14.1. Magnetic Fields http://www.ck12.org
14.1 Magnetic Fields
- Describe magnetic field lines and how they behave in the situation of permanent magnets and current carrying
wires. - Calculate the magnetic field at an arbitrary distance from the wire.
Students will learn the idea of magnetic field lines, how they behave in the situation of permanent magnets and
current carrying wires and also how to calculate the magnetic field at an arbitrary distance from the wire. Magnetic
fields are usually denoted by the letter B and are measured in Teslas, in honor of the Serbian physicist Nikola Tesla.
Key Equations
Bwire=μ 0 I
2 πr
Magnetic field at a distancerfrom a current-carrying wire
Whereμ 0 = 4 π× 10 −^7 Tm/A Permeability of Vacuum (approximately same for air also)Guidance
Permanent magnets (like refrigerator magnets) consist of atoms, such as iron, for which the magnetic moments
(roughly electron spin) of the electrons are “lined up” all across the atom. This means that their magnetic fields add
up, rather than canceling each other out. The net effect is noticeable because so many atoms have lined up. The
magnetic field of such a magnet always points from the north pole to the south. The magnetic field of a bar magnet,
for example, is illustrated below:
If we were to cut the magnet above in half, it would still have north and south poles; the resulting magnetic field
would be qualitatively the same as the one above (but weaker).
Charged particles in motion also generate magnetic fields. The most frequently used example is a current carrying
wire, since current is literally moving charged particles. The magnitude of a field generated by a wire depends on
distance to the wire and strength of the current(I)(see ’Key Equations’ section) :
Meanwhile, its direction can be found using the so calledfirst right hand rule: point your thumb in the direction
of the current. Then, curl your fingers around the wire. The direction your fingers will point in the same direction
as the field. Be sure to use your right hand!