Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

REVIEW OF PHYSICAL PRINCIPLES 35
In the cgs system, the unit for magnetic flux is called the maxwell, and the unit for
flux density is called the gauss, where
1T= 10,000 gauss.
Since joules = newtons × meters, the dimensions of μ0 may be written as
μ0 = 4π × 10 −7
J
A 2 · m .
Furthermore, since webers × amperes = joules, μ0 may also be expressed as
−7 Wb
μ0 = 4π × 10
A · m .
The magnetic flux density, symbolized by B, at a distance r from a wire carrying a
current i is given by
B = μ0 i
×
2π r
W EXAMPLE 2.15
Wb
= . (2.53)
m2 What is the magnetic flux density at a distance of 0.1 m from a wire that carries a
current of 0.25 A?
Solution
Substituting the numerical values into Eq. (2.53) yields
−7 Wb
4π × 10
B =
A · m
×
2π
0.25 A
0.1 m
−7 Wb
= 5 × 10
m2 = 5 × 10−7 T.
In comparison, the magnetic flux density at the equator is about 3 × 10 −5 T.
Any region in which there is a magnetic flux is called a magnetic field, and the
field intensity (or field strength) is directly proportional to the magnetic flux density.
Since magnetic flux has direction as well as magnitude, magnetic field strength is
a vector quantity. Michael Faraday, a Scottish experimental physicist, found in 1831
that electricity could be generated from a magnetic field and that electricity and
magnetism were related.