Electric force

Electric force

Lessons

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
Notes:

  • 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)

  • 1.
    Electric force in one dimension
    Three point charges are fixed in positions as shown. Find the net force acting on Q1Q_1.

  • 2.
    Electric force in two dimensions
    Three point charges are fixed in positions as shown. Find the net force acting on Q1Q_1 .