Magnetism and Electricity

1
Magnetism and Electricity
2
Magnets around the House

• Headphones Refrigerator magnets Computer
  speakers Telephone receiversPhone ringers
  Microwave tubesSeal around refrigerator door
• Floppy disk recording and reading headAudio
  tape recording and playback headVideo tape
  recording and playback headCredit card magnetic
  strip TV deflection coilComputer monitor
  Computer hard drive Power supply transformers

3
Magnets in your Car

• Starter motor
• A/C clutch
• Interior fan motor
• Electric door locks
• Windshield wiper motor
• Electric window motor
• Side-view mirror adjuster motor
• CD/tape player motor and playback
• Engine speed sensors
• Alternator
• Starter relay
• Windshield washer pump motor

4
Uses of magnets

• Magnets were first put to use in navigation
  because they always point north and south.

5
What is Magnetism?
Magnetism is the force of attraction or
repulsion of magnetic material due to the
arrangement of its electrons.
6
lab

• Magnets stick to some things but not to others.
• Find different things to test with your magnet.
• What is your conclusion?

7
Magnets

• not all objects are affected by the force of
  magnetism
• ex. wood, glass, paper, plastic
• common metals affected by magnetism are iron,
  nickel, and cobalt

8
Poles of a magnet always Come in pairs!
9
Magnetic Poles

• poles two ends of a magnet. every magnet has
  two poles north (N) pole and south (S) pole
• N S
• N S N S
• Break one bar magnet in half, and you have two
  smaller bar magnets!

10
Properties of Magnets

• if a north pole and a south pole are brought
  together, they will attract each other
• opposites attract

11
Properties of Magnets

• if the north pole of one magnet is brought near
  the north pole of another magnet, they will repel
  each other
• if two south poles are brought together, they
  will repel each other

12

• Ferromagnetic materials and magnetisation
• Use 2 paper clips. Put the paper clips close
  together and observe what happens.
• What happens to the paper clips?
• Are the paper clips magnetic?
• Now take a permanent bar magnet and rub it once
  along 1 of the paper clips.
• Remove the magnet and put the paper clip which
  was touched by the magnet close to the other
  paper clip and observe what happens.

13

• A magnetic field is a region in space where a
  magnet or object made of magnetic material will
  experience a non-contact force.

14

• Investigation Field around a Bar Magnet
• Take a bar magnet and place it on a flat surface.
  Place a sheet of white paper over the bar magnet
  and sprinkle some iron filings onto the paper.
• Give the paper a shake to evenly distribute the
  iron filings.
• In your workbook, draw the bar magnet and the
  pattern formed by the iron filings.

15

• Investigation Field around a Pair of Bar
  Magnets
• Take two bar magnets and place them a short
  distance apart such that they are repelling each
  other.
• Place a sheet of white paper over the bar
  magnets and sprinkle some iron filings onto the
  paper.
• Give the paper a shake to evenly distribute the
  iron filings.
• In your workbook, draw both the bar magnets and
  the pattern formed by the iron filings.
• Repeat the procedure for two bar magnets
  attracting each other and draw what the pattern
  looks like for this situation.

16
Like repels like
Opposites attract!
17

• Take a magnet and stroke it along the nail in ONE
  DIRECTION ONLY from thick to pointed end.
• Repeat this about 10 times.
• Now try to pick up the paper clip with the nail.

18
Demonstration

• Place two magnets with opposite poles facing each
  other in a test tube.
• Note what happens.

19
lab

• Use compass to determine the north, south, east
  and west directions.
• Taks a dry needle and Magnetize it by stroking
  with a magnet.
• Drop the needle in the container of water
• The needle won't sink if you drop it carefully.
• In what direction does it point?

20

• Hold magnet above the water with the needle in
  the water.
• Vary the distance of the magnet above the water
  and move it around
• Observe what happens to the needle.
• Repeat using the magnet above a compass.
• Repeat using a paper clip.

21

• If you dip a bar magnet into a cup of nails,
  nails will stick to it. But exactly, why? You
  know that magnets attract iron, but then you also
  note, some nails stick to other nails. Why?

22

• Ordinary iron turned into a magnet whenever it
  touched another magnet.

23

• allow two small nails to attach themselves to
  adjacent spots on one of the poles of a magnet,
  with your fingers holding them parallel to each
  other.
• Both nails are now temporary magnets with the
  same polarity--say, north-seeking or N--at the
  ends next to the magnet. The polarities of their
  other ends therefore must be the same--here,
  south-seeking or S, and those ends should repel
  each other. By spreading your fingers and
  allowing the ends of the nails to move apart, you
  can show that in fact they do.

24

• Use a compass, a D-cell and a short insulated
  wire. (insulated--just in case it gets hot).
• The D-cell should be fresh you will have to draw
  a large current from it, a short circuit really,
  though only for a very short time.
• The compass should point north. Then with your
  thumb press one end of the wire against the
  bottom of the D-cell. The wire should form a
  short loop, coming back to the other terminal of
  the battery, but not touching it.

25

• Move the wire so that the middle of the wire
  passes over the compass needle and is parallel to
  it.
• Then touch the other end of the wire to the
  other end of the cell--just a short touch (1-2
  seconds), it's a short circuit and not good for
  the cell, also it generates a lot of heat at the
  contacts. The needle will immediately pivot to
  stand at 90 degrees to the wire.
• Reverse the electrical contacts by turning the
  D-cell around.

26
Electromagnets

• electromagnet temporary magnet made by wrapping
  a current-carrying wire around an iron core
• the center of an electromagnet is called the core
• it is often made of iron

27
Electromagnets

• as long as current is flowing, an electromagnet
  has a magnetic field
• when current is turned off, there is no longer a
  magnetic field

28
Electromagnets

• there are two ways to make an electromagnet
  stronger
• increasing the number of coils
• increasing the amount of current

29
Electromagnets

• electromagnets are useful because they can be
  turned on and off
• electromagnets have many important uses
• ex. radios, telephones, computers

30
The ends of a magnet are where the magnetic
effect is the strongest.
31
(No Transcript)
32
Magnetic Fields
The region where the magnetic forces act is
called the magnetic field
33
An unmagnetized substance looks like this
34
While a magnetized substance looks like this
35
What are magnetic domains? Magnetic substances
like iron, cobalt, and nickel are composed of
small areas where the groups of atoms are aligned
like the poles of a magnet. These regions are
called domains. All of the domains of a magnetic
substance tend to align themselves in the same
direction when placed in a magnetic field. These
domains are typically composed of billions of
atoms.
36

• Atoms themselves have magnetic properties due
• to the spin of the atoms electrons.

• Groups of atoms join so that their magnetic
  fields
• are all going in the same direction

• These areas of atoms are called domains

37
When an unmagnetized substance is placed in a
magnetic field, the substance can become
magnetized. This happens when the spinning
electrons line up in the same direction.
38
Making a Magnet

• some magnets occur in nature
• these magnets are called natural magnets
• ex. magnetite (also called lodestone)

39
The Earth is a magnet
It exerts magnetic forces and is surrounded by
a magnetic field that is strongest near the North
and South magnetic poles
Geographic North Pole
Magnetic South Pole
Magnetic North Pole
Geographic South Pole
40
The Earth as a Magnet

• the Earth is surrounded by a magnetic field which
  extends far into space
• magnetosphere region of the Earths magnetic
  field

41
The Earth as a Magnet

• the magnetosphere traps charged particles from
  the sun
• when these particles enter the atmosphere, an
  aurora is formed
• auroras are also called the northern and southern
  lights

42
(No Transcript)
43

• Magnetic fields are produced by magnets and by an
  electric current in a wire.

44
Static Electricity

• Static electricity is the charge that stays on
  an object.
• Unlike charges attract each other, and like
  charges repel each other.

45

• Static electricity is the build up of electric
  charges on an object. (can occur by rubbing).
• For example if you rub a balloon against your
  head, then electrons from the atoms that make up
  your hair get transferred to the balloon.
• The balloon becomes negatively charged and your
  hair becomes positively charged. If you hold the
  balloon next to your hair, your hair will stand
  on end.

46

• Experiment Electrostatic Force
• You can easily test that like charges repel and
  unlike charges attract each other by doing a very
  simple experiment.
• Take a glass rod and rub it with a piece of silk,
  then hang it from its middle with a piece string
  so that it is free to move.
• If you then bring another glass rod which you
  have also charged in the same way next to it, you
  will see the rod on the string turn away from the
  rod in your hand i.e. it is repelled.
• If, however, you take a plastic rod, rub it with
  a piece of fur and then bring it close to the rod
  on the string, you will see the rod on the string
  turn towards the rod in your hand i.e. it is
  attracted.

47

• This happens because when you rub the glass with
  silk, tiny amounts of negative charge are
  transferred from the glass onto the silk, which
  causes the glass to have less negative charge
  than positive charge, making it positively
  charged.
• When you rub the plastic rod with the fur, you
  transfer tiny amounts of negative charge onto the
  rod and so it has more negative charge than
  positive charge on it, making it negatively
  charged.

48

• Going down the list materials have an increased
  tendency to lose electrons.
• brass
• copper
• silk
• lead
• fur
• wool
• glass

49
Static Electricity

• Occurs with materials which are insulators
• Rubbing adds or removes electrons
• Object becomes charged
• Like objects repel, unlike attract

50
What is static electricity? When two objects rub
against each other electrons transfer and build
up on an object causing it to have a different
charge from its surroundings. Like the shoes
rubbing against the carpet. Electrons are
transferred from the carpet to the shoes.
51
As electrons collect on an object, it becomes
negatively charged. As electrons leave an object
it attains a positive charges. Charges interact
with each other
Often when you remove clothes from the clothes
dryer, they seem to stick together. This is
because some of the clothes have gained electrons
by rubbing against other clothes. The clothes
losing electrons become positive. The negative
clothes are attracted to the positive clothes.
52
What causes you to be shocked when you rub your
feet across carpet?
An electrical discharge is the passing of an
electric current through the air from a
negatively charged object to a positively charge
object. This is what causes lightning!
53
How are static charges detected?
54
What is the difference between static electricity
and current electricity?
Static electricity is stationary or collects on
the surface of an object, whereas current
electricity is flowing very rapidly through a
conductor. The flow of electricity in current
electricity has electrical pressure or voltage.
Electric charges flow from an area of high
voltage to an area of low voltage.
Water pressure and voltage behave in similar ways.
55
(No Transcript)
56
What are batteries? Batteries are composed of a
chemical substance which can generate voltage
which can be used in a circuit. There are two
kinds of batteries dry cell and wet cell
batteries. Below is an example of a dry cell.
The zinc container of the dry cell contains a
moist chemical paste surrounding a carbon rod
suspended in the middle.
57
Wet cell batteries are most commonly associated
with automobile batteries.
A wet cell contains two connected plates made of
different metals or metal compounds in a
conducting solution. Most car batteries have a
series of six cells, each containing lead and
lead oxide in a sulfuric acid solution.
58
(No Transcript)
59
What are electric circuits? Circuits typically
contain a voltage source, a wire conductor, and
one or more devices which use the electrical
energy. A conductor is any material that allows
electrons to flow through it easily. The term
conductor is also used to refer to objects that
are good conductors of electricity, such as
copper wire. An insulator is a material which
does not allow an electric current to pass.
Nonmetals are good conductors of electricity.
Plastic, glass, wood, and rubber are good
insulators A resistor is a material that resists,
but doesnt stop the flow of current.
60

• Voltage is the electrical potential energy and is
  measured in volts.
• A good analogy is to think of a water hose. There
  is water pressure or potential energy on the
  other side of the faucet or outlet valve. Once
  you open the faucet, the pressure causes the
  water to rush through the hose.
• The unit symbol for volts is V, as in 110V.

61

• Current
• Current indicates the amount of electrons passing
  through the wire and is measured in amperes or
  amps for short. For some reason, they use I to
  indicate current instead of a different letter.
  The unit symbol for amps is A, as in 2.0A.

62

• Resistance
• Electrical resistance can be thought of as the
  "friction" on the movement of electrons in a
  wire. Resistance is measured in ohms, and the
  unit symbol for it is the Greek letter omega, O.
  Thus 3 ohms is often written as 3 O.
• Following the water hose analogy, resistance is
  similar to the friction inside the hose. But
  also, the resistance increases with a narrower
  hose, just like a thin copper wire has more
  electrical resistance than a thick wire.

63

• DC circuit
• The power source for a DC circuit could be a
  battery or DC generator. The () and (-) indicate
  the direction of the current.

64

• AC circuit
• A simple AC circuit is illustrated below. A
  circle with the sine wave symbolizes an AC
  generator with some given voltage.

65
What is the difference between an open circuit
and a closed circuit? A closed circuit is one in
which the pathway of the electrical current is
complete and unbroken. An open circuit is one in
which the pathway of the electrical current is
broken. A switch is a device in the circuit in
which the circuit can be closed (turned on) or
open (turned off).
66

• The steady flow of electricity is called an
  electric current. A current will move along a
  wire or a path called a circuit.
• Circuit means to go around.

67
CIRCUIT TYPES
The simplest type of circuit involves electricity
going around with no choices (electrons dont
really choose).
This is called a Series circuit. Draw the path
the electrons travel. The other main type of
circuit has two or more branches. This is called
a Parallel circuit. Draw on the electron flow.
What sort of circuit is this? A parallel but,
more importantly A short circuit.
68
Circuits

• A series circuit is a circuit that has only one
  path for the current.

• A parallel circuit has more than one path for
  current to travel.

69

• Lights in our homes are wired in parallel
  circuits.

70
Series circuit

• Has a single loop for electrons to travel round
• Components are connected one after another
• Current has to travel through all components
• Current is the same at all points
• Voltage is shared between components

71

• What is a series circuit?
• A series circuit is one which provides a single
  pathway for the current to flow. If the circuit
  breaks, all devices using the circuit will fail.

72

• In series
• I I1 I2 I3 ...
• V V1 V2 V3...
• RTotal R1 R2 R3...

73
What is a parallel circuit? A parallel circuit
has multiple pathways for the current to flow. If
the circuit is broken the current may pass
through other pathways and other devices will
continue to work.
74
Parallel circuit

• Has two or more paths for electrons to flow down
• Current is shared between the branches
• Sum of the current in each branch total current
• Voltage loss is the same across all components

75

• In a parallel circuit
• I I1 I2 I3 ...
• V V1 V2 V3...
• 1/ RTotal 1/ R1 1/R2 1/ R3...

76
Current (I)

• Current is the flow of electrons
• around a circuit
• DC direct current like battery
• Electrons flow in one direction
• AC Alternating current like mains
• Electron flow changes direction 50x per second

77
Ammeter

• Measures CURRENT(I)
• Unit Amp (A)
• Current is flow of electrons
• Connect in series at the point you wish to
  measure
• RED to RED and BLACK to BLACK

78
Voltmeter

• Measures voltage
• Unit Volt (V)
• Connect in parallel around a component

79
The unit for measuring resistance is the ohm (O).
80
Resistance (R)

• The amount that a component slows the current

• As the electrons are slowed by a resistor, energy
  is lost in the form of heat.
• This means that current, resistance and voltage
  must be linked.
• This is Ohms law
• The unit of resistance is the ohm, symbol ?

81

• Ohms Law
• The volt, ohm, and ampere are related to each
  other in a simple formula known as Ohms law
• Voltage current resistance, or E I
  R
• (1) Voltage current x resistance
• (2) Current voltage / resistance
• (3) Resistance voltage/ current

82

• If a current of 5 amps flows through a resistance
  of 40 ohms, the voltage across that resistor,
  according to the formula is
• Volts amps x ohms
• 5 amps x 40 ohms
• 200 volts

83
Electrical Calculations What is Ohms Law?
3 V
I 1.5 amps
I
2 O
84

• Using the information given in this diagram
  determine the reading on the ammeter.

85

• If the current of the circuit is 10 amps, what is
  the voltage of the battery?

86

• Solve for the current of the circuit

87

• In this circuit what is the reading on the
  ammeter?

88

• When a conductor has a potential difference of
  100 volts placed across it, the current through
  it is 5 ampere. What is the resistance of the
  conductor?

89

• If the potential difference across a 50- ohm
  resistor is 5 volts, what is the current through
  the resistor?
• (1)10A (2) .5A (3) 5A (4) .1A

90

• A generator supplies current in a circuit. If the
  resistance in the circuit is increased, the force
  required to keep the generator turning at the
  same speed is
• (1) decreased (2) increased (3) unchanged

91

• If the voltage across a 4-ohm resistor is 12
  volts, the current through the resistor is
• (1) .25 A (2) .48 A (3) 3.0A (4) 4.0A

92

• A resistor carries a current of .1 ampere when
  the potential difference across it is 5 volts.
  The resistance of the resistor is
• (1) .02 O (2) .5O (3) 5O (4) 50O

93

• Draw a circuit diagram to include a 60-V battery,
  an ammeter, and a resistance of 12.5 O in series.
  Determine the reading on the ammeter.

94

• (a) What is the total resistance of this circuit?
• (b) What is the current of the circuit?

95
Power

• Energy used by component per second
• Unit of power is the Watt, symbol is W
• One watt means that 1 joule of electrical energy
  is being used up per second.
• Current, voltage and power
• are linked

96
How is Electrical Power calculated? Electrical
Power is the product of the current (I) and the
voltage (v) The unit for electrical power is the
same as that for mechanical power in the previous
module the watt (W)
Example Problem How much power is used in a
circuit which is 110 volts and has a current of
1.36 amps? P I V Power (1.36 amps) (110 V)
150 W
97
How is electrical energy determined? Electrical
energy is a measure of the amount of power used
and the time of use. Electrical energy is the
product of the power and the time.
Example problem
E P X time P I V
P (2A) (120 V) 240 W E (240 W) (4 h)
960Wh 0.96 kWh
98
(No Transcript)
99
Multimeter

• DC Voltage Measurement Procedure
• 1. Insert the black test lead plug into the COM
  jack and the red test lead plug into the V/ jack.
• 2. Select a higher VDC range than you anticipate
  measuring.
• For example, to measure 700 VDC, select the
• 1000 VDC range. If the magnitude of voltage isnt
  known, select the highest range (1000 V).

100

• Note If you tried to measure 700 VDC on the 200
  VDC range, an overrange indication of 1 would
  be displayed.
• Conversely, you wouldnt measure 1.5 VDC on the
  1000 VDC range because accuracy would suffer.

101
DC Current Measurement Procedure

• Insert the red test lead plug into the A jack and
  the black test lead plug into the COM jack for a
  maximum measurement of current up to 200 mA. Turn
  OFF the power to the device being measured.
• Select a higher DCA range than you anticipate
  measuring.
• If the magnitude of current isnt known, select
• the highest range (200 mA) and reduce the
  setting until a satisfactory reading is obtained

102

• In this circuit, three resistors receive the same
  amount of voltage (24 volts) from a single
  source. Calculate the amount of current "drawn"
  by each resistor, as well as the amount of power
  dissipated by each resistor

103

• I1 ? 24 ampsI2 ? 12 ampsI3 ? 8 ampsP1 ?
  576 wattsP2 ? 288 wattsP3 ? 192 watts

104
(No Transcript)
105
Each resistor has 15 volts across it in this
circuit.

• Determine the amount of voltage impressed across
  each resistor in this circuit

106
IR(2.2k) 10.91 mAIR(4.7k) 5.11 mA

• According to Ohm's Law, how much current goes
  through each of the two resistors in this circuit?