Pick up Statics Packet You should start working on problems and it would be very helpful to read Physics Classroom: Electrostatics

ELECTROSTATICS ELECTROSTATICS the study of electric charges, forces and fields Two types of charges exists, arbitrarily named POSITIVE and NEGATIVE By Benjamin Franklin So what is positive and what is negative?

We know that charged particle exist in atoms Electrons negative Electrons are responsible for negative charges and Protons for positive charge Benjamin Franklin did not know about the existence of these particles but, he did investigate the behavior of static discharge and lightning.

Ben knew that if certain electrically neutral objects are rubbed, they can become charged. For example; when rubber is rubbed with a wool cloth, both become charged. The rubber scrapes electrons from fur atoms. So the rubber is negatively charged and the cloth is positively charged. or a comb through hair

Ben also knew that a charge separation occurs when a glass rod is rubbed with a silk cloth In the case of the glass and silk, the glass rod loses negative charge and becomes positively charged while the silk cloth gains negative charge and therefore becomes negatively charged.

Ben experimented with the interactions between the charge objects. He suspended one and brought other charged objects near Ben observed that like charged unlikeattract object repel & unlike charges attract repel attract

A NEGATIVE charge is an EXCESS of electrons POSITIVE SHORTAGE A POSITIVE charge is a SHORTAGE of electrons PRINCIPLE OF CONSERVATION OF ELECTRIC CHARGE moved aroundnot created or destroyed. In the process of rubbing two solid objects together, electrical charges are moved around, not created or destroyed. The negative charges So The negative charges are transferred between the two objects. Leaving one with an excess of positive charge and the other with an excess of negative charge. The quantity of excess charge on each object is exactly the same.

insulator insulator An insulator is a material in which the electrons are tightly held by the nucleus and are not free to move through the material. Examples of good insulators are: glass, rubber, plastic and dry wood. A material can be an electric conductor free to move conductor A conductor is a material through which electrons are free to move through the material. Examples of good conductors are: metals, such as silver, copper, gold and mercury.

Objects become charged by Friction Induction Conduction Electrons are rubbed off one insulator on to another insulator Charging an object WITHOUT touching a charged object Charging by CONTACT with a charged object

Electrons are rubbed off one insulator on to another insulator http://phet.colorado.edu/en/simulation/balloons

Charging by Conduction Charging by Conduction A charged object touches the uncharged object and some electrons leave the charged rod and move spread over sphere. same charge Charging by conduction results in an object with the same charge

POLARIZATON. Neutral objects can be temporarily attracted to charged objects by a process called POLARIZATON. To understand the last way to charge an object, we need to look at what happens to a neutral object when a charged object is brought near.

Electrons are free to move in metals. Nuclei remain in place; electrons move to bottom polarization Polarization occurs because the electrons are attracted or repelled by the charged object. This result is in a polarization or temporary separation of the charge, and attraction results.

Charging by Induction The rod does not touch the sphere. It pushes electrons out of the back side of the sphere and down the wire to ground. The ground wire is disconnected to prevent the return of the electrons from ground, then the rod is removed. The resulting charge on the object is opposite polarizationgrounding permanent charge

electroscope An electroscope is a device that detects static charge. The metal leaves of the electroscope move apart if a charged object is brought near the knob. Benjamin Franklin used a similar device when he investigated charges. Positively charged Negatively charged

Grounding is allowing charges to move freely along a connection between a conductor and the ground. The Earth (the ground) is a practically infinite reservoir of electric charge. Here a positively charge rod attracts electrons from the ground into the electroscope Here a negatively charge rod repels electrons into the ground from the electroscope

To review Induction results in an OPPOSITE CHARGE

Fine mist of negatively charged gold particles adhere to positively charged protein on fingerprint. Negatively charged paint adheres to positively charged metal.

How lightning forms YouTube how lightning works YouTube The Birth of a Lightning Bolt - YouTube

20 What exactly is CHARGE? It is physical property of matter. It comes in two flavors: plus and minus. What is the unit for charge? Coulombs (C)

21 What is the smallest charge possible? Millikan Oil Drop Experiment In 1910, Millikan was able to measure the charge of an electron. The smallest charge possible is: -1.602 x 10 -19 Coulombs (C).

22 Definition of Coulomb Abbreviation: C SI unit for charge One coulomb is NOT equal to the charge of 1 electron!!!! 1C ~ the charge of 6.25 x 10 18 electrons It is the amount of charge to pass through a cross-section of wire in 1 second when 1 Ampere (A) of current is applied. (Well cover the amp later.) Likewise the + charge of protons is associated with 6.25 x 10 18 protons

23 Elementary Particles ParticleCharge, (Coulombs per particle) # of particles in a Coulomb electron-1.6 x 10 -19 6.25 x 10 18 proton+1.6 x 10 -19 6.25 x 10 18

24 Coulombs Law Charles-Augustin de Coulomb used a torsion pendulum to establish his law.

25 Electric Force q charge, C (coulombs) r distance between charges, m F electric force, N k electrostatic constant 9.00 x 10 9 Nm 2 /C 2

26 Electric Force Coulombs Law: The electric force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between the charges.

What happens to F as charge increases? Increase What happens to F as r increases? Decreases by inverse square Look at k c. Is this a large or small value? large How is q described for a proton? positive For an electron? negative

28 The Product of q 1 and q 2 If the product, q 1 q 2,is negative then the force is attractive. If the product, q 1 q 2,is positive then the force is repulsive.

Ex 1: Two negatively charged balloons are 0.70m apart. If the charge of each is 2.0 x 10 -6 C, What is the electric force between the two balloons? q 1 = q 2 = 2.0 x 10 -6 C d = r = 0.70 m F = 9.0 x 10 9 N m 2 /C 2 (-2.0 x 10 -6 C) 2 (0.70m) 2 F = 0.073 N An attracting or repelling force?

30 ~ 1750 bells It consisted of two metal bells, one electrically connected to the earth (grounded) and the other connected to a lightning rod. Hanging between the two bells was a metallic ball suspended by an insulating (dielectric) thread. The lightning rod allows an electric charge to build up on one bell, which then attracts the metallic ball. When the ball hits this charged bell it becomes charged to the same potential and is immediately repelled. Since the grounded bell is charged oppositely, this attracts the ball towards it. When the ball touches and rings the grounded bell, the charge is transferred and the process repeats.groundedlightning roddielectricelectric charge Lightening Bells

Solve Ex 2 in your notes now

A negatively charged balloon is used to permanently charge an electroscope positively. This was done by A: Conduction B. Induction C: Neutralization Two identical objects are charged by contact with a charged balloon. The objects should: A: Attract each other B. Neutralize each other C: Repel each other A negatively charged balloon is used to permanently charge an electroscope negatively. This was done by A: Conduction B. Induction C: Neutralization What is the charge of one electron? Of one proton?

Ex.2: Two equally charged balloons repel each other with a force of 4.0 x 10 -3 N. If they are 0.015 m apart, what is the charge of the each balloon? F = 4.0 x 10 -3 N d = 0.015 m q 2 = (4x10 -3 N)(0.015m) 2 (9x10 9 Nm 2 /C 2 ) q 2 = Fd 2 k q 1 = q 2 = 1.0 x 10 -8 C

What happens there are more than two charged particles? We will assume that the electrical force between any two charged objects acts along the line joining the centers of the charges.

This is subatomic tug of war!!!

36 ABC How could you indicate displacement if green object B moved to point A? What if it moved to point C? Use + or -

Problem Two charges (q1 = -8C; q2 = 12C) are placed 120 mm apart in the air. What is the resultant force on a third charge (q3 = -4C), placed midway between the other charges?

38 What is the net force acting on green charge? -8.0 C-4.0 C12.0 C 0.06m + 200N How do you determine this?

39 What is the net force acting on green charge? -8.0 C-4.0 C12.0 C 0.06m0.06mm Charge is acted upon by charges left and right. From left, there is repelling to the right From right, there is attraction to the right. The sum of the forces will be additive to each other because they are in the same direction

Problem solving strategies Sketch the system Use Coulombs Law to calculate the magnitude of the individual forces acting on the center particle. Do not include signs of charge. Want the absolute value!!! The net force is the difference between the two calculated forces Direction is determined by the diagram.

41 What is the force acting on green charge from the left? -8.0 C-4.0 C12.0 C 0.06m F = k (8.0 x 10 -6 C)(4.0 x 10 -6 C) (0.06 m) 2 F = 80 N to the right, repelling

42 What is the force acting on green charge from the right? -8.0 C-4.0 C12.0 C 0.06 m F = k (4.0 x 10 -6 C)(12.0 x 10 -6 C) (0.06 m) 2 F = 120N to the right, attractive

43 What is the net force acting on green charge? -8.0 C-4.0 C12.0 C 0.06 m 80N + 120 N = 200 N 200N (towards the right)

44 What is the net force acting on green charge? -4.0 C+3.0 C-7.0 C 0.15 m0.20 m + 5.7 N How do you determine this?

45 What is the net force acting on green charge? -4.0 C+3.0 C-7.0 C 0.15 m0.20 m Charge is acted upon by charges left and right. From left, there is attraction to the left. From right, there is attraction to the right. The sum of the forces will be subtractive to each other because they are in opposite directions

46 What is the force acting on green charge from the left? -4.0 C+3.0 C-7.0 C 0.15 m0.20 m F = k (3.0 x 10 -6 C)(4.0 x 10 -6 C) (0.20 m) 2 F = -2.7 N (minus means left!!!!)

47 What is the force acting on green charge from the right? -4.0 C+3.0 C-7.0 C 0.15 m0.20 m F = k (3.0 x 10 -6 C)(7.0 x 10 -6 C) (0.15 m) 2 F = +8.4 N (positive means right!!!!

48 What is the net force acting on green charge? -4.0 C+3.0 C-7.0 C 0.15 m0.20 m -2.7 N + 8.4 N = 5.7 N + 5.7 N (towards the right) The -7.0 C charge WINS!!!

49 What is the net force acting on green charge? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 + 23 N at 24

Remember this problem? Socks Blue Patches

What method did we use to solve net force on a point?

53 What is the force acting on green charge from the right? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 F = k (4.0 x 10 -6 C) (5.0 x 10 -6 C) (0.10 m) 2 F = 18 N

54 What is the force acting on green charge from the top? Is the force really acting directly from the top? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 F = k (4.0 x 10 -6 C) (6.0 x 10 -6 C) (0.15 m) 2 F = 9.6 N

55 What is the net force acting on green charge? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 Along the x-axis (9.6 N)(cos 73) = 2.8 N added to 18 N = 21 N

56 What is the net force acting on green charge? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 Along the y-axis (9.6 N)(sin 73) = 9.2 N added to 0 N = 9.2 N

57 What is the net force acting on green charge? -6.0 C +4.0 C -5.0 C0.10 m 0.15 m 73.0 F 2 = F x 2 + F y 2 F 2 = (21 N) 2 + (9.2 N) 2 F = 23 N tan = y / x tan = 9.2 / 21 = 24

Two charges create a force on one another. If the charge of one object is doubled, how does the resulting force change? F will double What if charge of one object is tripled? F will triple

Two charges create a force on one another. If the distance between the objects is increased by a factor of 2, the force changes by a factor of? F will decrease by a factor of 4 What if distance between the objects is tripled? F will decrease by a factor of 9

Electric Fields

Symphony of Science - the Quantum World! - YouTubeSymphony of Science - the Quantum World! - YouTube

Force and Fields Contact forces What we mostly deal with Objects touch each other directly Ex. A tennis racket hits a tennis ball F=ma www.CartoonStock.comwww.CartoonStock.com.

Forces can occur without contact! Action at a distance Can you think of anything that applies a force without touching? 63

Gravity demonstrates action at a distance What happens if you get too far away from the mass exerting the force? The effects are less 64

What else applies an action at a distance? Magnets! 65

66 What else applies an action at a distance?

67 Attracting and repelling forces of charges

The space that surrounds these things is altered Examples: Magnets Sun Planets Electric charge

Action at a distance depends on a field of influence An object within the field may be affected by it Can be scalar or vector Magnitude only Ex. Heat Can be vector Magnitude and direction Ex. Gravity (one direction only since only attracts) Ex. Electric (more than one direction; attracts and repels 69

Fields are NOT Force, they exert the force Ex. A person pushes a box. The person is not the force, he exerts the force! 70

. A field is defined as a property of space in which a material object experiences a force. gravitational field at P. Above earth, we say there is a gravitational field at P. mass m Because a mass m experiences a downward force at that point. No force, no field; No field, no force! m F directionforce The direction of the field is determined by the force.. P

Electric field A field that exerts force that surrounds an electric charge or group of charges Magnitude and direction (vector)

3/18 Game Plan Quiz Get a formula chart and calculator Discuss Electrical Fields & Field Strength Pass Back Test Statics Lab Activity 73

Electric field How would you detect and measure an electric field around a charge? Place another one nearby and see what happens! Since all charges produce fields, come up with a model 74

Electric field model Source charge: charge producing the field Test charge: a mathematical creation Always positive Symbol: q Doesnt exist Infinitely small, thus produces no field of its own 75

What is the source charge if The test charge q moves towards it? Negative (attracts) The test charge q moves away from it? Positive (repels) How would I draw these? 76

Where do you think the field is strongest? 77

The Electric Field 1. Now, consider point P a distance r from +Q. 2. An electric field E exists at P if a test charge +q has a force F at that point. 3. The direction of the E is the same as the direction of a force on + (pos) charge. E 4. The magnitude of E is given by the formula: Electric Field + + + + + + + + Q. P r +q F +

Field is Property of Space E Electric Field + + + + + + + + Q. r The field E at a point exists whether there is a charge at that point or not. The direction of the field is away from the +Q charge The field E at a point exists whether there is a charge at that point or not. The direction of the field is away from the +Q charge. E Electric Field + + + + + + + + Q. r + +q+q - -q F F Force on +q is with field direction. Force on -q is against field direction.

Electric Field Lines + + + + + + + + Q - - - - - - - - -Q Electric Field Lines are imaginary lines drawn in such a way that their direction at any point is the same as the direction of the field at that point. Electric field line flow Out of positive charges and into Negative charges.

Examples of E-Field Lines Two equal but opposite charges. Two identical charges (both +). Notice that lines leave + charges and enter - charges. Also, E is strongest where field lines are most dense.

The electric field is strongest in regions where the lines are close together and weak when the lines are further apart.electric field

Threads floating on oil bath become polarized and align themselves with the electric field. These fields can be detected in lab

Electric Field Intensity (Strength) E - Electric Field Strength or Intensity (N/C) F - Force experienced by a test charge at that location (N) q - magnitude of the test charge placed at that location (C).

magnitude force per unit charge (N/C) The magnitude of the electric field intensity at a point in space is defined as the force per unit charge (N/C) that would be experienced by any test charge placed at that point Units: N/C http://phet.colorado.edu/simulations/sims.php?sim=Charges_and_Fields

Example 1. A +2 nC charge is placed at a distance r from a 8 C charge. If the charge experiences a force of 4000 N, what is the electric field intensity E at point P? Electric Field. - - - - - - - - -Q P First, we note that the direction of E is toward Q (down). 8 C E ++q+q E 4000 N +2 n C r E = 2 x 10 12 N/C Downward Note: The field E would be the same for any charge placed at point P. It is a property of that space.

SOLVE NOW A test charge of 5.0 x 10 -17 C is 6.5 cm from a proton. What is the field intensity at this point? If you were absent yesterday, you have a quiz to make up HW (Static Set): Complete both MC portions. #5 Magnitude only, not angle #8 Show how, but do not need to solve #9 Extension (for fun) Complete 1-6 after review as well. HW (Fields Set) Answer all problems (there are only 4)

SOLVE NOW A test charge of 5.0 x 10 -17 C is 6.5 cm from a proton. What is the field intensity at this point?

Static Activity What's happening with the Van De Graaf? When two insulators are rubbed together: Silk on glass glass (+) Who loses electrons? The glass Fur on rubber rubber (-) Who loses electrons? The fur

Electric Potential Chapter 19

Gravitational Work and Energy Work is done against gravity to move the block from A to B, a vertical height h. Work = Fd = mgh The external force F against the g-field increases the potential energy. If released the field gives work back. A B positive work negative work The external force does positive work; the gravity g does negative work.

Work changes potential energy! Work done can change the gravitational potential energy of an object W AB = GPE A - GPE B Work done can change the electrical potential energy of an object W AB = EPE A - EPE B

Electrical Work and Energy F+q againstqE An external force F moves +q from A to B against the field force qE. Work = Fd = (qE)d B EPE At level B, the electrical potential energy EPE is: EPE = qEd negative positive The E-field does negative work; External force does positive work. againstincreases The external force F against the E-field increases the potential energy. If released the field gives work back. B + + - - A + +q d qE E FeFe

Work and Negative Charges negative charge q Suppose a negative charge q is moved against E from A to B. No external force is required ! B + + - - A E d decreasing The E-field does positive work on q decreasing the potential energy. If released from B nothing happens. Work = Fd = (qE)d A At A, the EPE is: EPE = qEd qE -q -

Since the electric force is F = qE, the work that the electric force does as the charge moves from A to B depends on the amount of the charge. It is useful to express this work on a per charge basis. W AB = EPE A - EPE B q q q The electric potential or Voltage,V is defined in terms of the work to be done on a charge to move it against an electric field. W AB = V A - V B q V = W q Units: J/C = Volt (V)

Example 1: The work done on a test charge of 2C as it moves from A to B is 5.0 x 10 -8 J. (A) Find the difference in potential energies of the charge. (B) Determine the potential difference between point A and B W = EPE = 5.0 x 10 -8 J V = W q = 5.0 x 10 -8 J 2 x 10 -6 C = 0.025 V We say that positive charges naturally want to move from a point of high potential (B) to low potential (A), and we refer to the movement of the positive charges as current. We will return to voltage and current in the next chapter.

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Pick up Statics Packet You should start working on problems and it would be very helpful to read Physics Classroom: Electrostatics