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Electric Charge, Coulomb's Law, and Electric Fields

The atmosphere surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble in which we live. This protective bubble consists of several gases (listed in the table below), with the top four making up 99998% of all gases. Of the dry composition of the atmosphere, nitrogen by far is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins.

Figures (8)

Diagram showing field lines and equipotentials around an electron, a negatively charged particle. In an electrically neutral atom, the number of electrons is equal to the number of protons (which are positively charged), resulting in a net zero overall charge
Electric field induced by a positive electric charge
Electric field induced by a negative electric charge
Coulomb's torsion balance
Figure 5.14 The electrostatic force F→F→ between point charges q1q1 and q2q2 separated by a distance r is given by Coulomb’s law. Note that Newton’s third law (every force exerted creates an equal and opposite force) applies as usual—the force on q1q1 is equal in magnitude and opposite in direction to the force it exerts on q2q2. (a) Like charges; (b) unlike charges.
Figure 5.15 A schematic depiction of a hydrogen atom, showing the force on the electron. This depiction is only to enable us to calculate the force; the hydrogen atom does not really look like this. Recall Figure 5.7.
Figure 5.16 The eight source charges each apply a force on the single test charge Q. Each force can be calculated independently of the other seven forces. This is the essence of the superposition principle.
Figure 5.17 Source charges q1q1 and q3q3 each apply a force on q2q2.

Key Points

  • Electric charge (symbol q, sometimes Q) is a physical property of matter that causes it to experience a force when placed in an electromagnetic field.
  • The coulomb is defined as the quantity of charge that passes through the cross section of an electrical conductor carrying one ampere for one second.
  • Coulomb's inverse-square law, or simply Coulomb's law, is a scientific law of physics that describes the amount of force between two electrically charged particles at rest.
  • An electric field is a vector field that associates to each point in space the Coulomb force experienced by a unit test charge.
  • In the case of a single point charge at rest, the two laws are equivalent, expressing the same physical law in different ways.
  • Coulomb's law quantifies the electrostatic force between two particles by asserting that the force is proportional to the product of their charges, and inversely proportional to the square of the distance between them.

Terms

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