What is a Magnetic Field?
A magnetic field is the region around a magnet or current-carrying wire where a magnetic force can be detected. It's created by moving electric charges and exerts force on other moving charges or magnets.
A magnetic field, B, describes the strength and direction of magnetic influence at a point, measured in tesla (T). Around a long straight wire, B = μ₀I / (2πr).
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Step-by-step worked examples
A long straight wire carries 5 A. Find the magnetic field at 0.02 m from the wire.
B = μ₀I / (2πr) B = (2×10⁻⁷ × 5) / 0.02 B = 0.00005 T = 50 µT
A wire carries 10 A. Find the magnetic field 0.1 m away.
B = (2×10⁻⁷ × 10) / 0.1 B = 0.00002 T = 20 µT
A wire carries 15 A. Find the magnetic field 0.03 m away.
B = (2×10⁻⁷ × 15) / 0.03 B = 0.0001 T = 100 µT
Flashcards
Quick quiz
Q1.What is the unit of magnetic field strength?
Q2.As distance from a current-carrying wire doubles, the magnetic field…
Q3.What determines the direction of a magnetic field around a wire?
Q4.What produces a magnetic field?
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Common mistakes
Believing any electric charge creates a magnetic field. — Correct: Only moving charges (currents) and magnets create magnetic fields; static charges alone don't.
Assuming magnetic field increases with distance. — Correct: It decreases with distance — B ∝ 1/r for a straight wire.
Confusing magnetic field (B) with magnetic force. — Correct: B describes the field itself; force also depends on the charge/current and its motion or orientation.
Forgetting the field direction is circular around a wire, not straight. — Correct: Use the right-hand rule — the field forms circles around the current.
FAQ
What is a magnetic field?
A region around a magnet or current where magnetic force can be detected, measured in tesla.
What is the magnetic field formula for a wire?
B = μ₀I / (2πr), where I is current and r is distance from the wire.
What are examples of magnetic fields?
Earth's field, a bar magnet's field, and the field around a current-carrying wire or coil.
How do you calculate the magnetic field around a wire?
Use B = μ₀I/(2πr) with current I in amps and distance r in metres from the wire.