* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download An “electric field”
Survey
Document related concepts
History of quantum field theory wikipedia , lookup
History of electromagnetic theory wikipedia , lookup
Circular dichroism wikipedia , lookup
Casimir effect wikipedia , lookup
Superconductivity wikipedia , lookup
Work (physics) wikipedia , lookup
Speed of gravity wikipedia , lookup
Introduction to gauge theory wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Maxwell's equations wikipedia , lookup
Anti-gravity wikipedia , lookup
Electromagnetism wikipedia , lookup
Potential energy wikipedia , lookup
Field (physics) wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Lorentz force wikipedia , lookup
Transcript
Physics B Notes: Electrostatics Name:___________________ The atom Where is the positive charge in the atom? Is it mobile? Sample Problem How much positive charge resides in two moles of hydrogen gas (H2)? Where is the negative charge in the atom? Is it mobile? How much negative charge? Question How can a negative balloon pick up a neutral tissue? How much net charge? Sample Problem The total charge of a system composed of 1800 particles, all of which are protons or electrons, is 31x10-18 C. How many protons are in the system? Draw and label the electroscope How many electrons are in the system? Charge What are the two types of charge? Electric Force What condition must exist for an electrical force to be repulsive? What is meant by the elementary charge? What condition must exist for an electrical force to be attractive? Coulomb’s Law – form 1 Definition: Sample Problem A certain static discharge delivers -0.5 Coulombs of electrical charge. How many electrons are in this discharge? Equation: 6/30/2017 1 Bertrand What condition must exist for Coulomb’s law to hold? Definition: Sample Problem Coulomb’s Law – form 2 Equation: What is the force on the 4 C charge? y (m) 2.0 1.0 Define the relationship between k and o. -3 C 2 C 1.0 4 C 2.0 x (m) Sample Problem A point charge of positive 12.0 μC experiences an attractive force of 51 mN when it is placed 15 cm from another point charge. What is the other charge? Sample Problem What is the force on the 4 C charge? y (m) 2.0 -3 C 1.0 2 C 4 C 1.0 2.0 Sample Problem Calculate the mass of ball B, which is suspended in midair. A q = 1.50 nC x (m) R = 1.3 m B q = -0.50 nC Superposition 6/30/2017 2 Bertrand The Electric Field An “electric field” is created in the space around a charged particle or a configuration of charges. If a charged particle is placed in an electric field created by other charges, it will experience a force as a result of the field. Sometimes we know about the electric field without knowing much about the charge configuration that created it. We can easily calculate the electric force from the electric field. Field Vectors from Field Lines Why use fields? Force from Electric Field Equation: Field around + charge Sample Problem The electric field in a given region is 4000 N/C pointed toward the north. What is the force exerted on a 400 μg Styrofoam bead bearing 600 excess electrons when placed in the field? Field around - charge Sample Problem A 400 μg Styrofoam bead has 600 excess electrons on its surface. What is the magnitude and direction of the electric field that will suspend the bead in midair? Field between charged plates 6/30/2017 3 Bertrand Sample Problem A proton traveling at 440 m/s in the +x direction enters an an electric field of magnitude 5400 N/C directed in the +y direction. After 1.0 ms, find the velocity. Draw the field around a pair of charges Sample Problem A particle bearing -5.0 μC is placed at -2.0 cm, and a particle bearing 5.0 μC is placed at 2.0 cm. What is the field at the origin? For Spherical Electric Fields Equation: Does the Principle of Superposition apply to Electric Fields? Sample Problem A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero? Draw the field around an electric dipole Draw the field around a pair of + charges Sample Problem What is the charge on the bead? It’s mass is 32 mg. E = 5000 N/C 40o 6/30/2017 4 Bertrand What conditions would produce high positive electrical potential energy? Excess Charges on Conductors Where does the excess charge reside on a charged conductor? What conditions would produce low negative electrical potential energy? How does this help lightening rods to work? How would you get zero electrical potential energy? Electric Field Inside a Conductor Why is the field on the inside of an isolated conductor necessarily zero? Electric Potential What do we commonly call electrical potential? Electrical Potential and Potential Energy Equation: Draw an isolated conductor in an electric field, and show how the charge is distributed so as to nullify the external field. Electrical Potential and Potential Energy Positive charges like to __________ their potential (V ___ 0) Negative charges like to ___________ their potential. (V ___ 0) Electric Potential Energy Definition: 6/30/2017 5 Bertrand Sample Problem A 3.0 μC charge is moved through a potential difference of 640 V. What is its potential energy change? Sample Problem How much work would be done BY THE ELECTRIC FIELD in moving a 2 mC charge from A to C? From A to B? from B to C?. How much work would be done my an external force in each case? y(m) 1.0 Electrical Potential in Uniform Electric Fields Equation: Sample Problem An electric field is parallel to the x-axis. What is its magnitude and direction if the potential difference between x =1.0 m and x = 2.5 m is found to be +900 V? C A B 1.0 2.0 x(m) Conservation of Energy You know what Conservation of Energy is. We just need to work some problems. Sample Problem If a proton is accelerated through a potential difference of 2.000 V, what is its change in potential energy? Sample Problem What is the voltmeter reading between A and B? Between A and C? Assume that the electric field has a magnitude of 400 N/m. y(m) How fast will this proton be moving if it started at rest? C 1.0 6/30/2017 A B 1.0 2.0 x(m) 6 Bertrand Sample Problem A proton at rest is released in a uniform electric field. What potential difference must it move through in order to acquire a speed of 0.20 c? Electric Potential (spherical) Formula: Electric Potential Energy of Spherical Charges Equation: Sample Problem What is the electric potential at (2,2)? y (m) Sample Problem What is the potential energy of the configuration shown below? 2.0 -3 C 1.0 2 C 4 C 1.0 2.0 y (m) 2.0 1.0 2 C 4 C 1.0 2.0 x (m) x (m) Equipotential surfaces: parallel plate capacitor Sample Problem How much work was done in assembling the charge configuration shown below? y (m) 2.0 -3 C 1.0 2 C 4 C 1.0 2.0 6/30/2017 x (m) 7 Bertrand Equipotential surfaces: positive point charge Sample Problem Draw a negative point charge of -Q and its associated electric field. Draw 4 equipotential surfaces such that DV is the same between the surfaces, and draw them at the correct relative locations. What do you observe about the spacing between the equipotential surfaces? Equipotential surfaces: negative point charge Question What can you say about the intersection between field lines and equipotential surfaces? Fill in the following table for spherical charges Force Potential Energy Field Potential Sample Problem Draw field lines for the charge configuration below. The field is 600 V/m, and the plates are 2 m apart. Label each plate with its proper potential, and draw and label 3 equipotential surfaces between the plates. You may ignore edge effects. - - - - - -- - -- - -- - -- - -- - -- - -- - -- - -- - -- - - ++++++++++++++++++++++++++++++++ 6/30/2017 8 Bertrand Free Response Practice #1 A170B3. A small conducting sphere of mass 5 x 10 -3 kilogram, attached to a string of length 0.2 meter, is at rest in a uniform electric field E, directed horizontally to the right as shown above. There is a charge of 5x10-6 coulomb on the sphere. The string makes an angle of 30° with the vertical. Assume g = 10 meters per second squared. a. In the space below, draw and label all the forces acting on the sphere. b. Calculate the tension in the string and the magnitude of the electric field. c. The string now breaks. Describe the subsequent motion of the sphere and sketch on the following diagram the path of the sphere while in the electric field. 6/30/2017 9 Bertrand Free Response Practice #2 A176B2. Object I, shown above, has a charge of + 3 x 10 -6 coulomb and a mass of 0.0025 kilogram. a. What is the electric potential at point P, 0.30 meter from object I ? Object II, of the same mass as object I, but having a charge of + 1 x 10 -6 coulomb, is brought from infinity to point P, as shown above. b. How much work must be done to bring the object II from infinity to point P ? c. What is the magnitude of the electric force between the two objects when they are 0.30 meter apart? d. What are the magnitude and direction of the electric field at the point midway between the two objects? The two objects are then released simultaneously and move apart due to the electric force between them. No other forces act on the objects. e. What is the speed of object I when the objects are very far apart? 6/30/2017 10 Bertrand