coil,
producing a magnetic field. This field opposes the field of the magnet, and thus,
the force between coil and magnet is repulsive.
19.Motors If you unplugged a running vacuum cleaner from a wall outlet, you would be much
more likely to see a spark than you would if you unplugged a lighted lamp from the wall.
Why?
The inductance of the motor creates an EMF that causes the spark. The bulb has
very low self-inductance, so there is no EMF.
20.Transformers and Current Explain why a transformer may be operated only on AC.
Transformers rely on changing currents to induce changing magnetic fields.
DC always produces the same magnetic field and thus can’t induce a current in
another wire.

Step-up and Step-down Transformers
If the secondary potential difference is larger than the primary potential difference,
the transformer is called a step-up transformer.
If the secondary potential difference is smaller than the primary potential
difference, the transformer is called a step-down transformer.

The Ideal Transformer
• In an ideal transformer, the electrical power delivered to the
secondary circuit equals the power supplied to the primary
circuit:
Pp = Ps.

Is
Ip
=
Vp
Vs
=
Np
Ns
Transformer Equation

Isolation Transformers
The primary and secondary coils have the same
number of turns, so the input and output potential
differences are identical. These transformers, called
isolation transformers

21.Transformers Frequently, transformer coils that have only a few turns are made of very thick (low-
resistance) wire, while those with many turns are made of thin wire. Why?
22.Step-Up Transformers Refer to the step-up transformer shown in Figure 15. Explain what would
happen to the primary current if the secondary coil were short-circuited.
23.Critical Thinking Would permanent magnets make good transformer cores? Explain.

21.Transformers Frequently, transformer coils that have only a few turns are made of very thick (low-
resistance) wire, while those with many turns are made of thin wire. Why?
More current can go through the coil with fewer turns, so thick wires with capacity for large
currents are needed. Also, resistance must be kept low to prevent voltage drops and I2R power
loss and heating.
22.Step-Up Transformers Refer to the step-up transformer shown in Figure 15. Explain what would
happen to the primary current if the secondary coil were short-circuited.
According to the transformer equations, the ratio of primary to secondary current is equal
to the ratio of turns and doesn’t change. Thus, if the secondary current increases, so
does the primary.
23.Critical Thinking Would permanent magnets make good transformer cores? Explain.
No; induced EMF depends on a changing magnetic field through the core. Permanent
magnets are “permanent” because they are made of materials that resist such changes.

16- A transformer has a primary coil winding consisting of 7500 turns and a secondary coil
winding of 125 turns. The primary voltage is 7.2x103 V.
A) What is the voltage in the secondary circuit?
B) If the current in the secondary circuit is 36 A, what is the current in the primary circuit?

17- A transformer has a primary coil winding consisting of 300 turns and a secondary
coil winding of 90000 turns. The primary voltage is 60 V.
What is the voltage in the secondary circuit?
The current in the secondary circuit is 0.50 A. What current is in the primary circuit?

Electromagnetism
Revision
Lesson 3: ELECTRIC AND
MAGNETIC FIELDS IN SPACE

ELECTRIC AND MAGNETIC FIELDS IN SPACE
• A changing magnetic field produces
an electric field, and a changing
electric field produces a magnetic
field even if there is no matter
present.
• Electromagnetic waves are coupled,
oscillating electric and magnetic fields
that propagate through space and
matter.

• Electromagnetic waves are
transverse waves. Both electric
and magnetic fields oscillate
perpendicular to the direction the
wave travels.
• Furthermore, the electric and
magnetic fields are
perpendicular to each other.

l =
v
f
Wavelength
• The speed of an electromagnetic wave in a vacuum is approximately 3.00 × 108 m/s. It is denoted as c
and is commonly called the speed of light.
l =
c
f
Wavelength of Electromagnetic
Wave in a Vacuum

• The range of frequencies and
wavelengths that make up all forms
of electromagnetic radiation is
called the
electromagnetic spectrum.
• Energy that is carried by an
electromagnetic wave is called
electromagnetic radiation.
Textbook page 178-179
Important

Type of EM Wave Uses
Radio Waves • broadcasting information
Microwaves • cellular phones
• GPS
• microwave ovens
Infrared • infrared cameras and night-vision goggles
• heating buildings
Ultraviolet • ionizing molecules and atoms
• causing chemical reactions
• sterilizing instruments
X-rays • medical imaging
• cancer treatment
Gamma Rays • cancer treatment
Textbook page 178 - 179
Important

They travel at a speed of 3.00 × 108 m/s.
D
They require a medium through which to travel.
C
They are coupled, oscillating electric and magnetic fields.
B
They are transverse waves.
A
Which is NOT true about electromagnetic waves?
1.
CORRECT

8.00 × 1014 m
D
2.67 × 106 m
C
3.75 × 10−7 m
B
1.25 × 10−15 m
A
Which is the wavelength of an electromagnetic
wave that has a frequency of 8.00 × 1014 Hz?
CORRECT
c=3.00 × 108 m/s

38.What is the wavelength of green light that has a frequency of 5.701014 Hz?
39. An electromagnetic wave has a frequency of 8.2014 Hz. What is the wavelength of the
wave?
40. What is the frequency of an electromagnetic wave that has a wavelength of 2.2102 m?
41. CHALLENGE If an electromagnetic wave is propagating to the right and the electric field is in and out
of the page, in what direction is the magnetic field?

38.What is the wavelength of green light that has a frequency of 5.701014 Hz?
8
7
14
c 3.00 10 m/s
5.26 10 m
5.70 10 Hz
f
 

   

8
7
14
c 3.00 10 m/s
3.7 10 m
8.2 10 Hz
f
 

   

8
10
2
c
c 3.00 10 m/s
1.4 10 Hz
2.2 10 m
f
f

 


   

39. An electromagnetic wave has a frequency of 8.2014 Hz. What is the wavelength of the
wave?
40. What is the frequency of an electromagnetic wave that has a wavelength of 2.2102 m?
41. CHALLENGE If an electromagnetic wave is propagating to the right and the electric field is in and out
of the page, in what direction is the magnetic field?
up and down

• The range of frequencies and wavelengths that make up all forms of
electromagnetic radiation is called the
electromagnetic spectrum.
• Energy that is carried by an electromagnetic wave is called
electromagnetic radiation.
• A dielectric is a poor conductor of electric current whose electric
charges partially align with an electric field.
Electromagnetic Waves
v =
c
k

42.What is the speed of an electromagnetic wave traveling through air? Use c  299,792,458 m/s
in your calculation.
43.Water has a dielectric constant of 1.77. What is the speed of light in water?
44.The speed of light traveling through a material is 2.43108 m/s. What is the dielectric
constant of the material?

8
c 299,792,458 m/s
1.00054
2.99712 10 m/s
v
k
 
 
8
8
c 3.00 10 m/s
1.77
2.25 10 m/s
v
k

 
 
42.What is the speed of an electromagnetic wave traveling through air? Use c  299,792,458 m/s
in your calculation.
43.Water has a dielectric constant of 1.77. What is the speed of light in water?
44.The speed of light traveling through a material is 2.43108 m/s. What is the dielectric
constant of the material?
2
2 8
8
c
c 3.00 10 m/s
1.52
2.43 10 m/s
v
k
k
v

 

 
  
 
 

   

45.Challenge A radio signal is transmitted from Earth’s surface to the Moon’s surface,
376,290 km away.
What is the shortest time a reply can be expected? Round-trip distance is
752,580,000 m.
52.Dielectric Constant The speed of light traveling through an unknown material is 1.98108 m/s.
Given that the speed of light in
a vacuum is 3.00108 m/s, what is the dielectric constant of the unknown material?

752,580,000 m
2.5109 s
c 299,792,458 m/s
x
t   
45.Challenge A radio signal is transmitted from Earth’s surface to the Moon’s surface,
376,290 km away.
What is the shortest time a reply can be expected? Round-trip distance is
752,580,000 m.
 2.51034 s
52.Dielectric Constant The speed of light traveling through an unknown material is 1.98108 m/s.
Given that the speed of light in
a vacuum is 3.00108 m/s, what is the dielectric constant of the unknown material?
2
2 8
8
c
c 3.00 10 m/s
2.30
1.98 10 m/s
v
k
k
v

 

 
  
 
 

   

Transmitting Electromagnetic Waves
• Radio waves and microwaves are broadcast into space by
transmitters connected to antennas.
• A transmitter is a device that converts voice, music, pictures, or
data to electronic signals, amplifies the signals, and then sends
the signals to an antenna.
• The antenna creates the electromagnetic waves that propagate
through the air.
• receiver, which converts the sent potential deference to
usable information—sound, pictures, or data.
Textbook page 184
Important

Receiving Electromagnetic Waves
• Antennas capture electromagnetic
waves, converting the waves’ oscillating
electric fields back to potential
differences.
Textbook page 184
Important

• A wave’s electric field accelerates electrons in the metal of an
antenna, resulting in a potential difference across the antenna’s
terminals that oscillates at the frequency of the wave.
• The acceleration is largest when the antenna is parallel to the
direction of the wave’s electric field.
• A wire antenna is most efficient when its length is one-half the length
of the wave it is designed to detect.
Textbook page 184
Important

• A parabolic dish antenna reflects and
focuses signals off its surface and into a
horn.
• After an antenna converts the electric fields
of electromagnetic waves to potential
differences, it sends the potential
differences to a receiver, which converts
them to usable information—sound,
pictures, or data.
Textbook page 184
Important

• To select waves of a particular frequency, a
receiver uses a tuner, which has a coil-and-
capacitor circuit or a resonant cavity.
• You select a desired station by adjusting the
capacitance until the oscillation frequency of the
circuit equals the frequency of the desired wave.
Textbook page 184
Important

receiver
D
dielectric
C
transmitter
A
antenna
B
Which is the term for a device that converts voice, music,
pictures, or data to electronic signals, amplifies the signals, and
then sends the signals?
CORRECT

one-quarter
D
one-half
C
the same as
B
double
A
A wire antenna is most efficient when its length is
_______ the length of the wave it is designed to detect.
CORRECT

dielectric
D
receiver
C
transmitter
B
antenna
A
a device that converts oscillating potential
differences in an antenna to sound, pictures, or
data.
CORRECT

receiver
D
dielectric
C
transmitter
A
antenna
B
a device that propagates electromagnetic waves
through the air
CORRECT

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