Worksheet: Magnetic Field due to a Long Straight Conductor Carrying a Constant Current

In this worksheet, we will practice calculating the magnitude of the magnetic field produced by a long straight current-carrying wire at a given distance from the wire.


The magnitude of the magnetic field 50 cm from a long, thin, straight wire is 8.0 µT. What is the current through the wire?


A long, straight, horizontal wire carries a left-to-right current of 20 A. If the wire is placed in a uniform magnetic field of magnitude 4.0×10 T that is directed vertically downward, what is the resultant magnitude of the magnetic field 20 cm above the wire?

  • A4.5×10 T
  • B3.3×10 T
  • C5.0×10 T
  • D6.7×10 T
  • E4.0×10 T


A typical current in a lightning bolt is 4.0×10 A. Estimate the magnetic field 5.0 m from the bolt.

  • A9.6×10 T
  • B3.2×10 T
  • C4.2×10 T
  • D1.6×10 T
  • E16×10 T


A transmission line strung 3.00 m above the ground carries a current of 335 A. What is the magnetic field on the ground directly below the wire?

  • A4.48×10 T
  • B0.369×10 T
  • C6,360×10 T
  • D1.36×10 T
  • E2.24×10 T


The figure shows two long straight horizontal wires that are parallel and a distance 2𝑎 apart, where 𝑎=30cm. If the wires carry currents 𝐼=20A and 𝐼=30A, in opposite directions, at what distance between them from the top wire is the net magnetic field zero?

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