1. PHY 102: Lecture 1 1.1 Scotch Take Experiment
1.2 Atomic Nature of Electricity
1.3 Insulators / Conductors
1.4 Charging Objects
1.5 Force between Charges

2. PHY 102: Lecture 1 Electric Charge
1.1
Scotch Take Experiment

3. Isaac Newton (1643 – 1727) Force causes acceleration, F = ma
Force is a push or a pull
Acceleration is a change in velocity (motion)
Mass generates a force called “Gravity”
Mass is a property of matter
Some properties of gravitational force are:
force acts between masses
force is only attractive
action-at-a-distance (masses need not touch)

4. Do Scotch Tape Experiment - 1
Place 1 foot piece of Scotch Tape on table
Label this tape B for bottom
Place 1 foot piece of Scotch Tape on top of B tape
Label this tape T for top
Repeat for a separate set of B and T tapes

5. Do Scotch Tape Experiment - 2
Remove 1st set of B and T tapes from table
Separate the B and T tapes
Hold them near each other
What happens?
Remove 2nd set of B and T tapes from table
Hold the 1st B tape near the 2nd B tape

6. Results of Scotch Tape Experiment
B tape and T tape attract
B tape and B tape repel
T tape and T tape repel
Tapes lift up away from Earth

7. Force Exists Between Tapes
B and T tapes accelerate towards each other
B and B tapes accelerate away from each other
According to Isaac Newton, a force causes this acceleration

8. Force is Not Gravity The force on the tapes can’t be gravity:
The mass of the tapes are so small that gravity between tapes is almost zero
The force must be greater than gravity between Earth and tape because the tapes lift up away from the Earth
The force is both attractive and repulsive, while gravity is only attractive

9. New Force – Electric Force
The force between the tapes is a new force called the Electric Force
Electric comes from the Latin word electrum for amber and the Greek word elektron for amber
The Greeks first noticed this force when they rubbed amber with a cloth

10. Source of Electric Force Electric Charge - 1
The electric force is associated with another property of matter, Electric Charge
Because the force can attract and repel, there must be two types of electric charge
The two types of charge are named positive and negative

11. Source of Electric Force Electric Charge - 2
What is the source of the electric charge on the tapes?
Created when tapes are pulled apart, or
Already exists in tapes and are separated when tapes pulled apart
The 2nd is the source of charge
The electrical charge of matter is inherent in atomic structure

12. PHY 102: Lecture 1 Electric Charge
1.2
Atomic Nature of Electricity
Charged Objects

13. Atom An atom consists of a small, relatively massive nucleus
Nucleus contains proton, and neutron particles
Surrounding the nucleus is a diffuse cloud of orbiting particles called electrons
Mass of particles are:
Proton: x kg
Neutron: x kg
Electron: 9.11 x kg

14. Charges of Particles in Atom
Proton has a positive charge (“+”)
Electron has a negative charge (”-”)
Neutron has no electric charge (0)
The magnitude of the charge on the proton exactly equals the magnitude of the charge on the electron
The proton carries a charge +e, and the electron carries a charge of –e
The SI unit for the magnitude of an electric charge is the coulomb (C)
e has the value 1.60 x C

15. Net Charge on Atoms
In nature, atoms are normally found with equal numbers of protons and electrons
Usually, an atom carries no net charge
When an atom, or any object, carries no net charge, the object is said to be electrically neutral

16. Charged Objects - 1
The charge on an electron or a proton is the smallest amount of free charge that has been discovered
Protons and neutrons contain quarks
Quarks have charges of ±1/3 and ±2/3 of an electron
Quarks are not usually found to be free

17. Charged Objects – 2
Charges of larger magnitude are built up on an object by adding or removing electrons
A charge of magnitude q is integer multiple of e
q = Ne
Where N is an integer
Because any electric charge q occurs in integer-multiples of elementary, indivisible charges of magnitude e, electric charge is said to be quantized

18. Charged Objects - 3
It is possible to transfer electric charge from one object to another
Usually electrons are transferred and the body that gains electrons acquires an excess of negative charge
The body that loses electrons has an excess of positive charges

19. Charged Object - 4
Separation of charge occurs often when two unlike materials are rubbed together
Hard, black rubber rod is rubbed against animal fur
Some of the electrons from atoms of the fur are transferred to the rod
The rubber becomes negatively charged
The fur becomes positively charged

20. Charged Object - 5 A glass rod is rubbed with a silk cloth
Some of the electrons are removed from the atoms of the glass and deposited on the silk
The silk becomes negatively charged
The glass becomes positively charged

21. Charged Object - 6 A rubber rod is rubbed with animal fur
The rubbing process separates electrons and protons already present in the materials
No electrons or protons are created or destroyed
Whenever an electron is transferred to the rod, a proton is left behind on the fur

22. Charged Object - 7
The charges on the electron and proton have identical magnitudes but opposite sign
The algebraic sum of the two charges is zero
The transfer does not change the net charge of the fur/rod system
If each material contains an equal number of protons and electrons to begin with, the net charge of the system is zero initially and remains zero at all times during the rubbing process

23. Law of Conservation of Electric Charge
Electric charges are involved in chemical reactions, electric circuits, and radioactive decay
A great number of experiments have verified that in any situation, the law of conservation of electric charge is obeyed
Law of Conservation of Electric Charge
During any process, the net electric charge of an isolated system remains constant (is conserved)

24. Problem 1
How many electrons must be removed from an electrically neutral silver dollar to give it a charge of +2.4C?
Q = Ne
N = e / Q (number of electrons)
N = 2.4 x 10-6 C / 1.60 x C/electron
N = 1.5 x 1013 electrons

25. PHY 102: Lecture 1 Electric Charge
1.3
Insulators / Conductors

26. Insulators / Conductors
Electric charge can exist on an object
Electric charge can move through an object
Materials differ vastly in their abilities to allow electric charge to move (conducted) through them
Electrical Conductors - Substances that conduct electric charge
Metals such as copper, aluminum, silver, and gold
Electrical Insulators - Materials that conduct electric charge poorly
Rubber, many plastics, ceramics, and wood

27. Conductor Conductors and insulators differ in their atomic structure
Electrons in the outer orbits experience a weaker force of attraction to the nucleus than do those in the inner orbits
Outermost electrons can be dislodged more easily than inner ones
In a good conductor, some electrons become detached from a parent atom and wander more or less freely throughout the material, belonging to no one atom in particular
The exact number of electrons detached from each atom depends on the nature of the material, but is usually between one and three
When one end of a conducting bar is placed in contact with a negatively charged object and the other end in contact with a positively charged object, “free” electrons are able to move readily away from the negative and toward the positive end

28. Insulator
In an insulator there are very few electrons free to move through the material
Virtually, every electron remains bound to its parent atom
Without the free electrons, there is very little flow of charge when the material is placed between positively and negatively charged bodies

29. PHY 102: Lecture 1 Electric Charge
1.4
Charging Objects

30. Charging by Contact - 1
When a negatively charged rubber rod is rubbed on a metal object some of the excess electrons from the rod are transferred to the metal object
When the rod is removed the sphere is left with a negative charge distributed over its surface
When a positively charged rod is rubbed on a metal object some of the electrons from the metal object are transferred to the rod
When the rod is removed the sphere is left with a positive charge distributed over its surface
The process of giving one object a net electric charge by placing it in contact with another object that is already charged is known as charging by contact

31. Charging by Contact - 2

32. Charging by Induction - 1
A negatively charged rod is brought close to, but does not touch, a metal sphere
In the sphere, the free electrons closest to the rod, move to the other side
The part of the sphere nearest the rod becomes positively charged and the part farthest away becomes negatively charged
These positively and negatively charged regions have been “induced” to form because of the repulsive force between the negative rod and the free electrons in the sphere
If the rod were removed, the free electrons would return to their original places, and the charge regions would disappear

33. Charging by Induction - 2
The earth is a good electrical conductor
When a metal wire is attached between the sphere and the ground some of the free electrons leave the sphere and distribute themselves over the much larger earth
If the ground wire is then removed, followed by the rubber rod, the sphere is left with a positive net charge
The process of giving one object a net electric charge without touching the object to a second charged object is called charging by induction

34. Charging by Induction - 3

35. Charging by Induction - 4
If the sphere were made from an insulating material like plastic, instead of metal, the method of producing a net charge by induction would not work
Very little charge would flow through the insulating material and down the ground wire
The electric force would cause the positive and negative charges in the molecules of the insulating material to separate slightly, with the negative charge being “pushed” away from the negative rod
Although no net charge is created, the surface of the plastic does acquire a slight induced positive charge and is attracted to the negative rod

36. Charging by Induction - 5

37. PHY 102: Lecture 1 Electric Charge
1.5
Force between Charges

38. Forces on Electric Charges
Two electrically charged objects exert a force on one another
This is the Electric Force
Like charges repel and unlike charges attract each other

39. Charging by Rubbing - 1 Balloon is electrically neutral
Ten + and – charges
Sweater is also electrically neutral

40. Charging by Rubbing - 2 Rub balloon on sweater
Electrons (-) transfer from sweater to balloon
Balloon is negatively charged
Sweater is positively charges

41. Charging by Rubbing - 3 Separate the balloon and sweater
It is clear that the balloon is negatively charged
It is clear that the sweater is positively charged
Let the balloon go

42. Charging by Rubbing - 4
The negatively charged balloon is attracted to the positively charged sweater
Opposite charges attract

43. Charging by Induction - 1
Neutral balloon is brought close to wall
Nothing happens

44. Charging by Induction - 2
Negatively charged balloon is brought close to the wall
The negative charge on the balloon repels the negative charge on the wall
The wall is still neutral but the negative and positive charges separate slightly
The negative balloon is attracted to the closer positive charge on the wall

45. Charles Coulomb (1736 – 1806) 1784 French Engineer
Did static electricity experiments
Built from the work of Benjamin Franklin
Measured strength of electric force

46. Coulomb’s Law
The magnitude F of the electrostatic force exerted by one point charge q1 on another point charge q2 is
r is the distance between charges
k = 8.99 x 109 Nm2/C2 in SI units
Sometimes k = 1/(40)
0 is called the permittivity of free space
0 = 1/(4k) = x C2/(Nm2)
Electrostatic force is directed along the line joining the charges
It is attractive if the charges have unlike signs
It is repulsive if the charges have like signs

47. Coulomb’s Law
Force on a Point Charge Due to Two or More Other Point Charges
We have discussed the electrostatic force on a point charge (magnitude |q1|) due to another point charge (magnitude |q2|)
Suppose that a third point charge (magnitude |q3|) is present
What is the net force on q1 due to both q2 and q3?
First, find the magnitude and direction of the force exerted on q1 by q2 (ignoring q3)
Then, determine the force exerted on q1 by q3 (ignoring q2)
The net force on q1 is the vector sum of these forces

48. Problem 2 Two spheres are separated by distance of 1.80 x 10-3 m
The spheres are initially electrically neutral and are very small compared to the distance between them
Each sphere gets same negative charge due to addition of electrons
Each sphere experiences an electrostatic force that has a magnitude of 4.55 x N
How many electrons did it take to the produce charge on one sphere?
F =kqq/r2
4.55 x = 8.99 x 109 x q2 / (1.80 x 10-3)2
4.55 x x (1.80 x 10-3)2 / 8.99 x 109 = q2
1.64 x = q2
q = 1.28 x 10-18
q = Ne
N = number of electrons = 1.28 x / 1.60 x 10-19
N = 8

49. Problem 3
Two objects, whose charges are +1.0 C and -1.0 C, are separated by 1.0 km
Find the magnitude of the attractive forces that either charge exerts on the other
Equivalent to weight of 900 kg mass
Equivalent to weight of 2000 pounds

50. Problem 4
In the Bohr model of the hydrogen atom, the electron (charge=-e) is in a circular orbit about the nuclear proton (charge=+e) at a radius of 5.29 x m ( nm)
The mass of the electron is 9.11 x kg
Determine the speed of the electron
Centripetal force = Coulomb’s Force

51. Problem 5 - 1 Three point charges lie along the x axis
Determine the magnitude and direction of the net electrostatic force on q1

52. Problem 5 - 2
Magnitude of the forces
Direction of net force