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Practice Electrostatics
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Students will learn the inner workings of electrostatics: Why certain objects repulse, why certain objects attract and how to calculate the number of excess electrons or protons in an object.

Key Equations

q = Ne \text{ } Any object's charge is an integer multiple of an electron's charge.

  • Opposite charges attract and like charges repulse
  • The electron (and proton) is the fundamental charge unit. The charge of an electron and proton is  1.6 \times 10^{-19} C. One can determine the number of excess electrons (or protons if positive charge) by dividing the objects charge by the fundamental charge.
  • Most objects are electrically neutral (equal numbers of electrons and protons) and that's why gravity dominates on a macro scale.

Example 1

Question If an object has +0.003 C of charge, how many excess protons does the object have?

Answer q = Ne \text{ }  0.003 \text{C} = N \times 1.6 \times 10^{-19} N =  1.875 \times 10^{16} ; protons

Watch this Explanation


Balloons (PhET Simulation)

Time for Practice

  1. After sliding your feet across the rug, you touch the sink faucet and get shocked. Explain what is happening.
  2. What is the net charge of the universe? Of your toaster?
  3. As you slide your feet along the carpet, you pick up a net charge of +4 \;\mathrm{mC} . Which of the following is true?
    1. You have an excess of 2.5 \times 10^{16} electrons
    2. You have an excess of 2.5 \times 10^{19} electrons
    3. You have an excess of 2.5 \times 10^{16} protons
    4. You have an excess of 2.5 \times 10^{19} protons
  4. You rub a glass rod with a piece of fur. If the rod now has a charge of -0.6\ \mu C , how many electrons have been added to the rod?
    1. 3.75 \times 10^{18}
    2. 3.75 \times 10^{12}
    3. 6000
    4. 6.00 \times 10^{12}
    5. Not enough information

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