Take out a piece of paper

1
Take out a piece of paper

• Jot down what you know about magnets
• Where do we find magnets?
• What metals are magnetic?
• How can you make or destroy magnets?
• How do we use magnets?
• Andwhat would life be like without them?

2
Magnets

• Why do we discuss magnets along with electricity?

3
MAGnetIC Fields

• Why are we attracted to magnets?
• Or
• Animal Magnetism Myth or Misunderstanding

4
Discovery of Magnets

• Some debate one of the great controversies of
  our time!
• Story of sheep herder in Magnesia region of
  Greece.
• Or did the Chinese discover it thousands of years
  prior?

5
Magnetic Fields Overhead

• Look at magnets on overhead with iron filings
• Magnetic flux lines flow out of the north poles
  of magnets and into the south poles.
• Closed loops
• No overlap between 2 magnetic fields
• Remind you of anything?

6
Magnetic Fields Overhead

• Paper clips
• Soft vs hard
• How do you destroy a magnet?
• Can you see the magnetic fields?
• Magnetic Domains

7
Magnets interact with electricity

• Amperes the guy that current is named for
• Studied magnetism and electricity
• Observed current through a wire
• In a magnetic field
• Bounces or jumps

8
Current makes magnets?

• Current causes a magnetic field around the wire
• Ampere interaction between 2 magnetic fields
• One from a magnet
• One from electricity
• Electricity can create an electromagnet
• Like at the junk yard or

9
Magnetic Fields now that weve seen them

• The conventional way to think about magnetic
  fields
• Direction of concentric field lines based on RHR
• Grab the wire with your right hand
• The current flow lines up with your thumb
• Your fingers curve in the direction of the
  magnetic fields.

10
Magnetic field around a coil

• Take one loop of wire
• Trace your right hand around the loop and look
  where your fingertips go
• Resulting magnetic force inside the loop combines
  in one direction
• What happens when you have 2 loops?

11
Magnetic fields odds and ends

• Magnetic Fields
• Strength of the magnetic field
• B (vector)
• Into or out of page (x or .)
• Arrows

12
What next?

• Start Video?

13
Magnetic Force

• March 30/31, 2010

14
POTD

• Check homework
• Go over questions
• Magnetic Force Lecture
• Lab

15
HW

1. repel, attract, attract
2. two
3. a, b
4. parallel to the earths surface pointing towards
   the geographic north pole
5. Hard

1. Concentric circles
2. Smaller region inside
3. Opposite spin magnetic fields cancel

16
Last times Lab

• Please turn it in now! )

17
Whats the point?

• Last time
• Magnetism from Electricity
• We can see what happens!
• Must be a force acting on it
• Today
• Calculate the FORCE on a charge in a magnetic
  field

18
Magnetic Force

• Consider a charged particle
• Like an electron or a proton
• In the presence of a magnetic field
• The force felt by the charge
• F qvB
• q charge (Coulombs)
• V velocity (m/s)
• B strength of magnetic field (Tesla)

19
Strength of B

• The strength of a typical lab magnet
• 1.5 T
• The strength of the earths magnetic field
• Is it stronger or weaker than a lab magnet?
• Yes!! Nice call!!
• Earths is about 50 µT
• Or 5.0 x 10-5 T

20
Force as in VECTOR

• What direction is the force?
• Assume B is a uniform magnetic field
• The particle moves at a velocity of v
• Right hand rule 2
• Force comes out of wall at you

21
Force as in VECTOR

• Right hand rule 2
• Based on a positive charge
• If its negative
• Its the other direction

22
Example

• A charged particle
• Charge 1.7 x 10-6 C
• Is traveling north at 47 m/s
• Through a uniform magnetic field
• B 1.5 T straight up
• What is F?
• F qvB (1.7 x 10-6 C) x (47 m/s) x 1.5 T
• F 1.2 x 10-4 N (east)

23
Uniform Magnetic Field

• B is directed into the wall
• A particle travels perpendicular to B
• The force on the particle moves it
• Perpendicular to travel direction
• What is the path of the particle?

24
Is the particle or - ?
25
Wire in a Uniform Magnetic Field

• Similar to a chargeexcept
• Thumb along currents path
• Fingers up magnetic field
• RHR

26
The calculation

• Force qvB
• qv i x length
• Force Bil
• B magnetic field strength
• i current
• l length of wire in field

27
Example

• A 6.0 m wire carries a current of 7.0 A in the x
  direction.
• A magnetic force 7.0 x 10-6 N in the y
  direction
• What is the direction and magnitude of the
  magnetic field?
• 7.0 x 10-6 N B x 7.0 A x 6.0 m
• B F/(i l)
• B 1.7 x 10-7 T

28
Can you see it?

• What happens when you have 2 wires carrying
  current in the same direction.
• Check your answer with a friend
• What about if they are in the opposite direction?
• Same thing

29
Think about a loop

• In a magnetic field

30
Lab intro

• Discovery lab
• A magnet
• A solenoid
• A galvanometer
• To measure current (µA)
• Determine as much as you can
• Your grade will be based on your discoveries
• Dont skimp

31
Induced Current

• March 31 April 1, 2010

32
Turn in

• Lab
• Get homework out

1. 0.081 T
2. Up
3. 0.75 N
4. Attracts
5. Out of page

33
Todays Learning Goals

• What are the factors that affect induced current?
• Understand that induced current generates its own
  magnetic field
• Calculate the magnitude of an induced voltage

34
Right hand rules

• What are they?
• Check your understanding with a neighbor

35
Induced current

• Relative motion between a wire and a magnetic
  field
• Creates a potential difference in the wire
• Current flows if the circuit is complete!
• Recall a charged particle moving in a magnetic
  field experience a force.
• F qvB

36
F Bil Consider a wire

• Relative motion is required
• due to the force
• Pull it through a magnetic field
• Perpendicular to the field
• The magnetic force causes the positive ones to
  move
• According to the RHR
• And the negative charges to move
• According to the left hand rule

37
F qvB Consider a wire

• Separation of charges
• Like a potential difference or voltage
• If there is a completed circuit
• Then current flows

38
Motion is essential

• The voltage (and current) is maintained
• As long as the wire is moving relative to the
  magnetic field.
• The faster it moves the greater the current
• The greater the magnetic field, the greater the
  current

39
Orientation matters

• In a complete circuit/loop of wire
• more current flows if motion is perpendicular to
  the magnetic field.
• The number of field lines that you cut as it
  moves
• Increase that and you increase current!
• This means you can also rotate the coil in a
  magnetic field.

40
Faradays Law of Induction

• Factors that affect the induced voltage
• The number of coils in the circuit (N)
• The angle between B and the coil (?)
• Area of the coil (m2)
• B magnetic field strength (T)
• emf -N ?(AB(cos ?))/ ?t

41
Faradays Law of Induction

• emf -N ?(AB(cos ?))/ ?t
• If you vary the
• cross sectional area
• Magnetic field
• The angle of orientation
• With respect to a change in time
• THEN you will see an induced emf

42
Example

• A coil with 25 turns of wire
• Cross sectional area of 1.8 m2
• Magnetic field applied at a right angle to the
  plane of the coil
• The field is increased from 0.00 T to 0.55 T in
  0.85 seconds
• What is the magnitude of the induced emf?

43
The answer, please

• emf -N ?(AB(cos ?))/ ?t
• emf - 25 (1.8m2)cos0 (?B/?t)
• emf - 45 (0.55 0.00T/0.85 s)
• emf - 29 V
• Note that all factors are included in the answer.
  
• But something has to change!

44
Lenzs Law

• If you induce a current in a wire
• That current creates a magnetic field of its own!
• Lenz tells us that the induced current creates a
  magnetic field that opposes the applied magnetic
  field.
• The induced current tries to keep the field
  strength constant.

45
Todays Goals

• Factors that affect induced voltage/current
• Induced current generates its own
  _________________?
• Calculate the magnitude of an induced voltage

46
Lab!

• Todays lab DOES NOT connect directly with the
  lesson!!
• It does relate to magnetic fields generated by
  current carrying wire
• Have fun!

47
Transformers

• April 5/8, 2010

48
Transformers Saving the Planet

• We generate electricity at Bonneville
• Alternating current
• We use it in our homes
• 40 miles away
• Is that a problem?
• Line losses
• Big i big losses
• So how do we change the current??

49
How indeed

• Remember the door bell example
• The bell only works when there is a change in
  electrical current.
• The system is DC
• When connected, a constant current flows.
• What would happen if it was AC?

50
Same idea

• 2 coils connected with a soft metal core
• The current in the first coil creates a magnetic
  field.
• AC circuit - current changes
• 60 times a second (Hertz)
• The changing magnetic field induces a current in
  the secondary coil

51
Primary and secondary

• The coil that is hot is primary
• The coil that receives an induced current is
  secondary
• V1/N1 V2/N2
• V voltage
• N number of turns on the coil

52
Step up or step down?

• V1 110 V
• N1 75 turns
• V2 2400 V
• N2 ?
• N2 (N1xV2)/V1
• N2 (75x2400)/110
• N2 1600 turns

53
If you want more

• If you want the voltage to increase
• Will you have more primary or secondary windings?

54
The holy grail?

• No, there is no free lunch
• Power in on one side
• Equals power out on the other
• Vi Vi
• And that is the key to reducing line losses!

55
Another example

• A 120 volt system has 2 amps of current flowing
• What is the power in and the power out?
• If you want 60 volts on the secondary side
• How much current will flow?
• If the secondary has 20 wraps of wire
• How many will the primary have?

56
Change of pace?

• A worksheet to consider
• Nike scholarship?
• Homework clarification

57
The Grid
58
The Power Plant

• Start with some means of turning
• Hydro
• Wind
• Steam turbine
• Create electricity
• Typically about 5,000 V

59
Transmission

• Electricity is stepped up at a substation
• 5,000 V stepped up to 500,000 V
• Reduces power losses
• Feeds high voltage transmission lines
• Typical distances are 300 miles

60
Distribution Grid

• Once its where its needed
• It needs to be stepped-down to be useful
• First stop is the substation
• Stepped down to about 7,200 V
• Split into different paths

61
At your house

• A transformer steps down the power
• From 7,200 volts (typical)
• To 240 volts
• And through the meter

62
Last stop

• Into your circuit breaker panel
• And into the house
• As 240 V (washer, range, etc)
• And 120 V (everything else)

63
Got the picture?
64
Grid Facts
65
Grid Facts

• 30 of energy consumed in the US is in the form
  of electricity
• Average thermal efficiency of a power plant is
  30
• 157,000 miles of high voltage lines in the US

66
Unit 9 Review

• Electromagnetism

67
Homework

1. 55 turns
2. 3.5 x 104 turns
3. 25 turns
4. 1561

68
Magnets what are they good for?

• Spend a minute writing down what you know about
  magnets
• You should know
• How many poles
• How to destroy them
• What happens if you break them
• What metals can be magnetized
• What allows magnetism to occur

69
Faraday had a dream

• What were the 3 right hand rules?
• They explain
• The direction of magnetic field lines around a
  wire
• Interaction between electrical charges and
  magnetism
• Interaction between current and magnetism

70
Deep thoughts

• F Bil
• F qvB
• Faradays law
• Lenzs law

71
Induced emf

• We can cause current to flow through a circuit
• Interaction between magnetic field and circuit
• It explains a lot
• How generators and motors work
• Doorbell (DC)
• Transformers (AC)

72
Transformers

• V1/N1 V2/N2
• Power in Power out
• Vi (in) Vi (out)

73
Labs

• Magnetic field around a wire and a coil
• Which is stronger?
• Recall the shapes and direction of the field
  lines
• Electrical current generated by waving a magnet
  around
• How could you change the direction?
• How could you change the magnitude?
• Doorbell Lab electromagnetism