recap lecture 21 - cornell universityliepe/webpage/docs/ph… ·  · 2012-03-10recap lecture 21 ....

© 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