+ - The moment this switch is closed energizing this electromagnet time we expect the current jumps from zero to I = V/R time switch is closed
+ - However… as each coil of this electromagnet starts to pull current its contribution to the magnetic field shoves flux down the center of all the neighboring coils. each resists with an induced voltage to slow this change down... a small voltage countering the battery.
+ - Which means the current ramps up! time We reach the full current V/R only after a few moments: I = V/R time switch is closed
Current winds through the inductor as shown, creating an electromagnet with its North Pole on its 1) right end. 2) left end. 3) top surface. 4) bottom surface. + - A light bulb is run by a battery in series with an inductor.
When the switch is opened, current 1) stops abruptly. 2) gradually dies out. 3) continues flowing until the bulb cools. 4) reverses direction, returning charge to the battery. + - A light bulb is run by a battery in series with an inductor.
When plugged into an active household outlet, which of the light bulbs above will be brighter? 1) A 2) B 3) both the same
Self-Induction in Daily Life When you turn off your toaster by unplugging it current changes rapidly to zero rapidly changing B field produced very large voltage induced in toaster’s wires spark If you unplug your computer while it is still turned on, large voltages may be generated, possibly destroying the CPU chip. voltage difference large enough for electrons to flow through air
+
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Equilibrium position …antenna current zero The electric field in this region points: A. up. B. down. C. into the screen. D. out of the screen.
E field points down E field points up E field momentarily zero
E field points down E field points up E field zero strong weak zero E-field pointing down
Also, this OSCILLATION generates a B-Field! slowing to zero speed maximum speed As this positive charge moves up, the B-field in this region points INTO the screen
slowing to zero speed Also, this OSCILLATION generates a B-Field! maximum speed As this positive charge moves up, the B-field in this region points INTO the screen
B field is oscillating, continuously flipping the magnetic field in & out & in & out & in & out & · · · E field is oscillating, continuously flipping the electric field and at the same time… up & down & up & down & up & down & · · ·
c is used to stand for the “speed of light”
Before the days of cable, television sets often had two antennae on them, one straight, and one circular. One antenna picked up electric field oscillations, and the other picked up magnetic field oscillations. Which antenna picked up the magnetic oscillations? 1) the circular antenna 2) the straight antenna
Amplitude Modulation Carrier Frequency
Frequency Modulation
The loss of flux induces a current in the coil to partially replace the sudden loss. Though it soon fades, this current continues to flow in the same direction as the current from the battery had. QUESTION 1 QUESTION 2 The inductor slows down sudden changes …even the fluctuations of AC current! This means with the inductor in place, the AC current never makes it to quite as high as maximum values (before reversing direction) when with the inductor. the rms average current is also lower then, so I 2 R, the power consumed in the light-bulb is much less! QUESTION 3 QUESTION 4 QUESTION 5 Electric field lines point away from the positive charge, into the negative. At the moment pictured, the field right of the antenna points UP ( see next slide ). Set up to monitor the local magnetic fields…whenever the magnetic field passing through this circular loop CHANGES, a current is induced in it. 1) right end. 2) gradually dies out. One more example of induction easing sudden changes. 2) B A. up. 1) the circular antenna This is the type of antenna (see slide 23) your radio probably uses to pick up FM stations.