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Electric Fields The electric field is a vector quantity that relates the force exerted on a test charge to the size of the test charge. Huh? Electric Fields What does Coulomb’s law tell us? If an object is charged but nothing is close enough to feel it…does the charge go away? Why is the size of the test charge important? E= Fon q’ q’ Unit = ? Let’s practice! An electric field is to be measured using a positive test charge of 4.0 x 10-5 C. This test charge experiences a force of 0.60N acting at and angle of 10°. What is the magnitude and direction of the electric field at the location of the test charge? Answer: 1.5 X 104 N/C at 10° Picturing the Electric Field Electric field lines show direction and strength of the electric field. Will positive and negative field lines be the same? Lines start on a positive charge and terminate on a negative charge The closer together the lines are, the stronger the field -Lines of force are drawn perpendicularly away from the positively-charged object and perpendicularly into the negatively-charged object. -Electric field lines between the oppositely charged objects are bent…what dictates this shape? http://www.youtube.com/watch?v=gU0MUUVYyAU Review A positive test charge of 5.0 X 10-4C is in an electric field that exerts a force of 2.5 x 10-4N on it. What is the magnitude of the electric field at the location of the test charge? Suppose the electric field in the problem above is caused by a point charge. The test charge is moved to a distance twice as far from the charge. What is the magnitude of the force that the field exerts on the test charge now? You are probing the field of a charge of unknown magnitude and sign. You first map the field with a 1.0 x 10-6 C test charge, then you repeat your work with a 2.0 x 10-6 C test charge. a) Would you measure the same forces with the two test charges? Explain. b) Would you find the same fields? Explain.