phys141 principles of physical science chapter 8 electricity and magnetism instructor: li ma office:...
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Phys141 Principles of Physical Science
Chapter 8 Electricity and Magnetism
Instructor: Li Ma
Office: NBC 126Phone: (713) 313-7028Email: [email protected]
Webpage: http://itscience.tsu.edu/ma
Department of Computer Science & PhysicsTexas Southern University, Houston
Oct. 6, 2004
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Topics To Be Discussed
Electric Charge and CurrentVoltage and Electric PowerMagnetismElectromagnetismSkip
– §8.3– The Earth’s Magnetic Field in § 8.4– Motors and Generators in §8.5
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Electric Charge
Electric charge is a fundamental quantity (mentioned in Chapter 1)
The property of electric charge is associated with certain subatomic particles
Two types of charges:– positive (+)– negative (-)
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Electric Charge (cont)
All matter is made up of small particles called atoms
An atom is composed of electrons, protons and neutrons:– Electrons: negatively charged particles– Protons: positively charged particles– Neutrons: neutral particle– Table 8.1 on page 167
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Electric Charge (cont)
All three particles have certain massElectrons and protons possess electric
charges– Magnitudes are equal– Natures are different
When we have same number of electrons and protons– Total charge is zero – neutral situation
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Electric Charge (cont)
Unit of electric charge: coulomb (C)Electric charge is usually designated by
the letter q– An excess of positive charges: +q– An excess of negative charges: -q
When charge flows, or is in motion, we have electric current
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Electric Current
Electric current is defined as the time rate of flow of electric charge
Unit: ampere (A), 1 A = 1 C/s
current =charge
time
I =qt
or
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Electric Current (cont)
Electrical conductors: materials in which electric charge flows readily– The outer, loosely bound electrons of the
atoms: ex. metalsElectrical insulators: materials that do
not conduct electricity very well– Electrons are more tightly bound– ex. Wood, glass, plastic, rubber, etc
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Electric Current (cont)
Semiconductors: materials that are neither good conductors nor good insulators– ex. graphite (carbon)
Net charge q:– Electrons move randomly and chaotically
=> no net charge flows– More electrons move in one direction =>
net charges flow => current
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Electric Force
An electric force exists between any two charged particles
This kind of mutual forces may be either attractive or repulsive, depending on the types of charges (+ or -)
Law of charges:– Like charges repel, and unlike charges
attract
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Electric Force (cont)
The law of charges gives the direction of an electric force, what about the magnitude?
Coulomb’s law:– The force of attraction or repulsion
between two charged bodies is directly proportional to the production of two charges and inversely proportional to the square of the distance between them
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Electric Force (cont)
Where F is magnitude of the electric force
q1 is magnitude of the first charge
q2 is magnitude of the second charge
r is distance between charges
k is a proportionality constant:
k = 9.0 x 109 N-m2/C2
F =kq1q2
r2
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Electric Force (cont)
Compare Coulomb’s law and Newton’s law of universal gravitation:– Both forces depend on the square of the
separation distance– Coulomb’s law depends on charge, whereas
Newton’s law depends mass– Coulomb’s law can give rise to either an attractive
or a repulsive force; on the other hand, the force of gravitation is always attractive
– The electric forces are comparatively much stronger than the gravitational forces
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Voltage
It takes work to separate chargesThe more charges to separate, the
more work it takes => electric potential energy
Freely moving charge will move toward the charge of opposite sign:
electric potential energy=> kinetic energy
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Voltage (cont)
Voltage, or potential difference, is defined as the amount of work it would take to move a charge between two points, divided by the value of the charge
In other word, voltage (V) is the work (W) per unit charge (q) or the electric potential energy per unit charge
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Voltage (cont)
Unit: volt (V), 1 V = 1 J/CVoltage is caused by a separation of
charge
voltage =work
charge
V =Wq
or
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Voltage (cont)
Electric potential energy may be used to set up a current
When there is current, it meets with some opposition because of collisions within the conducting material
Resistance (R): the opposition to the flow of charge– Unit: ohm (Ω)
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Voltage (cont)
Ohm’s law:
voltage = current x resistance
or V = I·RIt is a simple relationship involving
voltage, current and resistanceOne ohm is one volt per ampere:
R = V / I
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Electric Power
When current exists in a circuit, work is done to overcome the resistance of the circuit, and power is expended
Electric power is defined in terms of current and voltage
P = I·V
or P = I2·R
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Electric Power (cont)
The power that is dissipated in an electric circuit is frequently in the form of heat
This heat is called joule heat or I2R losses
This heating effect is used in electric stoves, heaters, hair dryers, light bulb, and so on
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Magnetism
Two poles of the magnet: one at each end– The north pole (N): the north-seeking pole– The south pole (S): the south-seeking pole
Law of poles:– Like poles repel, and unlike poles attract– N-S poles attract, and N-N poles and S-S poles
repel– The strength of the magnetic force is inversely
proportional to the square of the distance between the poles
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Magnetism (cont)
All magnets have two poles: i.e. magnets are always dipoles
Every magnet produces a force on every other magnet
Magnetic field (B): a set of imaginary lines that indicates the direction in which a small compass needle would point if it were placed near a magnet
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Magnetism (cont)
Magnetic field is a force field: the field lines are indications of the magnetic force
In a magnetic field pattern:– The arrows in the field lines indicate the
direction in which the north pole of a compass would point
– The closer together the field lines, the stronger the magnetic force
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Electromagnetism
There is an electric field around chargesElectric field is the electric force per
unit chargeThe electric and magnetic fields are
vector quantitiesThe interaction of electrical and
magnetic effects is known as electromagnetism
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Electromagnetism (cont)
Two basic principles of this interaction:– Moving electric charges (current) give rise
to magnetic fields– A magnetic field may deflect a moving
electric chargeThe source of magnetism is moving and
“spinning” electrons – the 1st principleThere may be magnetic force on a
moving electric charge – the 2nd principle