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ScientistGrades 11–12⏱ 20 min

Electric charges and fields

Drag positive and negative charges, watch them push and pull by Coulomb’s law, and map the electric field around them.

Standards: HS-PS2-4

1Wonder

Rub a balloon on your hair and it sticks to the wall. Your hair stands up toward it. Nothing touches, yet something pulls. What is going on?

2Explore

Charge lab

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

+−+They attractr = 0.48 mq₁ = 2 μC, q₂ = −2 μCF = k q₁q₂ / r² = 0.156 NSillyChili
Try this
  • Make both charges positive. Which way do the orange force arrows point? Now make one negative.
  • Drag the charges twice as far apart. What happens to the length of the force arrows and the number for F?
  • Move the green test charge around. Which way does its arrow point near the + charge? Near the − charge?
  • Turn on Show field lines. Where do the lines start and where do they end?

3Explain

++−−+−SillyChili
Step 1 of 5

Two kinds of charge

Electric charge comes in two kinds, positive and negative. Like charges repel; unlike charges attract. The forces on the two charges are always equal and opposite.

Coulomb’s law and the electric fieldF=k q1q2r2E=Fq\begin{aligned} F &= \frac{k\,q_1 q_2}{r^2} \\ E &= \frac{F}{q} \end{aligned}

Like charges repel, unlike charges attract. The field points the way a positive charge would be pushed: away from + and toward −.

4Practice

Question 1 of 5

Two negative charges are placed near each other. What happens?

5Recap

  • Charges are positive or negative; like charges repel, unlike attract.
  • Coulomb’s law: F = k q₁q₂ / r², weaker with the square of the distance.
  • The electric field points away from + and toward −.
  • Field lines run from + to − and crowd together where the field is strong.