28.22 - Strength of the magnetic field around a wire
The strength of the magnetic field around a current-carrying wire can be calculated with
the equation to the right. The field strength is a function of two variables: the amount of
current and the distance from the wire. The field increases with current and decreases
with distance. The equation includes ȝ 0 , the permeability constant of free space. This
constant equals 4 ʌ×10í^7 N/A^2.
This equation is derived under the assumption that the wire is long and thin. It provides
accurate values when the distance from the wire is significantly less than the wire
length.
The direction of the magnetic field is determined using the right-hand rule for currents.
To review: The conventional current runs to the right, so the thumb points to the right
and the fingers wrap in the direction of the magnetic field.
To gain a sense of the strength of the magnetic field around a typical wire, you can look
at the example problem to the right. Calculations show that the magnetic field strength
15.0 cm away from a current of two amperes is 0.0267 G.
The field at this distance from the wire is much less than the average magnetic field
strength of the Earth, which is about 0.5 G. On the other hand, at a point 1.5 cm from
the wire, the field would be 0.267 G, or about half as strong as the Earth’s field.
Magnetic field around a wire
Strength of magnetic field
·Increases with amount of current
·Decreases with distance from wire