Transcript Electromagnetism

Magnetic Field
Patterns
A Quick Review of
Magnetic Fields
• http://www.youtube.com/watch?v=uj0DFDfQajw
• When an electric current passes along a wire, a
magnetic field is set up around the wire.
• For a straight wire, the magnetic field is a circle
around the wire.
• We can use our right hand to determine the
direction of the magnetic field.
• If a compass is placed next to a current carrying
conductor, what would happen if the current were
reversed?
• What would happen if you moved the compass
away from the current carrying conductor?
• When a long wire is wrapped in a coil it is called a
solenoid.
• Prediction: What do you think the magnetic field
around a solenoid would look like?
• To break it down we could imagine what the field
lines would look like for one loop.
• If we increase the number of loops next to each
other we can see that the magnetic field would
look like this:
• Describe the properties of the magnetic field in a
solenoid:
Conclusion
• The magnetic field lines around a wire are circles
centered on the wire.
• The magnetic field of a solenoid is uniform inside the
solenoid and like a bar magnet on the outside.
• Increasing the current increases the strength of the
magnetic fields; reversing the current reverses the
magnetic field lines.
• Homework pg. 203 #1,2
The Motor Effect
• When a current passes through a wire in a
magnetic field a force is exerted on the wire. This is
called the motor effect.
• We can use our left hand to determine the direct of
the Kinetic movement, Field, and Current (KFC).
Kinetic Movement
Field
Current
• Use the left hand rule to determine the direction of
motion for the following:
• How could we increase the force?
o Increase the current
o Use a stronger magnet
• How could we reverse the direction of the force?
o Reverse the direction of the field
o Reverse the direction of the current
• The Force depends on the angle between the wire
and the magnetic field lines. The greatest force
occurs when the wire is perpendicular to the field.
• The direction of the force is always at right angles to
the wire and field lines.
The Motor Effect
• A current carrying conductor in a magnetic field will
experience a force (the motor effect).
• In the motor effect, the force:
o Is increased if the current or the strength of the
magnetic field is increased.
o Is at right angles to the direction of the magnetic
field and to the wire.
o Is reversed if the direction of the current or the
magnetic field is reversed.
The Electric
Motor
• The motor effect can be used to create an electric
motor.
• The following is a diagram of a simple motor:
Brushes (metal or
graphite)
Split-ring commutator
• What direction is the current flowing in the wire?
Brushes (metal or
graphite)
Split-ring commutator
• What is the direction of the magnetic field?
Brushes (metal or
graphite)
Split-ring commutator
• Use the left hand rule to determine the way the wire will
move. Notice that the current in the wire is flowing in two
different directions between the magnets.
Brushes (metal or
graphite)
Split-ring commutator
• These opposing forces will cause the wire to turn.
Brushes (metal or
graphite)
Split-ring commutator
• This video demonstrates a working motor:
http://www.youtube.com/watch?v=yJyVTd_Ovw&feature=related
• Why does the motor need brushes and a split-ring
commutator?
Brushes (metal or
graphite)
Split-ring commutator
Brushes (metal or
graphite)
Split-ring commutator
• The split-ring commutator reverses the current round
the coil every half-turn. This means the coil is pushed
in the same direction every half-turn.
• What would happen if we increase the current?
• What would happen if we reverse the current?
Brushes (metal or
graphite)
Split-ring commutator
The Electric Motor
• A simple electric motor has a rectangular coil of
wire that spins in a magnetic field when a current
passes through a coil.
• The speed of an electric motor is:
o Increased if the current is increased
o Reversed if the current is reversed
• Homework: pg. 207 #1,2
Electromagnetic
Induction
• For the electric motor, when a current passes
through a magnetic field a force is created.
• For a generator, when a wire moves across a
magnetic field line a current is induced in the wire.
Motor
Field
Current
Movement
Generator
Field
Movement
Current
• If a wire is moved through a magnetic field we will
produce a current.
• Or, if a field is moved through a wire we will
produce a current.
Motor
Field
Current
Movement
Generator
Field
Movement
Current
Electromagnetic Induction
• When a wire passes through the lines of a magnetic
field, an emf is induced in a wire.
• If the wire is part of a complete circuit, the induced
emf causes a current in the circuit.
• The current is increased if the wire moves faster or a
stronger magnet is used.
• The direction of an induced current opposes the
change that causes it.
• Homework pg.213 #1,2
An AC Generator
AC Generator Graph
The AC Generator
• The simple ac generator consists of a coil that spins
in a uniform magnetic field.
• The slip rings and brush contacts enable the coil to
stay connected to the circuit.
• The peak value of the induced emf is when the
sides of the coil cut directly across the magnetic
field lines.
• When the sides of the coil move parallel to the field
lines, the induced emf is zero.
Transformers
Not these ones…
These ones.
Remember:
• When a current passes through a solenoid, a
magnetic field is created.
• The more coils the solenoid has the stronger the
magnetic field.
• A changing magnetic field creates an alternating
current. This creates an alternating voltage in the
conductor.
• The more turns in a coil, the larger the value of the
alternating voltage.
A Basic Transformer
• A transformer has two coils of insulated wire, both
wound round the same iron core as shown below:
A Basic Transformer
• When an alternating voltage is applied to the
primary coil and magnetic field is created in the
iron core.
A Basic Transformer
• This magnetic field will induce an alternating
voltage in the secondary coil.
Questions
Q: Will the alternating voltage in the secondary coil
be more or less than the primary coil?
A: It will be less because there are less turns in the coil.
Questions
Q: What would happen if both coils had the same number
of turns?
A: The primary and secondary voltage would be the same.
Questions
Q: Why does the primary voltage have to be alternating?
A: If it were only in one direction, the magnetic field would
not change. Only a changing magnetic field induces a
current.
The Transformer Equation
Vp N p
=
Vs N s
Where:
Vp – primary voltage
Vs – secondary voltage
Np – number of primary turns
Ns – number of secondary turns
Types of Transformers
• A step-up transformer has a secondary voltage that
is greater than the primary.
• A step-down transformer has a secondary voltage
that is less than the primary.
Example Problem:
• A transformer with 500 turns in the primary coil and
100 turns in the secondary coil has a secondary
voltage of 12V. What is the primary voltage? What
type of transformer is this?
Summary
• A transformer consists of a primary coil and a
secondary coil wrapped on the same iron core.
• Transformers only work using alternating current.
• The alternating current in the primary coil creates
an alternating magnetic field in the iron core which
induces an alternating voltage in the secondary
coil.
• The transformer equation is: Vp = N p
Vs N s
High-voltage
transmission of
electricity
Electricity is transferred using high-voltage
transmission. We will examine why it is more efficient to
transfer electricity using a high voltage.
Efficiency
• The efficiency of a system is the percentage of
energy entering the system that is turned into useful
energy.
• If I put 100J of electric energy into a lamp and 75J
of light energy comes out, I would say that my lamp
is 75% efficient.
• That means that 25% of the energy has gone
somewhere else. Where did it go?
Efficiency
• When current is traveling down a conductor why
does it lose energy?
• It loses energy because the conductor has
resistance. The current in the conductor causes it to
heat up, which means that electrical energy is lost
to heat energy.
• How could we transfer the same amount of
electrical energy, but reduce the amount of energy
lost?
• If we increase the voltage we can reduce the
current but transfer the same amount of energy.
Transformers
• Transformers are used to step up and down
voltages, so that a lower current can be used to
transfer electricity.
• Transformers are almost 100% efficient. That means
that almost all the electrical power supplied to the
primary coil is supplied to the secondary coil.
• Remember the equation for electrical power?
• P = IV
• So for a 100% efficient transformer Pp = Ps.
Efficiency Equation
IpVp = IsVs
Where
Ip – primary current
Is – secondary current
Vp – primary voltage
Vs – secondary voltage
Summary
• Transformers are used to step voltages up or down.
• High voltage transmission of electricity is much more
efficient than transmission at much lower voltages.
• Homework pg. 219 #1,2
Cathode Rays
A discharge tube
• Discharge tubes were invented in 1870 by William
Crookes.
• He showed that by applying a high voltage across
a low pressure gas, the gas would begin to glow.
A discharge tube
• Crookes also found that different gasses produce a
different colour.
A discharge tube
• In a discharge tube the negatively charged plate is
called the cathode, and the positively charged
plate is called the anode.
Investigating the tube
• By placing a paddle wheel inside the tube, Crookes
was able to show that the that the wheel rotates
due to radiation in the tube. He was able to show
that the radiation began at the cathode. He
named them cathode rays.
Investigating the tube
• John Thomson showed that cathode rays are
deflected by an electric field. By taking careful
measurements he showed that cathode rays are
made of negatively charged particles. They
became known as electrons.
• Cathode rays are beams of electrons.
Thermionic Emission
• An electron tube is a more controlled way to
produce an electron beam.
• A filament is heated so that the electrons gain
enough kinetic energy to leave. This is known as
thermionic emission.
Thermionic Emission
• If we have a small hole in the anode, some of the
electrons will go through the hole. This will create a
narrow beam.
Deflecting Cathode Rays
• Cathode rays are negatively charged. They will be
deflected by a magnetic field.
• We can use this fact to direct electron beams
where we want.
Deflecting Cathode Rays
• Increasing the pd between the anode and
cathode increases the speed of electrons.
• Changing the pd in the deflecting coils will change
the direction of the electron beam.
Cathode Rays Summary
• Cathode rays are electrons that travel towards the
anode (positive) after being emitted from the
cathode (negative).
• Thermionic emission is the emission of electrons from
a heated filament.
• A beam of electrons can be produced by
attracting electrons from a heated filament towards
a positive electrode (anode) in a vacuum tube.
• Cathode rays are deflected by electric and
magnetic fields.
• Homework pg. 209 #1,2
The Cathode
Ray
Oscilloscope
Using electric fields to
deflect cathode rays
• Cathode rays are beams of electrons. That means
they are negatively charge.
• If they pass between a positive and negative plate
they will be attracted towards the positive plate.
Oscilloscopes
• By using a horizontal and vertical electric field the
electrons can be deflected in any direction.
• The bigger the pd between the plates, the larger
the deflection.
• If the pd is reversed, the direction of deflection will
be reversed.
Oscilloscopes
We can use cathode rays
to create way paterns
What are they used for?
• They provide a way to visualize electrical voltage.
• The voltage can be compared to a change in time
or with other voltages.
• They can also show the time between two events,
even if those events occur very close together.
Oscilloscope Summary
• Cathode rays passing between oppositely charged
plates are deflected towards the positive plate.
• The deflection is increased if the pd is increased.
• In the cathode ray oscilloscope, a narrow beam of
electrons is deflected by two pairs of deflecting
plates.
• Homework pg. 211 #1