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

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**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?

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**We know that charged particle exist in atoms**

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.

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**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. or a comb through hair The rubber scrapes electrons from fur atoms. So the rubber is negatively charged and the cloth is positively charged.

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**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.

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**Ben observed that like charged object repel & unlike charges attract**

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

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**PRINCIPLE OF CONSERVATION OF ELECTRIC CHARGE**

In the process of rubbing two solid objects together, electrical charges are moved around, not created or destroyed. 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. A POSITIVE charge is a SHORTAGE of electrons A NEGATIVE charge is an EXCESS of electrons

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**A material can be an electric …**

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. 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.

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**Objects become charged by…**

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

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Friction Electrons are rubbed off one insulator on to another insulator

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**Charging by Conduction **

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

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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. Neutral objects can be temporarily attracted to charged objects by a process called POLARIZATON.

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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. Electrons are free to move in metals. Nuclei remain in place; electrons move to bottom

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**The resulting charge on the object is opposite**

Charging by Induction permanent charge polarization grounding 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

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**An electroscope is a device that detects static charge.**

Positively charged Negatively charged Before adding the description- ask what do you think is happening here? 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.

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**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. Examples of places that are grounded- 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

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To review… Induction results in an OPPOSITE CHARGE

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**Applications of Electrostatic charging**

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

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**How lightning forms – YouTube**

how lightning works – YouTube The Birth of a Lightning Bolt - YouTube

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**It is physical property of matter.**

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)

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**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: x Coulombs (C).

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**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 1018 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. (We’ll cover the amp later.) Likewise the + charge of protons is associated with 6.25 x 1018 protons

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**Charge, (Coulombs per particle) # of particles in a Coulomb**

Elementary Particles Particle Charge, (Coulombs per particle) # of particles in a Coulomb electron -1.6 x 10-19 6.25 x 1018 proton +1.6 x 10-19 23

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Coulomb’s Law Charles-Augustin de Coulomb used a torsion pendulum to establish his law.

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**Electric Force q charge, C (coulombs)**

r distance between charges, m F electric force, N k electrostatic constant x 109 Nm2/C2

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**Electric Force Coulomb’s 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.

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**What happens to F as charge increases?**

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

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The Product of q1and q2 If the product, q1q2 ,is negative then the force is attractive. If the product, q1q2 ,is positive then the force is repulsive.

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**An attracting or repelling force?**

Ex 1: Two negatively charged balloons are 0.70m apart. If the charge of each is 2.0 x 10-6C, What is the electric force between the two balloons? q1 = q2 = 2.0 x 10-6 C d = r = 0.70 m F = 9.0 x 10 9 N m2/C2 (-2.0 x 10-6 C)2 (0.70m)2 F = N An attracting or repelling force?

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~ 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. Lightening Bells 30

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**Solve Ex 2 in your notes now**

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**Two identical objects are charged by contact with a charged balloon**

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 positively. This was done by A: Conduction B. Induction C: Neutralization 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?

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**F = 4.0 x 10-3 N d = 0.015 m q2 = Fd2 k q2 = (4x10-3N)(0.015m)2**

Ex.2: Two equally charged balloons repel each other with a force of 4.0 x 10-3 N. If they are m apart, what is the charge of the each balloon? F = 4.0 x 10-3 N d = m q2 = Fd2 k q2 = (4x10-3N)(0.015m)2 (9x109Nm2/C2) q1 = q2 = 1.0 x 10-8C

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**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.

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**This is subatomic tug of war!!!**

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

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

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**What is the net force acting on green charge?**

0.06m 0.06m -8.0 µC -4.0 µC 12.0 µC + 200N How do you determine this?

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**What is the net force acting on green charge?**

0.06mm 0.06m -8.0 µC -4.0 µC 12.0 µC 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

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**Problem solving strategies**

Sketch the system Use Coulomb’s 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.

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**What is the force acting on green charge from the left?**

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

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**What is the force acting on green charge from the right?**

F = k (4.0 x 10-6 C)(12.0 x 10-6 C) (0.06 m)2 F = 120N to the right, attractive

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**What is the net force acting on green charge?**

0.06 m 0.06 m -8.0 µC -4.0 µC 12.0 µC 80N N = 200 N 200N (towards the right) 43

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**What is the net force acting on green charge?**

0.20 m 0.15 m -4.0 µC +3.0 µC -7.0 µC + 5.7 N How do you determine this?

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**What is the net force acting on green charge?**

0.20 m 0.15 m -4.0 µC +3.0 µC -7.0 µC 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

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**What is the force acting on green charge from the left?**

F = k (3.0 x 10-6 C)(4.0 x 10-6 C) (0.20 m)2 F = N (minus means left!!!!)

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**What is the force acting on green charge from the right?**

F = k (3.0 x 10-6 C)(7.0 x 10-6 C) (0.15 m)2 F = N (positive means right!!!!

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**What is the net force acting on green charge?**

0.20 m 0.15 m -4.0 µC +3.0 µC -7.0 µC -2.7 N N = 5.7 N + 5.7 N (towards the right) The µC charge WINS!!! 48

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**What is the net force acting on green charge?**

+ 23 N at 24˚ 0.15 m 73.0˚ 0.10 m -5.0 µC +4.0 µC

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Remember this problem? Socks Blue Patches

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

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**What is the force acting on green charge from the right?**

F = k (4.0 x 10-6 C) (5.0 x 10-6C) (0.10 m)2 -6.0 µC F = 18 N 0.15 m 73.0˚ 0.10 m -5.0 µC +4.0 µC

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**F = 9.6 N F = k (4.0 x 10-6 C) (6.0 x 10-6C) (0.15 m)2 -6.0 µC 0.15 m**

What is the force acting on green charge from the top? Is the force really acting directly from the top? F = k (4.0 x 10-6 C) (6.0 x 10-6C) (0.15 m)2 -6.0 µC F = 9.6 N 0.15 m 73.0˚ 0.10 m -5.0 µC +4.0 µC

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**What is the net force acting on green charge?**

Along the x-axis… (9.6 N)(cos 73˚) = 2.8 N added to 18 N = 21 N -6.0 µC 0.15 m 73.0˚ 0.10 m -5.0 µC +4.0 µC

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**What is the net force acting on green charge?**

Along the y-axis… (9.6 N)(sin 73˚) = 9.2 N added to 0 N = 9.2 N -6.0 µC 0.15 m 73.0˚ 0.10 m -5.0 µC +4.0 µC

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**What is the net force acting on green charge?**

F2 = Fx2 + Fy2 -6.0 µC F2 = (21 N)2 + (9.2 N)2 F = 23 N 0.15 m tan θ = y / x 73.0˚ θ = 24˚ 0.10 m -5.0 µC tan θ = 9.2 / 21 +4.0 µC

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**Two charges create a force on one another**

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

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**Two charges create a force on one another**

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

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Electric Fields

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

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**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

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**Forces can occur without contact!**

Action at a distance Can you think of anything that applies a force without touching?

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**Gravity demonstrates action at a distance**

What happens if you get too far away from the mass exerting the force? The effects are less

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**What else applies an action at a distance?**

Magnets!

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**What else applies an action at a distance?**

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**Attracting and repelling forces of charges**

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**The space that surrounds these things is altered Examples:**

Magnets Sun Planets Electric charge

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**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

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**Fields are NOT Force, they exert the force**

Ex. A person pushes a box. The person is not the force, he exerts the force!

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**The direction of the field is determined by the force.**

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

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

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**3/18 Game Plan Quiz Get a formula chart and calculator**

Discuss Electrical Fields & Field Strength Pass Back Test Statics Lab Activity

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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

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**Electric field model Source charge: charge producing the field**

Test charge: a mathematical creation Always positive Symbol: q’ Doesn’t exist Infinitely small, thus produces no field of its own

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**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?

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**Where do you think the field is strongest?**

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**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. r Electric Field + Q 3. The direction of the E is the same as the direction of a force on + (pos) charge. 4. The magnitude of E is given by the formula:

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**Field is Property of Space**

Electric Field + Q . r E Electric Field + Q . r Force on +q is with field direction. F + +q - -q F Force on -q is against field direction. 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.

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Electric Field Lines 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. + Q - -Q Electric field line flow Out of positive charges and into Negative charges.

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**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.

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

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**These fields can be detected in lab…**

Threads floating on oil bath become polarized and align themselves with the electric field.

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**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).

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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

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Example 1. A +2 nC charge is placed at a distance r from a –8 mC charge. If the charge experiences a force of 4000 N, what is the electric field intensity E at point P? Electric Field . - -Q P +2 nC + +q 4000 N E E r –8 mC First, we note that the direction of E is toward –Q (down). E = 2 x 1012 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.

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SOLVE NOW A test charge of 5.0 x μC 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)

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**SOLVE NOW A test charge of 5.0 x 10-17 μC is 6.5 cm from a proton.**

What is the field intensity at this point?

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**Static Activity What's happening with the Van De Graaf?**

When two insulators are rubbed together: Silk on glass Þ glass ® (+) Who loses electrons? The glass Fur on rubber Þ rubber ® (-) The fur

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90

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Electric Potential Chapter 19

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**Gravitational Work and Energy**

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

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**Work changes potential energy!**

Work done can change the gravitational potential energy of an object WAB = GPEA- GPEB Work done can change the electrical potential energy of an object WAB = EPEA- EPEB

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**Electrical Work and Energy**

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

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**Work and Negative Charges**

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

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**ΔV = W q WAB = EPEA- EPEB q q q WAB = VA - VB q Units: J/C = Volt (V)**

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. WAB = EPEA- EPEB q q q WAB = VA - VB q ΔV = W 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. Units: J/C = Volt (V)

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Example 1: The work done on a test charge of 2μC as it moves from A to B is 5.0 x 10-8 J. (A) Find the difference in potential energies of the charge. W = ΔEPE = 5.0 x 10-8 J (B) Determine the potential difference between point A and B ΔV = W q = 5.0 x 10-8 J 2 x C = 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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