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I am a high school student. Recently I have read about magnetism. Each time I found magnetic field has to be perpendicular with electric field.

Is there any explanation why is it?

Dumb_kaz
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  • It doesn't have to be in general. Imagine a charged capacitor and a bar-magnet next to it. You may arrange both however it pleases you and the fields don't have to be orthogonal. – denklo Oct 07 '19 at 13:50

3 Answers3

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It isn't.

Consider, for example, an infinite horizontal sheet of positive charge with an infinite current-carrying wire, parallel to the sheet, suspended above it.

The electric field from the charged sheet points upward everywhere above it. The magnetic field curls around the wire. This means that, in the plane that contains the wire and is parallel to the charged sheet, the magnetic field is parallel (or antiparallel) to the electric field in this case.

probably_someone
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  • Sorry for my poor question statement. But actually I want to know, in your explanation, why the magnetic field curls around the wire? I know about the right hand rule which gives the direction of magnetic field. Is there any explanation other than this? – Dumb_kaz Oct 07 '19 at 15:07
  • @Dumb_kaz An infinite current-carrying wire looks the same under translation along the wire, so the magnetic field must be at the same value along any line parallel to the wire. An infinite current-carrying wire also looks the same under rotation about the wire, so the magnetic field must also look the same when it's rotated. This means that, for any circle about the wire, the radial and tangential components of the magnetic field must be the same at every point. – probably_someone Oct 07 '19 at 15:13
  • @Dumb_kaz Gauss's Law for Magnetism forbids any radial component of the magnetic field, since a magnetic field pointing radially outward from a point can only be made by a magnetic monopole, which isn't observed to exist. This means that the magnetic field must have zero radial component, which means it must be tangent to any circle around the wire. In other words, it curls about the wire. – probably_someone Oct 07 '19 at 15:17
  • @Dumb_kaz Note that this argument works for any configuration that has cylindrical symmetry, not just an infinite wire. So, for example, a collection of an arbitrary number of concentric cylindrical shells of current will also have a magnetic field that curls around their mutual center. – probably_someone Oct 07 '19 at 15:49
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Magnetic field arises when charged particles move with some velocity. They can be elctrons in a conductor, which is known as current. The direction of magnetic field is found out by curling your finger while keeping your thumb in the direction of or current.

For further reading, see How do moving charges produce magnetic fields?

Mathboi
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In the classical description of the production of an electromagnetic wave, a charged particle is oscillating back and forth. This introduces a distortion into the preexisting radial electric field. The distortion has field components that are parallel to the direction of motion. The motion of charge also generates a magnetic field which wraps around the direction of motion. Off to the side, the magnetic field is perpendicular to the distortion, and the two of them move away from the charge at a rate which is determined by how they interact with each other.

R.W. Bird
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