State the behaviour of electric charges: like charges repel and opposite charges attract.
Key Concepts
Electric charge is a fundamental property of matter.
Charges are of two types: positive (+) and negative (‑).
Like charges repel each other; unlike charges attract each other.
Charge Interactions
The force between two point charges is described by Coulomb’s law:
\$F = k \frac{|q1 q2|}{r^2}\$
where \$F\$ is the magnitude of the force, \$k = 9 \times 10^9 \,\text{Nm}^2\text{C}^{-2}\$, \$q1\$ and \$q2\$ are the charges, and \$r\$ is the separation.
Summary Table
Charges Involved
Force Direction
Positive – Positive
Repulsion
Negative – Negative
Repulsion
Positive – Negative
Attraction
Examples
Two positively charged spheres pushed together will move apart.
A negatively charged rod brought near a positively charged electroscope causes the leaves to collapse (attraction).
Two negatively charged balloons will repel each other.
Suggested diagram: Two positive charges repelling each other and a positive charge attracting a negative charge, with arrows indicating the direction of the forces.
Common Misconceptions
Thinking that “opposite” refers to the direction of force rather than the type of charge.
Assuming that neutral objects have a charge; they are uncharged.
Check Your Understanding
What will happen if a positively charged rod is brought near a negatively charged rod? Explain using the table.
Two identical charges are separated by a distance \$r\$. If the distance is halved, how does the magnitude of the force change? Use Coulomb’s law to justify.