An inductor is a wire coil usually wrapped around an iron core

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Presentation transcript:

An inductor is a wire coil usually wrapped around an iron core Inductors Toroidal core An inductor is a wire coil usually wrapped around an iron core

Self Inductance A coil can induce a voltage in itself !!!??

Predict what happens when the switch closes.

What happens when the switch opens?

An inductor is designed to oppose a changing current This is because it can induce an EMF in itself. This is called Self Inductance.

Direction of EMF opposes current change Current Increasing Increasing current Causes increasing magnetic field Causes increasing flux through coil Causes induced EMF Increasing current Direction of EMF opposes current change Induced EMF

Self Inductance A coil can induce a voltage in itself. L is called the self inductance Self Inductance (L) is measured in henries (H) Circuit Symbol for inductor: Considering resistance:

This induced EMF (or back EMF) opposes the increase in current, so the current rises… SLOWLY This is called Self Inductance (The coil induces an EMF in itself)

current I = V/R R is the ohmic resistance of the inductor Close switch time

current Close switch Back EMF Close switch

Imax 0.63 Imax time Close switch Meaning of time constant : The time taken for the current to increase to 63% of the maximum value.

Larger inductance produces larger back EMF with the same change rate of magnetic flux as in . It opposes the EMF of the power supply, resulting in longer time to reach maximum current. Larger resistance results in smaller maximum current. So it takes shorter time to reach 63% of the maximum current, resulting in smaller τ.

How would the graph change if: Inductor had higher inductance? Inductor had higher resistance (be careful) current Time Close switch

Direction of EMF opposes current change. Current Decreasing Decreasing current Causes decreasing magnetic field flux decreasing Causes induced EMF Direction of EMF opposes current change.

This induced EMF (or back EMF) opposes the decrease in current, so the current drops… SLOWLY This is called Self Inductance Key idea: Inductors oppose a changing current but have no effect on constant current. n.b. a real inductor has resistance

When the switch opens, the current drops to zero rapidly Causes a large flux change rate Induces a very large EMF Causes a spark across the switch

Energy stored in an inductor Current in an inductor produces a magnetic field, therefore energy is stored in the magnetic field. L: self inductance in henries (H), I: current in amps (A)

Inductor radio

Example: As shown, the coil has an inductance of 0. 20H Example: As shown, the coil has an inductance of 0.20H. When the switch S is open, the current falls from 5.0A to zero in 1.0s. a. What is the induced voltage? b. What is the resistance, R, of the coil c. What is the energy stored when S is closed? d. What is the time constant of the coil?

Ex. (a) What is the steady current flowing when the switch has been closed for a long time? (b). What is the voltage across the inductor at the instant the witch is closed? (c) find the time constant. (d)How long does it take for the current to reach a value of 0.021A?

Mutual Inductance http://phet.colorado.edu/web-pages/simulations-base.html