Feynman ’ s interview about magnets: http://www.youtube.com/watch?v=wMFPe-DwULM
Electromagnet crane http://www.youtube.com/watch?v=6yhNOXQkMpY&feature=related
Wiley Coyote: “ Beep Prepared ” http://www.youtube.com/watch?v=My1Kzy_cDV0

ConcepTest 20.1c Magnetic Force III
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
1) out of the page
2) into the page
3) zero
4) to the right
5) to the left
→ → → → →
→ → → → →
→ → → → → q
→ → → → →

ConcepTest 20.1c Magnetic Force III
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
1) out of the page
2) into the page
3) zero
4) to the right
5) to the left
Using the right-hand rule, you can see that the magnetic force is directed into the page . Remember that the magnetic force must be perpendicular to BOTH the B field and the velocity .
→ → → → →
→ → → → →
→ → → → →
F
× q
→ → → → →

Electromagnetism
What you need to know
•   All Magnetic Fields are due to electric charges in motion
•   Force on an Electric Charge in a Magnetic Field
•   Force between Two Parallel Wires
•   Solenoids and Electromagnets
•
Electromagnetic Induction
•   Applications: Motors, Loudspeakers, Mass Spectrometer
•   Electric Generators
•   Transformers and Transmission of Power
•   Applications of Induction: Sound Systems, Computer Memory,
Seismograph, GFCI

How to calculate the Force on a Currentcarrying wire
•   Symbol for Magnetic Field: B
•   Units: Tesla (T) or Gauss (10 -4 T)
•   The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation.
F = qvB sin
θ (think why this is the same thing)
This equation defines the magnetic field B .
If the wire is a right-angles to the field, then the maximum force is felt max

Does the Earth ’ s Magnetic Field exert a significant force on current carrying wires (e.g. power lines) ?
If B earth
= 0.5x10
-4 T, what is the magnetic force on a 100m length of cable carrying 30A ?
A.
0.15 N
B.
150 N
C.
1500 N
D.
No Force

ConcepTest 20.1d Magnetic Force IV
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
1) out of the page
2) into the page
3) zero
4) to the right
5) to the left
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ v
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑

ConcepTest 20.1d Magnetic Force IV
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
1) out of the page
2) into the page
3) zero
4) to the right
5) to the left
The charge is moving parallel to the magnetic field , so it does not experience any magnetic force .
Remember that the magnetic force is given by: F = v B sin( θ ) .
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑ v
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑
F = 0
↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑

Force on Electric Charge Moving in a
Magnetic Field
•   If a charged particle is moving perpendicular to a uniform magnetic field, its path will be a circle.
•   This happens because the magnetic force points at right angles to the particle’s direction of motion
•   Hence the particle feels a centripetal force.
•   By combining the equations for centripetal and magnetic force, and measuring the radius of the circle, you can calculate the mass of charged particles
•   E.g. Mass Spectrometer

ConcepTest 20.2 Atomic Beams
A beam of atoms enters a magnetic field region.
What path will the atoms follow?
1 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
4 x x x x x x x x x x x x
2
3

ConcepTest 20.2 Atomic Beams
A beam of atoms enters a magnetic field region.
What path will the atoms follow?
1 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
4 x x x x x x x x x x x x
2
3
Atoms are neutral objects whose net charge is zero .
Thus they do not experience a magnetic force.
Follow-up: What charge would follow path #3? What about path #1?

ConcepTest 20.3 Magnetic Field
A proton beam enters into a magnetic field region as shown below. What is the direction of the magnetic field B ?
1) + y
2) – y
3) + x
4) + z (out of page)
5) – z (into page) y x

ConcepTest 20.3 Magnetic Field
A proton beam enters into a magnetic field region as shown below. What is the direction of the magnetic field B ?
1) + y
2) – y
3) + x
4) + z (out of page)
5) – z (into page)
The picture shows the force acting in the + y direction . Applying the right-hand rule leads to a B field that points into the page . The B field must be out of the plane because B ⊥ v and B ⊥ F .
Follow-up: What would happen to a beam of atoms? y x

ConcepTest 20.4a Mass Spectrometer I x x x x x x x x x x x x
Two particles of the same mass enter a magnetic field with the x x x x x x x x x x x x same speed and follow the paths shown. Which particle has the x x x x x x x x x x x x bigger charge ? x x x x x x x x x x x
1 x 2
3) both charges are equal x x x x x x x x x x x
4) impossible to tell from the picture x x x x x x x x x x x x

ConcepTest 20.4a Mass Spectrometer I x x x x x x x x x x x x
Two particles of the same mass enter a magnetic field with the x x x x x x x x x x x x same speed and follow the paths shown. Which particle has the x x x x x x x x x x x x bigger charge ? x x x x x x x x x x x
1 x 2
3) both charges are equal x x x x x x x x x x x
4) impossible to tell from the picture x x x x x x x x x x x
According to this equation, the
R = mv qB bigger the charge, the smaller the radius.
Follow-up: What is the sign of the charges in the picture?

ConcepTest 20.4b Mass Spectrometer II
A proton enters a uniform magnetic field that is perpendicular to the proton ’ s velocity. What happens to the kinetic energy of the proton?
1) it increases
2) it decreases
3) it stays the same
4) depends on the velocity direction
5) depends on the B field direction x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

ConcepTest 20.4b Mass Spectrometer II
A proton enters a uniform magnetic field that is perpendicular to the proton ’ s velocity. What happens to the kinetic energy of the proton?
1) it increases
2) it decreases
3) it stays the same
4) depends on the velocity direction
5) depends on the B field direction
The velocity of the proton changes direction but the magnitude (speed) doesn ’ t change. Thus the kinetic energy stays the same . x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

Solenoids and Electromagnets
So much for straight wires, it “ turns out ” that coils are much more interesting and useful.
A solenoid is a long coil of wire. If it is tightly wrapped, the magnetic field in its interior is almost uniform:

20.7 Solenoids and Electromagnets
If a piece of iron is inserted in the solenoid, the magnetic field greatly increases. Such electromagnets have many practical applications.

Which of the following must be true?
A.
The things being picked up are Iron and Steel
B.
The Crane uses an Iron magnet
C.
The crane uses a direct current coil as an electromagnet
D.
All of the above
E.
A & C only

20.10 Applications: Galvanometers,
Motors, Loudspeakers
An electric motor also takes advantage of the torque on a current loop, to change electrical energy to mechanical energy.

Applications: Motors, Loudspeakers
Loudspeakers use the principle that a magnet exerts a force on a current-carrying wire to convert electrical signals into mechanical vibrations, producing sound.

•   Electric Currents (moving charges) produce magnetic fields.
–   Think of a compass deflected by a nearby electric wire.
•   Coiling a wire tightly into a “ solenoid ” intensifies the magnetic field.
•   Just as like-poles of magnets repel each other, the magnetic field caused by a current in a wire or coil exerts a force upon a magnet.
–   This is how electric motors (and loudspeakers) work.
•   A magnet moving near a conductor (a wire or a coil of wire) causes an electric current to flow. (Induction)
–   This is how electric generators and microphones work.
•   The magnitude of these magnetic fields and electric currents are related by
Faraday ’ s Law and Ampere ’ s Law.
–   Motors: More loops of wire and a larger currents produce bigger fields, and hence a more powerful motor.
–   Generators: Stronger magnets and more coils of wire produce larger currents

21.1 Electromagnetic Induction
Almost 200 years ago, Faraday looked for evidence that a magnetic field would induce an electric current with this apparatus:

When does this ammeter indicate a current?
A. When the switch is open
B. When the switch is closed
C. As the switch closes
D. As the switch opens
E. Never, its not connected to the battery

Transformers and Transmission of
Power
•   A transformer consists of two coils, either interwoven or linked by an iron core. A changing emf in one induces an emf in the other.
The ratio of the emfs is equal to the ratio of the number of turns in each coil:
This is a step-up transformer – the emf in the secondary coil is larger than the emf in the primary:

21.7 Transformers and Transmission of Power
Transformers work only if the current is changing; this is one reason why electricity is transmitted as ac.

Applications of Induction: GFCI
•   A ground fault circuit interrupter (GFCI) will interrupt the current to a circuit that has shorted out in a very short time, before a dangerous current flows, preventing electrocution.
•   Relies on the the hot and neutral wires carry exactly equal but opposite currents when a device is plugged into the outlet and switched on.
•   Anti-parallel currents produce opposite magnetic fields, so a short distance away from the wires, there in NO NET Magnetic Field.
•   If even a tiny current leaks out of the device (say to your wet finger as you reach for a hair dryer), the incoming and outgoing currents become imbalanced.
•   Unbalanced currents means a net magnetic field appears, causing an induced EMF - Current flows in the sensing coil, and a second coil opens a switch.
A GFCI will save your life, possibly without you even noticing!
Note: a fuse won ’ t blow until your body gets shocked with 15 Amps enough to kill

21.5 Electric Generators
A generator is the opposite of a motor – it transforms mechanical energy into electrical energy. This is an ac generator:
The axle is rotated by an external force such as falling water or steam.
The brushes are in constant electrical contact with the slip rings.

Electric Generators - Wind Turbine
Generators that use magnets and coils , plus a source of mechanical power are often called
Alternators - there ’ s one in your car!
Alternators are the simplest and most efficient type of generator.
They produce alternating current.
Coils Magnets

Electric Generators and AC
A sinusoidal emf is induced in each coil in the generator ( N is the number of turns, and A the area of the loop):
(21-5)
•   The frequency of the AC waveform depends on how fast the generator rotates.
•   A power station can be designed to give 60 Hz, but wind is more problematic …

ConcepTest 21.10 Generators
A generator has a coil of wire rotating in a magnetic field.
If the rotation rate increases , how is the maximum output voltage of the generator affected?
1) increases
2) decreases
3) stays the same
4) varies sinusoidally

ConcepTest 21.10 Generators
A generator has a coil of wire rotating in a magnetic field.
If the rotation rate increases , how is the maximum output voltage of the generator affected?
1) increases
2) decreases
3) stays the same
4) varies sinusoidally
= NBA ω sin( ω t )
The maximum voltage is the leading term that multiplies sin( ω t ) and is given by ε
0
= NBA ω . Therefore, if
ω increases , then ε
0
must increase as well.

21.8 Applications of Induction: Sound
Systems, Computer Memory,
Seismograph, GFCI
This microphone works by induction; the vibrating membrane induces an emf in the coil

21.8 Applications of Induction: Sound
Systems, Computer Memory,
Seismograph, GFCI
Differently magnetized areas on an audio tape or disk induce signals in the read/write heads.

21.8 Applications of Induction: Sound
Systems, Computer Memory,
Seismograph, GFCI
A seismograph has a fixed coil and a magnet hung on a spring (or vice versa), and records the current induced when the earth shakes.

Calculating the Force on a Charge Moving in a Magnetic Field
Problem solving: Magnetic fields – things to remember
1.
The magnetic force is perpendicular to the magnetic field direction.
2.
The right-hand rule is useful for determining directions.
3.
Equations in this chapter give magnitudes only. The right-hand rule gives the direction.

ConcepTest 21.1a Magnetic Flux I
In order to change the magnetic flux through the loop, what would you have to do?
1) drop the magnet
2) move the magnet upwards
3) move the magnet sideways
4) only (1) and (2)
5) all of the above

ConcepTest 21.1a Magnetic Flux I
In order to change the magnetic flux through the loop, what would you have to do?
1) drop the magnet
2) move the magnet upwards
3) move the magnet sideways
4) only (1) and (2)
5) all of the above
Moving the magnet in any direction would change the magnetic field through the loop and thus the magnetic flux.

ConcepTest 21.1b Magnetic Flux II
In order to change the magnetic flux through the loop, what would you have to do?
1) tilt the loop
2) change the loop area
3) use thicker wires
4) only (1) and (2)
5) all of the above

ConcepTest 21.1b Magnetic Flux II
In order to change the magnetic flux through the loop, what would you have to do?
1) tilt the loop
2) change the loop area
3) use thicker wires
4) only (1) and (2)
5) all of the above
Since Φ = B A cos θ , changing the area or tilting the loop (which varies the projected area) would change the magnetic flux through the loop.

ConcepTest 21.12a Transformers I
What is the voltage across the lightbulb?
1) 30 V
2) 60 V
3) 120 V
4) 240 V
5) 480 V
120 V

ConcepTest 21.12a Transformers I
What is the voltage across the lightbulb?
1) 30 V
2) 60 V
3) 120 V
4) 240 V
5) 480 V
The first transformer has a 2:1 ratio of turns, so the voltage doubles .
But the second transformer has a
1:2 ratio , so the voltage is halved again. Therefore, the end result is the same as the original voltage .
120 V 240 V
120 V

ConcepTest 21.12b Transformers II
Given that the intermediate current is 1 A, what is the current through the lightbulb?
1) 1/4 A
2) 1/2 A
3) 1 A
4) 2 A
5) 5 A
120 V
1 A
240 V
120 V

ConcepTest 21.12b Transformers II
Given that the intermediate current is 1 A, what is the current through the lightbulb?
1) 1/4 A
2) 1/2 A
3) 1 A
4) 2 A
5) 5 A
Power in = Power out
240 V × 1 A = 120 V × ???
The unknown current is 2 A .
120 V
1 A
240 V
120 V

ConcepTest 21.12c Transformers III
A 6 V battery is connected to one side of a transformer.
Compared to the voltage drop across coil A, the voltage across coil B is:
1) greater than 6 V
2) 6 V
3) less than 6 V
4) zero
6 V
A B

ConcepTest 21.12c Transformers III
A 6 V battery is connected to one side of a transformer.
Compared to the voltage drop across coil A, the voltage across coil B is:
1) greater than 6 V
2) 6 V
3) less than 6 V
4) zero
The voltage across B is zero .
Only a changing magnetic flux induces an EMF. Batteries can only provide DC current .
A B
6 V