Electric Charge and Electric Force
Introduced| English | Meaning |
|---|---|
| charge/tʃɑːdʒ/ | An electrical property that causes attraction or repulsion. |
| Coulomb's law/ˈkuːlɒmz lɔː/ | The rule that the electrostatic force between point charges is proportional to their charge product and inversely proportional to the square of their separation. |
| coulomb/ˈkuːlɒm/ | The SI unit of electric charge, equal to the charge transferred by a current of one ampere in one second. |
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:
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Like charges repel — two negatives push apart.
Electric charge is measured in ____.
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Charge is measured in coulombs ($\text{C}$).
A positive and a negative charge will:
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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?
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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.
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$F = kq_1q_2/r^2$ 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.
The electric force between two charges weakens in direct proportion to their separation.
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False. Like gravity, the electric force follows an inverse-square law: doubling the separation divides the force by 4.
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.
Write from a force of 4 N
- Two charges repel with 36 N when they are a distance r apart.
- They are moved to 3r. Find the new force.
Two point charges repel with 36 N at separation r. They are moved to separation 3r, and the charges stay the same. What is the new force, in newtons?
A triple separation divides the force by 9, and 36 divided by 9 is 4.
What is the force at separation 3r?
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Tripling the separation divides an inverse-square force by 9. Other options — 36 N (the original force); 324 N (the force multiplied by 9).