Electric Charges and Fields Interaction of Charges Opposite
- Slides: 34
Electric Charges and Fields
Interaction of Charges • Opposite charges attract (positive to negative) • Like charges repel each other (positive to positive OR negative to negative)
Newton’s Third Law and Charges • The force felt by one object is identical to the force felt by the other object.
Interaction of Charges Practice • If a balloon is known to have a negative charge and is attracted to another balloon, what must the charge on the second balloon be? • If I put a metal rod which I do not know the charge of next to a negatively charged sphere and the sphere rolls away from it what can I infer the charge of the rod is? • Two balls are interacting, one has a charge of -3 and one is neutral. Which experiences a greater electrostatic force? • A rod with an unknown charge attracts a ball. Which of the following could describe the situation? (A) The rod has negative charge and the pith ball has positive charge. (B) The rod has positive charge and the pith ball has negative charge. (C) They both have positive charge. (D) They both have negative charge. (E) Both A and B above.
Charge Interactions are Forces The two fundamental charge interactions are: • oppositely charged objects attract • like charged objects repel. These mutual interactions resulted in an electrical force between the two charged objects. **This force is MUCH greater than the force of gravity!**
Electrostatic Force The magnitude of the electrostatic force of attraction or repulsion between two charges is: • directly proportional to the product of the magnitudes of charges • inversely proportional to the square of the distance between them.
Coulomb's Law The magnitude of the electrostatic force of attraction or repulsion between two charges is: • directly proportional to the product of the magnitudes of charges If either charge goes up, the total force goes up
Coulomb's Law The magnitude of the electrostatic force of attraction or repulsion between two charges is: • inversely proportional to the square of the distance between them. F d
Force as a Vector Quantity • Being a force, the strength of the electrical interaction is a vector quantity which has both magnitude and direction. _________ • If your force is negative: They are attracting • If your force is positive: They are repelling • Negative X Negative = Positive • Positive X Positive = Positive • Negative X Positive = Negative
Practice A) B) C) D) Two uniformly charged spheres are attached to frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces on the two spheres? E) Answer: D) The forces are equal in magnitude and opposite in direction, by Newton's Third Law, and by the form of the Coulomb force equation.
Practice Two charges that are 2 meters apart repel each other with a force of 2 x 10 -5 newton. If the distance between the charges is decreased to 1 meter, what will the force of repulsion be?
Practice A repulsive electrostatic force of magnitude F exists between two metal spheres having identical charge q. The distance between their two centers is r. Which combination of changes would produce no change in the electrostatic force between the two spheres? 1. doubling q on one sphere while doubling r 2. doubling q on both spheres while doubling r 3. doubling q on one sphere while halving r 4. doubling q on both spheres while halving r
Calculating Electrostatic Force: Coulomb's Law
Practice • Suppose that two point charges, each with a charge of +1. 00 Coulomb are separated by a distance of 1. 00 meter. Determine the magnitude and direction of the electrical force of repulsion between them.
What is an Electric Field? • How can an apple reach across space and fall toward Earth? • The Earth creates a Gravitational field. Other masses in that field would feel its effect in the space. Whether a mass object enters that space or not, the gravitational field exists.
What is an Electric Field? Electric Fields behave in the same way as gravitational fields! • How can a balloon reach across space and pull a second balloon towards it or push it away? • A charged object creates an electric field. Other charges in that field would feel its effect in the space. Whether a charged object enters that space or not, the electric field exists. • This electric field is much stronger than gravity which is why you can pick up a piece of paper with a charged balloon
Drawing an Electric Field Rules 1. 2. 3. 4. 5.
Electric Field Lines • Lines are directed away from positively charged source charges and toward negatively charged source charges. • The closer lines near the charge indicating stronger field.
Rules for Drawing Electric Field Patterns 1. Surround more charged objects by ___________. more lines.
greatest 2. The electric field is __________ at locations closest to the surface of the charge and least at locations further from the surface of the charge.
3. never cross Electric field lines should ___________. • Every single location in space has its own electric field strength and direction associated with it; consequently, the lines representing the field cannot cross each other at any given location in space.
Electric Field Lines for Configurations Two Opposite charges Two same charges 0
Unequal amount of charges
Practice The diagram shows the electric field in the vicinity of two charged conducting spheres, A and B. What is the static electric charge on each of the conducting spheres? 1. A is negative and B is positive. 2. A is positive and B is negative. 3. Both A and B are positive. 4. Both A and B are negative.
Lightning
Lightning • Perhaps the most known and powerful displays of electrostatics in nature is a lightning storm. • What is the cause and mechanism associated with lightning strikes?
Static Charge Buildup in the Clouds The precursor of any lightning strike is the tops of the storm clouds are known to acquire an excess of positive charge and the bottom of the storm clouds acquire an excess of negative charge.
• When the difference in negative and positive charges between ground and cloud gets large enough, a lightning bolt begins. The excess electrons on the bottom move towards the positive charge of the ground. • As electrons travel close to the Earth, it encounters the positive charges traveling upward, when the two types of charges meet, lightning begins. • The enormous and rapid flow of charge along this pathway between the cloud and Earth heats the surrounding air, causing it to expand violently. The expansion of the air creates a shockwave which we observe as thunder
Types of Lightning
Nat. Geo Lightning Documentary
Electric Field Strength • Electric field strength is determined by the force of the field and the charge of the field.
Electric Field Strength • Direct Relationship between Electric Field Strength and Force • Inverse Relationship between Electric Field Strength and Charge
Electric Field strength • Electric field strength is a vector quantity. it has both magnitude and direction. Since the direction of this arrow is going to the right, it must be a positive quantity. If the arrows were going left, it would be a negative quantity. Notice that the arrows for each are pointing the same way
Electric Field Calculation Practice What is the magnitude of the electric force acting on an electron with a charge of 1. 6 × 10 -19 C located in an electric field with an intensity of 5. 0 x 103 N/C? A proton with a charge of 1. 6 x 10 -19 C is observed to experience an Electrostatic Force to the left; the force has a magnitude of 3. 9 x 10 -15 N. What is the magnitude of the Electric Field at the location of this proton? What is the charge on a particle that is in an electric field with a strength of 2. 5 x 104 N/C experiencing a force of 1. 4 x 10 -13 N?
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