Download lecture 25 circuits applications

LECTURE 25
CIRCUITS APPLICATIONS
Instructor: Kazumi Tolich
Lecture 25
2
¤ Circuits
applications
Can you start a car?
3
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If the car battery (12 V) is dead, can you start a car using eight D-cell 1.5-V
“flashlight” batteries?
NO! They do not provide enough current to start a car (a several hundred
amps).
A flashlight batteries normally has an internal resistance of ~0.1 Ω.
The terminal voltage of a battery decreases as the current through it
increases since 𝑉 = ℇ − 𝐼𝑟.
The maximum current a flashlight battery can supply can be estimated by
ℇ
+.- /
setting the terminal voltage to be zero: 𝐼'() = =
= 15 A.
*
¨
0.+ 1
DO NOT TRY THIS AT HOME! If the car battery is not completely flat, it
could send a large current through the flashlight batteries, causing them to
explode!
Example: 1
4
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The equivalent resistance between points A and B
of the resistors shown is Req = 26 Ω. An ideal
battery with a terminal voltage of V = 12 V is
connected to terminals A and B.
a)
Find the value of resistance R3.
b)
Find the current in each resistor.
c)
Find the power dissipated in each of the
resistors.
d)
If R3 is increased, state whether the current
through each resistor increases or decreases.
R1 = 12 Ω
R2 = 55 Ω
R3
Clicker question: 1 through 3
5
Example: 2
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Three different circuits, each containing a switch and
two capacitors, initially charged as shown. The
switches are then closed, allowing charge to move
freely between the capacitors. Rank the circuits in
order of increasing final charge of the left plate of
a)
the upper capacitor and
b)
the lower capacitor.
Example: 3
7
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A flash unit for a camera has a
capacitance of C = 1500 µF.
What resistance is needed in
this RC circuit if the flash is to
charge to 90 % of its full
charge in t = 21 s?
Demo: 1
8
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Relaxation oscillator (RC circuit)
¤ A
capacitor, resistor, and 90V battery are hooked in series with a neon bulb
in parallel with the capacitor.
¤ The capacitor charges to about 80V (the breakdown voltage of the neon
bulb), then discharges through the bulb and begins the cycle again.
Pacemakers
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When heart’s natural pacemaker fails, you may need an electronic pacemaker, which produces a
regular voltage pulse that starts and controls the frequency of the heartbeat.
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The electrodes are implanted in or near the heart.
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The circuit contains a capacitor and a resistor.
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The charge and the capacitor increases to a certain value, then discharges. Then it starts charging
again.
The pulsing rate depends on the time constant of the circuit.