What We’ve Observed An increasing magnetic field induces a negative emf A decreasing magnetic field induces a positive emf A magnetic field that alternates.

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Presentation transcript:

What We’ve Observed An increasing magnetic field induces a negative emf A decreasing magnetic field induces a positive emf A magnetic field that alternates by increasing and decreasing causes current to move back and forth

Lenz’s Law Heinrich Lenz Image obtained from:

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field B B

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field B inc. B induced I × × × × × × I B inc.

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field B initial B induced B inc. B net B still increases, but was opposed by B from induced current Negative feedback

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field If B is decreasing, current will flow to try and increase B field B B

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field If B is decreasing, current will flow to try and increase B field B dec. B induced I I B dec.

Lenz’s Law A current induced by a changing B field opposes the change in the B field If B is increasing, current will flow to try and decrease B field If B is decreasing, current will flow to try and increase B field B initial B induced B dec. B net B still decreases, but was opposed by B from induced current Negative feedback

Lenz’s Law A helpful analogy: Inertia: mass resists changes to its velocity If velocity is = 0 m/s, wants to remain at 0 m/s If velocity is ≠ 0 m/s, wants to keep moving with that velocity B B

Lenz’s Law A helpful analogy: Inertia: mass resists changes to its velocity If velocity is = 0 m/s, wants to remain at 0 m/s If velocity is ≠ 0 m/s, wants to keep moving with that velocity B B

Lenz’s Law A helpful analogy: Inertia(?): charge resists changes to its current If velocity is = 0 m/s, wants to remain at 0 m/s If velocity is ≠ 0 m/s, wants to keep moving with that velocity B B

Lenz’s Law A helpful analogy: Inertia(?): charge resists changes to its current If current is = 0 A, wants to remain at 0 A If velocity is ≠ 0 m/s, wants to keep moving with that velocity B B

Lenz’s Law A helpful analogy: Inertia(?): charge resists changes to its current If current is = 0 A, wants to remain at 0 A If current is ≠ 0 A, wants to keep flowing with that current Lenz’s Law describes how current in wires do this B B

Inductance Suppose you have the following circuit: Inductor- resists changes in current If connected to source, keeps current from flowing for a while If disconnected from source, keeps current flowing for a while

Inductance Assume switch has just been closed Current flowing through inductor was 0 A Current now increasing through inductor Lenz’s Law: inductor opposes change by inducing a current in opposite direction of increasing current Acts like a temporary battery

Inductance Assume switch has just been closed Current flowing through inductor was 0 A Current now increasing through inductor Lenz’s Law: inductor opposes change by inducing a current in opposite direction of increasing current Acts like a temporary battery

Inductance After letting this run for a while, inductor operates like a normal wire

Inductance After letting this run for a while, inductor operates like a normal wire But what happens to a solenoid with a current flowing through it? Strong B field inside inductor B

Inductance Now suppose switch is opened B

Inductance Now suppose switch is opened Was current flowing through inductor Current now decreasing through inductor Lenz’s law: inductor opposes change by inducing a current in same direction as decreasing current Acts like a temporary battery B

Inductance Now suppose switch is opened Was current flowing through inductor Current now decreasing through inductor Lenz’s law: inductor opposes change by inducing a current in same direction as decreasing current Acts like a temporary battery B

Inductance After letting this run for a while, inductor operates like a normal wire B

Inductance After letting this run for a while, inductor operates like a normal wire Where did the current come from? Strong B field inside inductor is no longer there Hmm…

Inductance Inductors store energy in a B field When current first flows into inductor, some current gets stored in B field When current is cut off, current stored in B field released Capacitors & Inductors Capacitors store charge in E field Inductors store current in B field

Inductance

Image obtained from: Joseph Henry

Inductance Image obtained from: Joseph Henry