1. Moving conductor – eddy currents
A increases  F increases  Induced current creates Bint opposite to external Bext
I down
Now we have a current in magnetic field  there is a force acting on it
The direction of this force is opposite to v
Conservatism of nature
Currents created in conductors moving through the magnetic field – eddy currents – work to resist the change
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2. Electric generator –counter torque
Loop is rotating cw
Induced currents experience force in magnetic field  resultant torque on the loop ccw – counter torque
Nature resists change.
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3. Electric motor – counter emf
Current loop in magnetic field
Magnetic field creates a torque that rotates the loop
Changing flux  emf
Based on conservatism – this emf will try to create a current in the opposite direction to the original current - counter emf.
Current is large at the beginning and is decreased later on.
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4. AC circuits
Physics 122
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5. Math review Integrals and derivatives of trig. functions:
Relations between trig functions:
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6. Self inductance Magnetic field in a solenoid
It creates a magnetic flux through itself
Self inductance of a solenoid
In general:
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7. Direction of induced emf
In accordance to Lenz law
I – increase F increase  induced magnetic field in the opposite direction to initial magnetic field  emf in the opposite direction to original emf
I – decrease F decrease  induced magnetic field in the same direction to initial magnetic field  emf in the same direction to original emf
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8. Current and voltage in AC circuit
Drop of voltage over resistor (V) follows I
Current and voltage in phase
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9. Current and voltage in AC circuit
I goes up – V>0 – loose voltage
I goes down – V<0 – gain voltage
Inductor: current lags voltage
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10. Current and voltage in AC circuit
t=0, current flows to capacitor
 gain charge  gain voltage
Current changes sign
 drain charge  loose voltage
Capacitor: voltage lags current
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11. Energy in AC circuit
Energy of the magnetic field stored in an inductor
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12. Energy in AC circuit
Energy of the electric field can be stored in a capacitor
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13. LC circuit Two forms of energy:
Electric – in a capacitor
Magnetic - in solenoid
Analogy with a mass on a spring, two forms of energy
Kinetic
Potential
Oscillator!
No energy is lost, it is just changing its form
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14. Energy in AC circuit Power dissipated: P=I2R=RI20cos2wt
Energy dissipated
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15. LCR circuit Resistors dissipate energy
Convert electrical energy into thermal energy
Resistor acts like friction for a weight on a spring
Damped oscillator!
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