coulombs law

• Coulomb's law


  Equation that describes the relationship between the electric force, the magnitude of the charges, and the distance between the two charges
• At the center of an atom is the nucleus, which contains protons and neutrons
• Protons

  Subatomic particles in the nucleus that carry a positive charge
• Neutrons


  Subatomic particles in the nucleus that are neutral in charge
• Electrons


  Particles that orbit the nucleus and carry a negative charge
• The charge of a proton is 1.6 x 10^-19 coulombs
• Electric charge is quantized, it exists in discrete amounts
• Electron charge


  Negative 1.6 x 10^-19 coulombs
• Opposite charges


  Attract each other
• Like charges


  Repel each other
• k


  Proportionality constant = 9 x 10^9 N·m^2/C^2
• r


  Distance between the two charges
• Increasing the magnitude of one charge


  Doubles the electric force
• Doubling the distance between charges

  Reduces the force by a factor of 4
• Distance units


  • 1 centimeter = 0.01 meters, 1 millimeter = 0.001 meters
• Solving Coulomb's law problems


  Draw diagram

  2. Identify charges, distances, and forces

  3. Plug values into Coulomb's law equation

  4. Calculate the unknown value
• Point charges


  Charges that are concentrated at a single point in space
• 0 newtons exists between two identical point charges separated by a distance of 40 centimeters
• Calculating magnitude of two point charges

  1. Draw picture

  2. Identify charges as q

  3. Convert distance to meters

  4. Use formula: f = k * q^2 / r^2

  5. Solve for q
• Coulomb


  Unit of electric charge
• Converting charge in micro coulombs to number of electrons


  1. Convert micro coulombs to coulombs

  2. Use conversion factor: 1 electron = -1.6 x 10^-19 coulombs

  3. Divide charge in coulombs by electron charge
• Converting number of protons to charge in micro coulombs

  1. Use conversion factor: 1 proton = 1.6 x 10^-19 coulombs

  2. Multiply number of protons by proton charge

  3. Divide by 1 x 10^-6 to get micro coulombs
• Calculating net electric charge on a metal sphere


  1. Find difference between number of protons and electrons

  2. Multiply difference by charge of a proton
• Net electric force


  The vector sum of all the electric forces acting on a charge
• Calculating net electric force on a 100 micro coulomb charge

  1. Draw picture showing charges and positions

  2. Calculate force between 100 micro C and 200 micro C charges

  3. Calculate force between 100 micro C and -50 micro C charges

  4. Add the two forces to get net force
• The net electric force acting on the 200 micro coulomb charge is -8.75 newtons in the negative x direction