Definition: The integral around a closed path of
the component of the magnetic field tangent to
the direction of the path equals µ0 times the
current intercepted by the area within the path
Or, in a simplified scalar form:
Thus the line integral (circulation) of the magnetic
field around some arbitrary closed curve is
proportional to the total current enclosed by that
curve
exercise: If we know that an electric current of 12
A flows through an empty 5 cm solenoid and the
magnetic field created inside it is 0.1 T. How many
turns is the solenoid made up of?
To determine the number of turns,
it suffices to apply the formula of the
magnetic field generated inside a
solenoid and replace the values that
we know:
In order to apply Ampère’s Law all
currents have to be steady (i.e. do not
change with time)
Only currents crossing the area inside
the path are taken into account and have
some contribution to the magnetic field
Currents have to be taken with their
algebraic signs (those going “out” of the
surface are positive, those going “in” are
negative)- use right hand’s rule to
determine directions and signs
The total magnetic circulation is zero
only in the following cases: -the
enclosed net current is zero -the
magnetic field is normal to the selected
path at any point -the magnetic field is
zero
Ampère’s Law can be useful when
calculating magnetic fields of current
distributions with a high degree of
symmetry (similar to symmetrical
charge distributions in the case of
Gauss’ Law)
Applications of the Ampere Law
Magnetic Field of a Solenóide
Magnetic Field of a Toróide
The toróide is a very useful device
that is used in many areas, from tape
heads to tokamaks.