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Concept Summary

Electric Fields & Potential. Concept Summary. Electric Fields. An electric charge creates a disturbance in the space around it - an electric field . The electric field extends infinitely far but weakens with distance .

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Concept Summary

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  1. Electric Fields & Potential Concept Summary

  2. Electric Fields • An electric charge creates a disturbance in the space around it - an electric field. • The electric field extends infinitely far but weakens with distance. • This electric field exerts a force on other charges within the field.

  3. The Electric Field • One can think of electric force as establishing a “field” telling particles which way to move and how fast Electric “field lines” tell a positive charge which way to move. For example, a positive charge itself has field lines pointing away from it, because this is how a positively-charged “test-particle” would respond if placed in the vicinity (repulsive force). + Run Away! + 3

  4. Electric Fields • Electric fields are vector fields. • Direction at any point is the direction that a positive charge would move if placed there. • Magnitude at any point, E = F/q • Units of E are Newtons/Coulomb • Force a charge feels, F = qE

  5. Electric Field Lines • Point in the direction a positive charge would move • Field is stronger where field lines are closer together. • Also called “lines of force”

  6. Analogy The electric field is the space around an electrical charge just like a gravitational field is the space around a mass.

  7. Electric Field • Space around a charge.

  8. What is the difference?

  9. Electric Field Lines

  10. Electric Field Lines of two Positive Charges

  11. Electric Field Lines • Lines that indicate the strength and direction of the electric field. • The more dense the lines, the stronger the field. • Electric field vectors are tangent to the curve.

  12. Rye High School: Physics Field Lines The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge. Electric field lines never cross each other. The electric field is stronger where the field lines are closer together. 12

  13. Which field is stronger? • A B

  14. Charged Conductors • If a conductor has a static charge: • All of the charge resides on the surface of the conductor • The electric field everywhere inside the conductor is zero. • If the conductor is not spherical, the charge distribution will not be uniform.

  15. Electric Potential Energy • You have to do work to move a charge in an electric field. • The electric potential energy of a charge at a point in an electric field = -(work done by the electric field when the charge is moved there).

  16. Electric Potential • Electric potential = electric potential energy per unit charge • Electric potential is commonly called voltage .

  17. Electrostatics • Electrostatics is the study of electric charge at rest. • (Or more or less at rest, in contrast with current electricity.)

  18. Electrical Charges • Electric charge is a fundamental property of matter. • Two types of electric charges • Positive charge - every proton has a single positive charge. • Negative charge - every electron has a single negative charge.

  19. Electrical Charge • An object with an excess of electrons is negatively charged. • An object with too few electrons (too many protons) is positively charged. • An object with the same number of electrons and protons is neutral.

  20. Electrical Forces • Like charges repel. • Opposite charges attract.

  21. Elementary Charges • Protons carry the smallest positive charge. • Protons and uncharged neutrons generally reside in an atom’s nucleus. • Protons are held in the nucleus by the strong force.

  22. Elementary Charges • The charges carried by the proton and electron are equal in size. • The mass of the proton is about 2000 times the mass of the electron.

  23. Charge is Conserved • Electric charge is conserved - • Electric charge moves from one place to another - no case of the net creation or destruction of electric charge has ever been observed. • In solids, only electrons can move. • In liquids, gasses, and plasmas, both positive and negative ions are free to move.

  24. “Creating” an Electric Charge • When you “create” an electric charge (by rubbing your feet on a carpet) you are actually separating existing charges - not creating charges. • One object ends up with an excess of electrons (- charge), and the other a deficit of electrons (+ charge).

  25. Charging by Friction • If one neutral material has more affinity for electrons than another (neutral) material, it will attract electrons from the other. • One material becomes negatively charged, the other positively charged.

  26. Charging by Contact • If a charged object is brought in contact with a neutral object, charges will be repelled from (or attracted to) the charged object. • The neutral object will gain a charge of the same sign as the charged object.

  27. Grounding • Providing a path from a charged object to the Earth is called grounding it. • Charges will be attracted from (or repelled to) the Earth by the charged object. • Since the Earth is so large, both the charged object and the Earth are neutralized.

  28. Polarization • Bringing a charged object near (but not touching) a neutral object polarizes (temporarily separates) the charge of the neutral object. • Like charges in the neutral object are repelled by the charged object. • Unlike charges in the neutral object are attracted by the neutral object. • The neutral object returns to normal when the charged object is removed.

  29. Electric Dipoles • An object that is electrically neutral overall, but permanently polarized, is called an electric dipole. • Example: H20 molecule

  30. Charging by Induction • Bring a charged object near (but not touching) a neutral object. • Ground the neutral object. • Remove the ground. • Remove the charged object • The neutral object now has a charge opposite to the charged object.

  31. Voltage • A location can have a voltage whether or not a charge is located there. • The voltage difference (potential difference) between two points tells you: • The energy available when a unit charge is moved between the points, or • The work that needs to be done to move a unit charge between the two points.

  32. In a battery the potential difference (voltage) between the positive and negative terminals sets up an electric field. This electric field causes electrons to accelerate in the direction of the positive terminal thus causing current to flow.

  33. Electric Circuit A path over which electrons travel, out through the negative terminal, through the conductor, back in through the positive terminal.

  34. http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htmhttp://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htm

  35. Voltage • The voltage between two points (potential difference) tells you the amount of energy 1 Coulomb of charge will lose when it moves between the two points.

  36. Storing Electrical Energy • Since unlike charges attract, it takes work to separate unlike charges. • This work is stored as electrical potential energy

  37. Capacitors • A capacitor is a device that contains 2 parallel conducting “plates” insulated from each other. • It is used to store electrical energy, by holding positive charges on one plate and negative charges on the other. • Capacitors can be charged to high voltages and store large amounts of energy.

  38. Van de Graaff Generator • The Van de Graaff Generator is a device for creating large voltages. • Voltages of 150,000 V + are common

  39. Voltage and Danger • ENERGY DOES WORK! • The Van de Graaff gives each charge a large amount of energy (high voltage), but there aren’t many charges - therefore the Van de Graaff delivers very little total energy.

  40. Voltage and Danger • An electrical outlet (120 Volts) gives comparatively little energy to a charge, but it can deliver LOTS of charge - at least 15 Coulombs of charge per second. • Therefore, the electrical outlet can deliver LOTS of energy - enough to kill you!

  41. Resistance Depends on 4 Factors: 1.    Temperature Increasing the temperature causes an increase in the collision of particles, which then in turn increases the resistance.  2.    Length of Material Increasing the length of the material causes an increase in the number of collisions, which in turn increases the resistance .

  42. 3.    Cross Sectional Area of Wire Increasing the space, decreases the number of collisions, which then decreases the resistance. 4.       Nature of the Material Metals differ in both the number of the free electrons and the space available for electron movement. The number that indicates the resistance of materials per meter in length of 1m2 cross-sectional area, at a given temperature, is called resistivity of the material.

  43. Insulators have very high values of resistivity whereas good conductors have very low resistivities. Resistors are materials with a mid-range resistivity.

  44. Resistance http://www.youtube.com/watch?v=WzUTOeoxREM&feature=related

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