Electric Charge and Electric Force
Introduced| English |
|---|
| charge/tʃɑːdʒ/ |
| Coulomb's law/ˈkuːlɒmz lɔː/ |
| coulomb/ˈkuːlɒm/ |
Rub a balloon on your hair — now it sticks to the wall
- Rub a balloon on your hair and it clings to a wall, defying gravity.
- Rubbing moved invisible charge 电荷 from one surface to the other.
- Charges come in two kinds, and they push and pull without ever touching.
- The rule for that push and pull is Coulomb's law 库仑定律.
Two kinds of charge
- There are positive and negative charges (protons are +, electrons are −).
- Like charges repel; opposite charges attract — the fundamental rule.
- Charge is measured in coulombs 库仑 ($\text{C}$); the electron's charge is tiny.
- An object is charged when it has unequal numbers of protons and electrons.
Two negative charges are placed near each other. They will:
Like charges repel — two negatives push apart.
Electric charge is measured in ____.
Charge is measured in coulombs ($\text{C}$).
A positive and a negative charge will:
Opposite charges attract.
Coulomb's law
- The force between two point charges is $F = \dfrac{k q_1 q_2}{r^2}$.
- It grows with each charge and falls off as the inverse square of the distance.
- Double the distance and the force drops to a quarter — just like gravity.
- $k$ is Coulomb's constant; the direction is along the line joining the charges.
The field of a charge
Change the charge and see the electric field it creates around itself.
Two charges are brought from $2\ \text{m}$ apart to $1\ \text{m}$ apart. By what factor does the force increase?
Halving $r$ multiplies $1/r^2$ by $4$ — the force is four times larger.
Select all changes that increase the electric force between two charges.
$F = kq_1q_2/r^2$ grows with either charge and with closeness. Farther apart weakens it.
Just like gravity, but stronger
- Coulomb's law has the same inverse-square shape as gravity.
- But the electric force is vastly stronger and can repel as well as attract.
- Gravity only ever pulls; the electric force does both, depending on the signs.
- This is why chemistry and electronics are governed by electric forces, not gravity.
Like gravity, the electric force follows an inverse-square law with distance.
Both $F = kq_1q_2/r^2$ and gravity go as $1/r^2$; the electric force can also repel.
Coulomb's law is inverse-square ($1/r^2$), not $1/r$. Doubling the separation quarters the force, not halves it. And remember the force can be a push or a pull depending on whether the charges are alike or opposite.
Two charges are moved from $2\ \text{m}$ apart to $1\ \text{m}$ apart. How does the force change?
- Halving $r$ multiplies $\dfrac{1}{r^2}$ by $4$.
- The force becomes four times stronger.
There are two kinds of charge (+ and −): like repel, opposite attract. Coulomb's law gives the force, $F = \dfrac{kq_1q_2}{r^2}$ — an inverse-square law like gravity, but far stronger and able to both attract and repel.