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Electric Fields http://www.falstad.com/vector3de/ Electric Field • The electric field is a vector field that describes how charges modify the space around them. • The electric field direction at any location is the direction of the force experienced by a positive test charge imagined to be placed at that location. How can we quantify electric field? • Electric field is defined as 𝐹𝑡 𝐸= 𝑞𝑡 • For a single point charge (source): 𝑞𝑠 𝑞𝑡 𝑘 2 𝐹𝑡 𝑞𝑠 𝑟 𝐸= = =𝑘 2 𝑞𝑡 𝑞𝑡 𝑟 Exercise 22, p. 557 An electron is acted on by an electric force of 3.2 x 10-14N. What is the magnitude of the electric field at the electron’s location? (e = 1.60 x 10-19C) ANSWER: 2.0 x 105 N/C Exercise 24, p. 557 What is the magnitude and direction of the electric field at a point 0.75cm away from a point charge of +2.0pC? ANSWER: 3.2 x 102 N/C away from the source Superposition Principle For a configuration of charges, the total, or net, electric field at any point is the vector sum of the electric fields due to individual charges of the configuration. Example 15.6, p. 542 Two point charges are placed on the x-axis as in the figure below. Find all locations on the axis where the electric field is zero. ANSWER: 0.20m FOLLOW-UP EXERCISE: Repeat this example, changing only the sign of the right-hand charge. Example 15.7, p. 542 The figure below shows a configuration of three point charges. a) In what quadrant is the electric field at the origin: (1) the first quadrant, (2) the second quadrant, or (3) the third quadrant? Explain your reasoning, using the superposition principle. b) Calculate the magnitude and direction of the electric field at the origin due to these charges. ANSWER: 1.69 x 103 N/C at 30° What is the force on a -1µC charge placed at the origin? (magnitude and direction)