A.7
INDUCTANCE AND CAPACITANCE
587
Figure A.10
A magnetic field surrounding a solenoid with an air-core.
Figure A.10 shows a spool on which we can support the looped wire turns. Some will
call this spool arrangement a solenoid. If a solenoid is very long when compared with its
diameter, the field intensity in the air inside the solenoid is directly proportional to the
product of the number of turns and the current and inversely proportional to the length
of the solenoid. This can be expressed by the following relationship:
H = k(NI)/l,
(A.17)
where
l is the length of the solenoid, N is the number of turns, and I is the current.
Parameter
k adjusts the equation to the type of unit system used. When I is in amperes
and
l is in meters, then k = 1. H is defined as ampere-turns per meter. Of course, H is
the field intensity.
A.7
INDUCTANCE AND CAPACITANCE
A.7.1
What Happens When We Close a Switch on an Inductive Circuit?
Figure A.11 shows a simple circuit consisting of an air-core inductance, a battery emf
supply, and a switch. The resistance across the coil or inductance is 10
and the battery
supply is 24 V. We close the switch, and calculate the current flowing in the circuit. Using
Ohm's law:
I = E/R = 24/10 = 2.4 A.
Off hand, one would say that at the very moment the switch is closed, the 2.4 A are
developed. That is not true. It takes a finite time to build from the zero value of current
Figure A.11
A simple circuit with an inductance or coil.