Electrical Energy Fields Or Fields of Dreams 2

An electric charge changes the space around it so that other charges… Feel an electrical force – Electrical force field – Measured by electrical force (F E ) and electrical field (E) – Vector quantities and addition Have an electrical potential energy – Electrical energy field – Measured by electrical potential energy (PE E ) and Electrical potential or potential or voltage – Scalar quantities All assume a positive test charge

Electrical Energy Field Variables Electrical PE Potential Energy charge has due to its position near another electrical charge Units of Joules Scalar quantity Usually measured relative to a reference value  PE E = Q test  V Varies w/ test charge, source charge and distance Electrical Potential, Potential or Voltage Potential energy for each coulomb of charge due to test charge’s position near another electrical charge Units of Joules/ Coulomb or Volts Scalar quantity Usually measured relative to a reference value  V =  PE E / Q test Varies w/ source charge and distance only

Check Question Do a or b have the greater electrical PE? How about Voltage? Do b or c have the greater electrical PE? How about Voltage A B C

Representing Electrical Energy Fields Equipotential Diagrams – Equipotential lines connect locations of equal potential energy – Equipotential lines never cross one another – Close equipotential lines indicate strong fields – Test charges move from high to low potential – Lines are perpendicular to force lines Faraday Diagrams – Equate potential to elevation like on contour map – Test charges roll downhill – Positive points are hills, Negative points are holes, positive plates are plateaus and negative plates linear valleys.

Check Question Which charge has the stronger electric field? Which charge is positive? Which direction would a proton move if placed between the charges? A B

Examples of Energy Fields Point Charges – Equipotential diagrams – Faraday diagrams – Equations  PE E = K c Q source Q test /d  V= K c Q source /d Opposite Plate Charges – Diagrams – Potential decreases as move towards negative plate in linear fashion –  V=E  d only for this case (uniform electric field)

Conservation of Energy Like charges – Potential greatest near + charge – Potential energy defined to be zero at infinity Opposite charges – Potential least near + charge – Potential energy defined to be negative infinity at infinity and zero near charge Test charges conserve energy when moved. That is the PE stored will become KE when charge is released Capacitor – Stores energy in electrical field – Charges to battery voltage, charge flows off when two plates are connected by a wire. – Charge – PE E =1/2 Q  V – C=Q/  V – C=    d for parallel plate capacitors PE d Like charges Opposite charges

Check Question It takes 100 joules of work to push a test charge 2 cm closer to a positive plate. The charge is then released. – What is the potential energy change of the charge after the push? – What is the potential energy change of the charge when it returns to its initial position? – What is the kinetic energy of the charge when it returns to its initial position? – What is the velocity of the charge when it returns to its initial position if its mass is 2 kg?

Check Question List 4 reasons why the electric charges move inside the fluorescent bulb when held perpendicular to the plasma sphere.