Electric Fields
Introduced| English | Meaning |
|---|---|
| electric field/ɪˈlektrɪk fiːld/ | A region in which an electric charge experiences an electric force. |
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.
Map the field
Change the charge's sign and size and watch the field lines flip and thicken.
A $2\ \text{C}$ charge sits in a field of $5\ \tfrac{\text{N}}{\text{C}}$. What force does it feel, in $\text{N}$?
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$F = qE = 2 \times 5 = 10\ \text{N}$.
The electric field at a point is defined as:
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$E = F/q$ — the force a unit positive charge would feel.
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.
Electric field lines point:
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Lines show where a positive test charge is pushed: away from + and toward −.
Where the field lines are closer together, the field is ____.
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Denser lines mean a stronger field.
Select all true statements about electric field lines.
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Lines run + → − and crowd where the field is strong. They never cross.
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.
An electric field exists around a charge even when no test charge is present.
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The source charge sets up the field; a test charge only reveals it.
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 $E = kq/r^2$.
Write from a force of 12 N
- A 4 C charge sits in a field of 3 N per C.
- Find the force on the charge.
A charge of 4 C sits in an electric field of 3 N per C. What force does it feel, in newtons?
4 times 3 is 12.
What force does the 4 C charge feel?
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Charge times field is 4 times 3. Other options — 10 N (the lesson force); 3 N (the field strength used as a force).