Electric Field-Line Reasoning
Field lines are an oriented visualization of a vector field. A charged particle follows Newton's law, so inertia generally prevents its path from coinciding with a curved field line.
Why this shows up in the exam
Inferring charge signs · Comparing charge magnitudes from line counts · Rejecting impossible line diagrams
Learn the idea
A field line is tangent to E; its density suggests strength, but it is not a particle trajectory. Lines begin on positive charge and end on negative charge or infinity. They never cross because the field has only one direction at a point.
🧠 Memory hook: Field lines guide the force now, not the whole future path.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- tangent to line || E — local direction represented by a field line
- F = qE — actual particle acceleration, whose sign depends on q
How to approach it
- 1Check start and end points
- 2Check tangency and non-crossing
- 3Separate field geometry from particle dynamics
Common slip-ups that cost marks
- •Treating a field line as a material path
- •Allowing lines to cross
- •Forgetting that negative charges accelerate opposite E
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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