Long answer·Hard exam skill (4/5)·No calculator
Difficulty guide
The label describes the problem, not your ability.
1/5 - Foundational
Warm-up setup; checks one basic idea.
2/5 - Routine exam skill
Normal syllabus skill; usually one main rule or interpretation.
3/5 - Standard multi-step
Exam-style problem with multiple steps or a common trap.
4/5 - Hard exam skill
High-end exam problem; combines methods carefully.
5/5 - Challenge
Stretch problem beyond normal exam pressure.
Question
Use Ampere's circuital law to obtain the magnetic field at distance from a very long straight wire carrying current . Then use the result to compare fields at and .
Total: 5 marks
(a) Choose a suitable Amperian loop and write Ampere's law. (2 marks)
(b) Derive the expression for B at distance r. (2 marks)
(c) Find the ratio . (1 mark)
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