Electric force

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  1. Electric force in one dimension
    Three point charges are fixed in positions as shown. Find the net force acting on Q1Q_1.
    1. Electric force in two dimensions
      Three point charges are fixed in positions as shown. Find the net force acting on Q1Q_1 .
      Topic Notes
      In this lesson, we will learn:
      • Coulomb's law, which gives the electric force that one charged object exerts on another
      • Calculating electric force for different arrangements of charges

      • Electrostatics deals with electric charges that are at rest ("static")
      • Charge is a scalar quantity. It can be positive or negative. The positive or negative character of a charge is called polarity.
      • Like gravity, electric forces act at a distance. Unlike gravity, which always pulls objects together, electric forces can either push apart or pull together charges.
        • Like charges (both positive or both negative) will repel each other
        • Opposite charges (one positive and one negative) will attract each other.
      • Coulomb's law describes electric force (Fe).

      • Fe=kQ1Q2r2|F_e| = k \frac{|Q_1 Q_2|}{r^2}

        • k (Coulomb's constant) is an experimentally determined constant that relates the size of the charges (Q1 and Q2) and radius (r¸ distance between charges) to the magnitude Fe.
        • Coulomb's law only gives the magnitude of Fe and not the direction, indicated by the absolute value sign on |Fe|. Notice that k, Q, and r2r^2 are all scalars: there are no vectors on that side of the equation that could give Fe a direction. The direction of Fe must be found by considering if the charges involved would be attracted or repelled, based on their polarities.

      Coulomb's Law (Electric Force)

      Fe=kQ1Q2r2|F_e| = k \frac{|Q_1 Q_2|}{r^2}
      Fe:|F_e|: magnitude of electric force, in newtons (N)
      k:k: Coulomb's law constant, 9.00×109Nm2/C29.00 \times 10^9 N\centerdot m^2 / C^2
      Q1,Q2:|Q_1|, |Q_2|: magnitude of each charge, in coulombs (C)
      r:r: distance between charges, in meters (m)