Transcript Document 7239437

Magnets around the House:
Headphones
Refrigerator magnets
Computer speakers
Telephone receivers
Phone ringers
Microwave tubes
Seal around refrigerator door
Floppy disk recording and reading head
Audio tape recording and playback head
Video tape recording and playback head
Credit card magnetic strip
TV deflection coil
Computer monitor
Computer hard drive
Power supply transformers
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
Uses of magnets
• Magnets were first put to use in navigation
because they always point north and south.
What is Magnetism?
Magnetism is the force of
attraction or repulsion of
magnetic material due to
the arrangement of its
electrons.
lab
• Magnets stick to some things but not to
others.
• Find different things to test with your magnet.
• What is your conclusion?
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
Poles of a
magnet always
Come in pairs!
Magnetic Poles
– poles – two ends of a magnet. every magnet
has two poles: north (N) pole and south (S)
pole
N
N
S
S
N
S
Break one bar magnet in half, and you have
two smaller bar magnets!
Properties of Magnets
• if a north pole and a south pole are brought
together, they will attract each other
– opposites attract
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
• 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.
• A magnetic field is a region in space where a
magnet or object made of magnetic material
will experience a non-contact force.
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.
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.
Like repels like…
Opposites attract!
• 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.
Demonstration
• Place two magnets with opposite poles facing
each other in a test tube.
• Note what happens.
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?
• 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.
• 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?
• Ordinary iron turned into a magnet whenever
it touched another magnet.
• 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.
• 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.
• 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.
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
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
Electromagnets
• there are two ways to make an electromagnet
stronger
– increasing the number of coils
– increasing the amount of current
Electromagnets
• electromagnets are useful because they can
be turned on and off
• electromagnets have many important uses
– ex. radios, telephones, computers
The ends of a magnet are where
the magnetic effect is the
strongest.
Magnetic Fields
The region where the magnetic forces
act is called the “magnetic field”
An unmagnetized substance
looks like
this…
While a magnetized substance looks
like this…
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.
Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic fields
are all going in the same direction
These areas of atoms are called “domains”
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.
Making a Magnet
• some magnets occur in nature
• these magnets are called natural magnets
– ex. magnetite (also called lodestone)
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
Magnetic South Pole
Geographic South Pole
Geographic North Pole
Magnetic North Pole
The Earth as a Magnet
• the Earth is surrounded by a magnetic field
which extends far into space
• magnetosphere – region of the Earth’s
magnetic field
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
• Magnetic fields are produced by magnets and
by an electric current in a wire.
Static Electricity
Static electricity is the
charge that stays on
an object.
Unlike charges attract
each other, and like
charges repel each
other.
• 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.
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.
• 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.
• Going down the list materials have an
increased tendency to lose electrons.
• brass
• copper
• silk
• lead
• fur
• wool
• glass
Static Electricity
•
•
•
•
Occurs with materials which are insulators
Rubbing adds or removes electrons
Object becomes charged
Like objects repel, unlike attract
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.
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.
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!
How are static charges detected?
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.
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.
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.
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 doesn’t stop the flow of
current.
• 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.
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.
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, Ω.
Thus 3 ohms is often written as 3 Ω.
• 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.
DC circuit
• The power source for a DC circuit could be a
battery or DC generator. The (+) and (−)
indicate the direction of the current.
AC circuit
• A simple AC circuit is illustrated below. A circle
with the sine wave symbolizes an AC
generator with some given voltage.
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).
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.”
CIRCUIT TYPES
The simplest type of circuit involves electricity going around
with no “choices” (electrons don’t 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.
Circuits
• A series circuit • A parallel
is a circuit that circuit has
has only one
more than one
path for the
path for
current.
current to
travel.
• Lights in our
homes are
wired in
parallel
circuits.
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
• 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.
•
•
•
•
In series:
I = I1 = I2 = I3 ...
V = V1 + V2 + V3...
RTotal = R1 + R2 + R3...
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.
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
•
•
•
•
In a parallel circuit :
I = I1 + I2 + I3 ...
V = V1 = V2 = V3...
1/ RTotal =1/ R1 + 1/R2 + 1/ R3...
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
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
Voltmeter
• Measures voltage
• Unit = Volt (V)
• Connect in parallel around a component
The unit for measuring resistance is the ohm
(Ω).
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 
V
I × R
• Ohm’s Law
• The volt, ohm, and ampere are related to each
other in a simple formula known as Ohm’s
law:
Voltage = current resistance, or E = I R
• (1) Voltage = current x resistance
• (2) Current = voltage / resistance
• (3) Resistance = voltage/ current
• 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
Electrical Calculations – What is Ohm’s Law?
I=
3V
2Ω
I = 1.5 amps
• Using the information given in this diagram
determine the reading on the ammeter.
• If the current of the circuit is 10 amps, what is
the voltage of the battery?
Solve for the current of the circuit
• In this circuit what is the reading on the
ammeter?
• 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?
• 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
• 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
• 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
• 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 Ω (2) .5Ω (3) 5Ω (4) 50Ω
• Draw a circuit diagram to include a 60-V
battery, an ammeter, and a resistance of 12.5
Ω in series. Determine the reading on the
ammeter.
• (a) What is the total resistance of this circuit?
• (b) What is the current of the circuit?
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
P
I × V
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=IV
Power = (1.36 amps) (110 V) = 150 W
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=IV
P = (2A) (120 V) = 240 W
E = (240 W) (4 h) = 960Wh = 0.96 kWh
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
isn’t known, select the highest range (1000 V).
• Note: If you tried to measure 700 VDC on the
200 VDC range, an overrange indication of “1”
would be displayed.
• Conversely, you wouldn’t measure 1.5 VDC on
the 1000 VDC range because accuracy would
suffer.
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 isn’t known, select
the highest range (200 mA) and reduce the
setting until a satisfactory reading is obtained
• 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:
• I1 Ω = 24 amps
I2 Ω = 12 amps
I3 Ω = 8 amps
P1 Ω = 576 watts
P2 Ω = 288 watts
P3 Ω = 192 watts
Each resistor has 15 volts across it
in this circuit.
• Determine the amount of voltage impressed
across each resistor in this circuit:
•
IR(2.2k) = 10.91 mA
IR(4.7k) = 5.11 mA
• According to Ohm's Law, how much current
goes through each of the two resistors in this
circuit?