Remember, what self-magnetic field in a DC circuit is changed at moments of actuation and de-actuation circuit, and if you change it current. This means, in these moments in the chain must occur e. d. with. induction. The appearance of e. d. with. induction in the circuit, which is caused by the change of the magnetic field, the current in this same circuit, called the phenomenon of self-induction, and the electromotive force e. d. with. self-induction.

Find out more, what happens when closing the circuit. Let there be an open circuit (rice. 23.12), consisting of a source of electrical energy B and the follower of the light bulbs M and coils with the core made of a ferromagnetic S. On closing the circuit the bulb is on with some delay. This is due to the emergence in the coil considerable e. d. with. self-induction, which according to Lenz's law prevents a rapid buildup of current in the circuit (rice. 23.13; I₀ — DC current in the circuit).

Note, what energy source, expended to overcome the counter e. d. with. self-induction, accumulates in the magnetic field of this chain, mainly inside the coil core's. (Why?) When the current in the circuit becomes constant, then the energy of the magnetic field circuit is not changed. The magnetic field energy of the circuit depends not only on the current, but from the look of chain, t. e. from its inductance L. Strong electromagnets in the magnetic energy is particularly high.

To observe the phenomenon of self-inductance by opening constitute a chain, shown in figure. 23.14. By opening this circuit the key To remains closed the circuit of the coil S and M light bulbs. Since the current in the coil begins to subside (rice. 23.15), it creates e. d. with. self-induction, which slows the decline in current. In this case, the coil for a short time becomes a source of energy, which creates the current in the bulb M. At the moment of breaking the circuit the current in the lamp drops to zero and, changing the direction, abruptly increases to this magnitude, which may be significantly more, what is the strength of the current in the lamp before opening. Therefore, the lamp at the time of opening or vividly flashing, and even burn out.

The phenomenon of self-induction creates a spark in the wrong place, where there is an open circuit. If the chain has powerful electromagnets, in which stored magnetic energy is large, the spark can turn into arc discharge and ruin the switch. For breaking such circuits at power stations are oil breakers and used other precautions.

Derive a formula to calculate e. d. with. self-induction. Since every e. d. with. induction can be found by the formula: K_ind =—D/Δt, and a=LI, we R_c=—Δ(LI)/Δt, where

K_c = — LΔI/Δt. (23.7)

E. d. with. inductance in a circuit is directly proportional to the rate of change of current in this circuit.