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© Matthias Liepe, 2012
Recap Lecture 21
Today:
• Inductors and their inductance
• RL circuits
• Energy density of a magnetic field
Inductors and Inductance L:
i (increasing)
RL circuit: Rise of current
R
L
b
a
• At time t 0 move the switch to position a.
i
i i
• Current i begins to flow but the self-induced emf L in
the inductor L opposes the rise in current.
-> Current starts out at 0 at t=0 and then increases until it approaches a steady state value asymptotically.
L
i (t→)?
R
L
b
a
At time t 0 move the switch to position a.
i
i i
After a very long time what will be the magnitude of the steady state current in the circuit?
L
A. 0 B. |L|R C. R D. ( |L|)/R E. Both answers C & D above.
At time t 0 move the switch to position a.
i (increasing)
R
L
a
i
i i
L
RL circuit: Rise of current
RL circuit: Rise of current
i (decreasing)
R
L
b i
i i
RL circuit: Decay of current
• The switch has been in position a for a very long time.
• At time t 0 move the switch to position b.
• Current i begins to decrease, but the self-induced emf L
in the inductor L slows down the decease in current.
-> Current starts out at the equilibrium value, and then decays to zero over time.
i (decreasing)
R
L
b
a
i
At time t 0, switch to position b using a make-before-break switch.
L
RL circuit: Decay of current
RL circuit: Decay of current