recall and use F = qE for the force on a charge in an electric field
Electric Fields and Field Lines ⚡️
What is an Electric Field?
An electric field E is a region around a charge where other charges feel a force. Think of it like an invisible wind that pushes or pulls on a tiny ball (the test charge).
Field Lines ✨
Field lines are a visual aid. They show the direction a positive test charge would move:
- Lines start on positive charges and end on negative charges.
- Closer lines mean a stronger field.
- They never cross.
Force on a Charge: $F = qE$ 🔬
Here, q is the test charge (in coulombs) and E is the electric field (in newtons per coulomb).
- Identify the magnitude of the field E.
- Choose the test charge q (positive or negative).
- Multiply: $F = qE$.
- Direction: if q is positive, F points in the same direction as E; if negative, it points opposite.
| Symbol | Meaning | Units |
|---|---|---|
| $q$ | Charge of the test particle | Coulombs (C) |
| $E$ | Electric field strength | Newtons per Coulomb (N/C) |
| $F$ | Force on the charge | Newtons (N) |
Exam Tip 💡
When you see a problem asking for the force on a charge in a given field:
- Write down $F = qE$.
- Check the sign of q to determine direction.
- Remember units: N = C × N/C.
- Show your work clearly; even if the answer is correct, partial marks are awarded for correct steps.
Quick Practice Problem 🚀
A positive charge of +2 µC is placed in a uniform electric field of 5 kN/C pointing to the right. What is the magnitude and direction of the force?
Answer: $F = (2\times10^{-6}\,\text{C})(5\times10^{3}\,\text{N/C}) = 10^{-2}\,\text{N}$ to the right.
Revision
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