1. Voltage in Electrical Systems
1.3.1

2. 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.

3. 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).

4. Gravitational Force 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.

5. Formula for Gravitational Force
G = universal gravitational constant = 6.67 x 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

6. 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.

7. Notes on numbers with exponents:
When you multiple number with exponents :
1. multiply the numbers together as usual
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
Divide the numerator by the denominator
Subtract the exponent value in the denominator from the exponent value in the numerator

8. 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
cancel out the m2 and the kg2 with those in G (N·m2)
kg2
Your remaining unit is N (Newton) which is the correct unit of force.

10. Set up the problem and solve
Set up the problem and solve. Use our standard procedure and the notes from slides (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.

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.

13. 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.

14. Origin of Charge Charge can transferred
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.

15. 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.

16. Electrical Force SI unit for charge is the Coulomb (C).
Elementary charge of one electron or proton is x C
q1 and q2 are the charges on two objects.
d = distance between charged objects
K = constant = 9.0 x 109 N•m2/C2

17. 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.

19. Example 1.12 is on the next slide
Set up the problem and solve. Use our standard procedure and the notes from slides (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.

21. 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.

22. 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.

23. Gravitational Field

24. Electrical Field

25. Gravitational Field
Electrical Field
What happens when you substitute the respective laws into each equation above?
E does not depend on size of test charge.
g does not depend on size of test mass.

26. 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

27. Forces, units and Formulas
G = universal gravitational constant = 6.67 x 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
Gravitational force
Gravitational Field
SI unit for charge is the Coulomb (C).
Elementary charge of one electron or proton is x 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
Electrical Field