irectly proportional to 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: (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 So 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 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. REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS when the switch is open to the 2.4-A value when the switch is closed. This is a change of state, from off to on. For every electrical change of state there is a finite amount of time required to fully reach the changed-state condition. Imagine a steam-locomotive starting off. It takes considerable time to bring the railroad train to full speed. When there is an inductance in a circuit, such as in Figure A.11, the buildup is slowed still further because a counter emf is being generated in the coil (L). This is due to the dynamics of the buildup. As current starts to flow in the circuit once the switch is closed, a magnetic field is developing in the coil windings. By definition, this field generates a second field just in the windings themselves. This second magnetic field generates a voltage in the opposite direction of the voltage resulting from the primary fie