| English | 意味 |
|---|---|
| electric field/ɪˈlektrɪk fiːld/ | 電場 |
A charge feels a push with nothing touching it
- Hold a charge near another and it feels a force — across empty space, nothing in between.
- How? Each charge fills the space around it with an invisible electric field 电场.
- Any charge placed in that field feels a force from it.
- The field is the "reach" of a charge, mapped out everywhere around it.
Field is force per charge
- The electric field $E$ is the force per unit charge: $E = \dfrac{F}{q}$.
- Its units are $\tfrac{\text{N}}{\text{C}}$, and it is a vector with a direction at every point.
- Put a charge $q$ in a field $E$ and it feels a force $F = qE$.
- The field exists whether or not a test charge is there to feel it.
電場のマッピング
電荷の符号や大きさを変えると、电场線が反転したり密度が高まったりする様を見よ。
$2\ \text{C}$ の電荷が $5\ \tfrac{\text{N}}{\text{C}}$ の電場の中に置かれている。この電荷が感じる力の大きさ是多少、$\text{N}$ で答えてください。
難易度 2/5 · 140 コイン + 今日初回完了時に小ボーナス付与
$F = qE = 2 \times 5 = 10\ \text{N}$.
ある点における電場の強さは以下のように定義される:
難易度 1/5 · 100 コイン + 今日初回完了時に小ボーナス付与
$E = F/q$ — 単位正電荷が感じる力である。
Reading field lines
- Field lines show the field's direction: they point the way a positive test charge would be pushed.
- They point away from positive charges and toward negative charges.
- Where lines are closer together, the field is stronger.
- Lines never cross — the field has one direction at each point.
電場線は方向として:
難易度 1/5 · 100 コイン + 今日初回完了時に小ボーナス付与
線は正の試験電荷が押される方向を示す: + から外へ、− へ向かう。
電場線が密集している場所では、電場は ____ 。
難易度 1/5 · 100 コイン + 今日初回完了時に小ボーナス付与
線が密集しているほど、電場は強い。
電場線に関する正しい記述をすべて選択せよ。
難易度 2/5 · 140 コイン + 今日初回完了時に小ボーナス付与
線は + → − に走り、電場が強い箇所で密集する。交差することは決してない。
The field of a point charge
- A single point charge makes a field $E = \dfrac{kq}{r^2}$ pointing radially outward (if +).
- It weakens with the inverse square of distance, just like the force.
- Near the charge the field is intense; far away it fades.
- Combine several charges by adding their field vectors at each point.
試験電荷が存在しなくても、電荷の周囲には電場が存在する。
難易度 1/5 · 100 コイン + 今日初回完了時に小ボーナス付与
源電荷が電場を作り出し、試験電荷はそれを検出するだけである。
The field $E$ exists on its own, set by the source charges — it is there even with no test charge present. And field lines point the way a positive charge would move: away from +, toward −. A negative charge feels a force opposite to the field.
A charge of $q = 2\ \text{C}$ sits in a field of $E = 5\ \tfrac{\text{N}}{\text{C}}$. What force does it feel?
- $F = qE = 2 \times 5 = 10\ \text{N}$, in the direction of the field.
A negative charge of the same size would feel $10\ \text{N}$ the opposite way.
An electric field $E = F/q$ (in $\tfrac{\text{N}}{\text{C}}$) is the force per unit charge that surrounds every charge; a charge in it feels $F = qE$. Field lines point away from + and toward −, and are denser where the field is stronger. A point charge gives $E = kq/r^2$.