Transcript Voltage in Electrical Systems

Voltage in Electrical Systems
1.3.1
Objectives
• Explain the similarities and differences between
Newton’s law of universal gravitation and
Coulomb’s law.
• Explain how force between two like charges and
the force between two unlike charges are
different.
• Describe how to create an electric field and
interpret the information given in a drawing of an
electric field.
Universal Forces
• Include: gravitational and electrical
– Gravitational force acts between two or more masses
– Electrical force acts between two or more charges
• Called universal because each force behaves the
same anywhere in the observable universe.
• The two forces are field forces and act over a
distance (the masses or charges do not have to be
touching – example – gravitational forces affect the earth
and the moon or magnetic forces affecting two magnets).
Gravitational Force
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Why cover gravitational force?
17th century, Isaac Newton.
View video clip (control+click)
Newton’s universal law of gravitation
– Every object in the universe attracts every
other object with a force that is directly
proportional to the mass of each body and
inversely proportional to the square of the
distance between them.
Formula for Gravitational Force
m1m2
Fg  G 2
d
G = universal gravitational constant = 6.67 x 10-11 N•m2/kg2
m1 = mass of first body in kilograms
m2 = mass of second body in kilograms
d = distance between the two bodies in meters
Fg = the gravitational force in Newtons
Using the formula for gravitational forces
•Use the standard procedure of writing down the givens,
solving for and formulas needed
•Convert masses to kilograms and distance to meters
•Write down the formula
•Rearrange the formula if necessary to solve for needed
component
If you are solving for Fg and set up the problem correctly you
will be able to m2 from the distance and kg2 from the masses
and then cancel out the m2 and the kg2 with those in G
(Nm2/kg2)so that your remaining unit is N (Newton) which is
the correct unit of force.
Notes on numbers with exponents:
When you multiple number with exponents :
1. multiply the numbers together as usual
2. add the exponents together
– two positive exponents give you a larger positive exponent
– two negative exponents give you a larger negative exponent
- when exponents are of different signs you find the difference between
them and give the sign of the larger exponent.
When you have numbers with exponents in the numerator and
denominator
1. Divide the numerator by the denominator
2. Subtract the exponent value in the denominator from the exponent
value in the numerator
m1m2
Fg  G 2
d
Formula for Fg
If you are solving for Fg and set up the problem correctly
you will be able to calculate m2 from the distance and kg2
from the masses
Fg  G
kg 1kg 2
m2
cancel out the m2 and the kg2 with those in G (N·m2)
kg2
Nm
Fg 
kg 2
2
kg1kg2
m2
Your remaining unit is N (Newton) which is the correct unit
of force.
Set up the problem and solve. Use our standard procedure
and the notes from slides 5 - 8. (I will check your work in
class when I check your notes.)
Check your answer by sliding the brown box away from
example 1.11.
Electric Charge
• Electrostatic forces, the comb and the CRT.
• Charge – property of and object that causes
electrical force.
• Two types of charge: positive and negative
• Electrical forces are either attractive or repulsive.
– Like charges repel
– Opposite charges attract.
Origin of Charge
• Structure of atom.
• Charge of electron is
equal in magnitude to
proton but opposite in
sign.
• Normal atom is
neutral because
number of protons
and electrons are
equal.
Origin of Charge
Net Charge = #protons – #electrons
• Charge can transferred
– Comb example
– Balloon example (in class)
• Principle of conservation of charge – net
charge in an isolated system never
changes.
Electrical Force
• In 18th century, French scientist Charles
Coulomb discovered the relationship
between force, charge and distance.
• Coulomb’s law
– The electrical force between two charged
bodies is directly proportional to the charge
on each body and inversely proportional to the
square of the distance between them.
Electrical Force
q1q2
FE  K 2
d
•SI unit for charge is the Coulomb (C).
•Elementary charge of one electron or proton is
1.60 x 10-19 C
•q1 and q2 are the charges on two objects.
•d = distance between charged objects
•K = constant = 9.0 x 109 N•m2/C2
Electrical Force
• Coulomb’s law similar to Newton’s
universal law of gravitation.
– But, gravitational force is always attractive.
– Direction for electrical force depends on
charge of particles.
Example 1.12 is on the next slide
Set up the problem and solve. Use our standard procedure
and the notes from slides 5 - 8. (I will check your work in
class when I check your notes.)
Check your answer by sliding the brown box away from
example 1.11.
Scale and Universal Forces
• Small distance and mass as in atoms, electrical
forces are important and gravitational force is
insignificant.
• Large distance and mass, significance reverses.
• Thus
– Electrical forces govern the structure of atoms,
molecules, solids, liquids and gases.
– Gravitational forces govern the structure of planets,
stars, galaxies and the universe.
Gravitational and Electrical
Fields
• Field forces are alterations in space around
the body creating the field.
– They are models used by scientists to help
them understand and predict how forces are
transmitted from one object to another.
• The field forces are vector quantities.
Gravitational Field
Gravitational Field
g
Fg
m
Electrical Field
FE
E
q
What happens when you substitute the
respective laws into each equation
above?
g does not depend
on size of test mass.
E does not depend
on size of test
charge.
Field Line Diagrams
• Illustration of a field can
be done with field lines.
• Direction of field at any
given point is tangent.
• Lines are close, field is
strong
• Lines are far apart, field
is weak
Forces, units and Formulas
Gravitational force Fg  G
m1m2
d2
G = universal gravitational constant = 6.67 x 10-11 N•m2/kg2
m1 = mass of first body in kilograms
m2 = mass of second body in kilograms
d = distance between the two bodies in meters
Gravitational Field
Electrical force
g 
Fg
FE  K
q1q2
d2
Fg = the gravitational force in Newtons
m
•SI unit for charge is the Coulomb (C).
•Elementary charge of one electron or proton
is 1.60 x 10-19 C
•q1 and q2 are the charges on two objects.
Electrical Field
F
E E
q
•d = distance between charged objects
•K = constant = 9.0 x 109 N•m2/C2