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Ch 22 Electric Fields 2 The Electric Field. This is the electric force per unit charge at a point in space. If a test particle with ⃗ at that point. Units: [ ] charge at a point feels net electric force ⃗ , then ⃗⃗ Table 22-1 Some Electric Fields 3 Electric Field Lines. These lines of force enable us to visualize patterns in electric fields. The density of lines in a small region is proportional to the magnitude of ⃗⃗ , and the direction of the tangent to a line at a point gives the direction of ⃗⃗ . The lines extend away from positive charge (where they originate) and toward negative charge (where they terminate). Additional rules for drawing electric field lines: a) They start at positive charges or at infinity, b) They end at negative charges or at infinity, and c) The number of lines entering or leaving a charge is proportional to the magnitude of the charge. 4 The Electric Field Due to a Point Charge. The field of a positive (negative) charge points away from | |⁄ (toward) the charge. The magnitude of the field is . Superposition of Fields: Fields from different charges are added in the same way as forces. ⃗⃗ ⃗⃗ ⃗⃗ ⃗⃗ Checkpoint 1 Sample Problem: Net electric field due to three charged particles 5 The Electric Field Due to an Electric Dipole. A dipole moment ⃗ represents the combination of two charges of equal magnitude and opposite sign, separated by a distance . The magnitude is and the direction points from the negative to the positive charge. Consider a point on the line joining the two charges, on the side of the positive charge, at a distance from the dipole’s center. The electric field points away from the dipole; its magnitude is approximately given by Sample Problem: Electric dipole and atmospheric sprites 8 A Point Charge in an Electric Field. The electrostatic force ⃗ acting on a charged particle located in an external electric field ⃗⃗ has the direction of ⃗⃗ if the charge of the particle is positive and has the opposite direction if is negative: ⃗ ⃗⃗ . Checkpoint 3 Sample Problem: Motion of a charged particle in an electric field 9 A Dipole in an Electric Field. If a dipole ⃗ is in a uniform external field ⃗⃗ , the torque acting on it is ⃗⃗ and its potential energy is ⃗ ⃗ ⃗ ⃗⃗ . Checkpoint 4 Sample Problem: Torque and energy of an electric dipole in an electric field