Transcript 11. Electric Power

Outline of Electricity and
Magnetism
1. Positive and Negative Charge
2. What is Charge?
3. Coulomb’s Law
4. Force on an Uncharged Object
5. Matter in Bulk
6. Conductors and Insulators
7. Superconductivity
8. The Ampere
9. Potential Difference
10. Ohm's Law
11. Electric Power
12. Magnets
13. Magnetic Field
14. Oersted's Experiment
15. Electromagnets
16. Magnetic Force on a Current
17. Electric Motors and
Generators
18. Electromagnetic Induction
19. Transformers
1. Positive and
Negative Charge
Opposites attract-Same charges
repel
Let’s try it!
1. Positive and Negative Charge
Electrons are stripped
from one component
and transferred to the
other to cause both to
be oppositely charged.
Fig. 1.
Ben Franklin discovered
electric charge.
2. What is Charge?
Atoms are composed of protons (+), electrons (-)
and neutrons. The nucleus contains the protons
and neutrons and the electrons surround the
nucleus.
2. What is Charge?
Protons are much larger than electrons but have
an equal and opposite charge.
The coulomb (C) is the unit of electric charge.
The basic quantity of electric charge (e) is 1.6 x 10-19 C.
3. Coulomb’s Law
Q 1Q 2
F = K ------R2
Charles Coulomb
(1736-1806)
4. Force on an Uncharged
Particle
Initially the paper is uncharged, but the
comb polarizes the charges in the paper.
5. Matter in Bulk
Coulomb's law resembles the law of gravity; however,
gravitational forces are always attractive, whereas electric
forces may be attractive or repulsive.
Coulomb’s Law
Q 1Q 2
F = K ------R2
Law of Gravity
M 1M 2
F = G ------R2
Gravitational forces dominate on a cosmic
scale; electric forces dominate on an atomic
scale.
6. Conductors and Insulators
A conductor is a substance through which electric charge flows
readily. An insulator is a substance that strongly resists the flow of
electric charge. Semiconductors are substances whose electrical
conductivity is between that of conductors and insulators.
Semiconductors
Transistors are switches that conduct
electricity only when a second source of
electricity is energized.
A
B
John Bardeen
Nobel Prizes in 1956
and 1972 for developing
the transistor and
superconductivity.
What a
Transistor
looks like.
C
A current will not flow from A to B unless C is energized.
Fig. 2.
Enlargement
of 5 mm
square
computer
chip.
7. Superconductivity
Superconductivity
refers to the loss of all
electrical resistance by
certain materials at very
low temperatures.
Substances that are
superconducting at
150K are now known
which is warmer than
liquid nitrogen (77K).
Maglev Train in Japan
8. The Ampere
The Ampere is a measure of how
much electrical current is flowing
and is measured in units of amps.
Q
I = ---t
8. The Ampere
The current varies depending on the force behind
the current and the resistance to flow.
9. Potential Difference
Alessandro Volta
(1745-1827)
Potential difference, or voltage, is
the electrical potential energy per
coulomb of charge.
J
V = ---C
Fig. 3. & 4.
10. Ohm’s Law
André Marie Ampére
(1775-1836)
V
I = ---R
Resistance is a
measure of
opposition to the
flow of charge
and is measured
in ohms ()
Georg Ohm (1787-1854)
Fig. 5
DC Circuit Concept Map
Parallel and Series Circuits
•
•
•
•
•
The Parallel Circuit
The Series Circuit
PhysicsLAB: An Introduction to DC Circuits
Capacitance
Capacitance of a Capacitor
Fig. 6.
11. Electric Power
P= IV
The power of an electric current is the rate at which
it does work and is equal to the product of the
current and the voltage of a circuit:
The unit of electric power is the watt.The
commercial unit of electric energy is the
kilowatthour (kWh).
11. Electric Power
Typical Power Ratings
Appliance
Stove
Clothes Dryer
Heater
Dishwasher
Photocopier
Iron
Vacuum Cleaner
Coffee Maker
Refrigerator
Portable Sander
Fan
Personal Computer
TV Receiver
Fax Transmitter/Receiver
Charger for Electric Toothbrush
Power (W)
12,000
5,000
2,000
1,600
1,400
1,000
750
700
400
200
150
150
120
65
1
12. Magnets
Every magnet has a north pole
and a south pole.
13. Magnet Field
How to make a magnet:
Heat in magnetic field.
Magnetic force lines.
14. Oersted’s
Experiment
Hans
Christian
Oersted
(1777-1851)
Hans Christian Oersted discovered in 1820 that an
electric current near a compass causes the compass
needle to be deflected. Oersted's experiment showed
that every electric current has a magnetic field
surrounding it.
14. Oersted’s Experiment
According to the right-hand rule, the electron current
in a wire and the magnetic field it generates are
perpendicular to each other.
14. Oersted’s Experiment
All magnetic fields originate from moving electric
charges. A magnetic field appears only when relative
motion is present between an electric charge and an
observer. Electric and magnetic fields are different
aspects of a single electromagnetic field.
15. Electromagnets
An electromagnet consists of an iron core placed
inside a wire coil. The magnetic field strength of a
wire coil carrying an electric current increases in
direct proportion to the number of turns of the coil.
15. Electromagnets
An electromagnet can
be used to move large
quantities of metal.
When the current is on
the magnet will pick up
the metal. When you
want to drop it you turn
off the power and the
electromagnet is
disabled and the metal
drops.
16. Magnetic Force on a Current
A magnetic field exerts a
sideways push on an
electric current with the
maximum push occurring
when the current is
perpendicular to the
magnetic field. Currents
exert magnetic forces on
each other. The forces are
attractive when parallel
currents are in the same
direction and are repulsive
when the parallel currents
are in opposite directions.
Fig. 7.
The experimental
Japanese Maglev train
uses magnetic forces
for both support and
propulsion.
How a TV works.
17. Electric Motors
An electric motor uses
the sideways push of
a magnetic field to
turn a current-carrying
wire loop. Electric
motors use a
commutator to
change the direction
of the current in the
loop. Alternating
current electric motors
do not use
commutators.
How Does A Motor Do Work?
The Force
Theory Behind a Working Motor
The Torque
The Applications of a Motor
What about a Generator ?
• The current running through coils of wire
would produce magnetic field that attracts or
opposes the existing permanent magnet, which
then drives the coil of wire to move or rotate.
• Reversely, coils of wire move or rotate in a
magnetic field would also produce a current.
This is what a generator does.
How can Motors and Generators
help us?
• With the theory of conservation of energy, we know that the
energy will not disappear. Rather, energy would change from
one form to another. In our discussion today, we have seen
electrical energy turn into mechanical energy and also from
mechanical to electrical.
• Motors would help us to utilize the electrical energy that we
have to do work through the form of mechanical energy.
• Generator does the opposite. It transforms mechanical energy
into electrical energy.
• Generator is able to provide us with the source of electricity
when we provide other forms of energy or materials that could
react to release energy.
How Does a Generator Work
18. Electromagnetic Induction
The effect of producing an
induced current is known as
electromagnetic induction.
The direction of the induced
current can be reversed by
reversing the motion of the wire
or reversing the field direction.
The strength of the current
depends on the strength of the
magnetic field and the speed of
the wire's motion.
Fig. 8.
Michael Faraday (17911867) built the first electric
motor and discovered
magnetic induction.
The stationary
windings of a
large electric
motor. magnetic
forces underlie
the operation of
such motors.
Fig. 9.
Sharks navigate
with the help of
the earth’s
magnetic field.
They detect the
field using
electromagnetic
induction.
Alternating and Direct Current
Alternating current (ac) is current that flows in a back-andforth manner; household current changes direction 120 times
each second (60 Hz). Direct current (dc) flows in one
direction.
The ac generator (or alternator) produces an ac current and can be
modified to produce dc current by
1. Use of a commutator.
2. Use of a rectifier which permits current to pass through it in only
one direction.
19. Transformers
A transformer is a device composed of two unconnected
coils, usually wrapped around a soft iron core, that can
increase or decrease the voltage of ac current.
19. Transformers
A transformer is used to step the voltage
down and the power up (P=IV) so that we
can use it. Low power is desired for the
transport of electricity long distances to
avoid loss of energy to heat loss.
A moving coil
activated by voice
vibrations is used
as a microphone.
The coil induces a
current in the
magnet that can be
amplified or
recorded.
19. Transformers
A taperecorder records signals from a microphone on
magnetic tape which then can be run across a magnet and
played back.
What we discussed about
Electricity and Magnetism
1. Positive and Negative Charge
2. What is Charge?
3. Coulomb’s Law
4. Force on an Uncharged Object
5. Matter in Bulk
6. Conductors and Insulators
7. Superconductivity
8. The Ampere
9. Potential Difference
10. Ohm's Law
11. Electric Power
12. Magnets
13. Magnetic Field
14. Oersted's Experiment
15. Electromagnets
16. Magnetic Force on a Current
17. Electric Motors and
Generators
18. Electromagnetic Induction
19. Transformers