Chapter 27
Introduction to circuits
Phys 272
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Q: When does current flow through a wire?
A: When there is a voltage difference between the ends
+
-
i
Electric field inside the wire -> force that drives the motion of
charges
To create voltage difference between ends, make a circuit with a
battery
SLIDE FROM MONDAY MARCH 7
Charging the capacitor
Connect the capacitor to a battery
A BATTERY is a device that maintains a certain
potential difference across its terminals
Circuit diagram: “wire” + devices; constant potential
on “wire” between devices
E
+
+
-
Ideal battery maintains specified voltage E btwn + and – terminal
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E
+
+
i
-
Ideal battery maintains specified voltage E btwn + and – terminal
Same voltage difference between the ends of the resistor
Current i flows from + to – voltage
i = E /R
Assume Ohm’s law: i doubles if E doubles
I decreases by factor of 2 if R doubles.
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E
+
+
i
-
Ideal battery maintains specified voltage E btwn + and – terminal
charges arrive at small terminal at rate given by current i
battery does work q E on charges to place them at + terminal
Rate of energy delivery by battery = i E
charges flow through the resistor and lose energy q E
Rate of energy loss = i E = E2/R
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Mechanical analogy
Parent lifts toddlers from ground to top of slide
Toddlers slide down slide (with friction) & stop at bottom
Toddler = charge
Parent = battery
Slide = resistor
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Resistors that obey Ohm’s law V=iR
V -> I = V/R
I -> V = iR, decreasing in direction of
current
Bulbs obey Ohm’s law
brightness = power = IV = V2/R
Bulbs usually labeled by power (100W)
not resistance
Any conducting object in a circuit will be a resistor
e.g. human body
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i-clicker:
Only difference between bubs A & B is
that B’s filament is thicker. Each is in a simple
circuit with a 120 V battery.
A)
B)
C)
D)
E)
B will be brighter because it has higher resistance.
B will be dimmer because it has higher resistance.
B will be brighter because it has lower resistance
B will be dimmer because it has lower resistance
Equal brightness.
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i-clicker:
Only difference between bubs A & B is
that B’s filament is thicker. Each is in a simple
circuit with a 120 V battery.
A)
B)
C)
D)
E)
B will be brighter because it has higher resistance.
B will be dimmer because it has higher resistance.
B will be brighter because it has lower resistance
B will be dimmer because it has lower resistance
Equal brightness.
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Resistors in series
CURRENT IS THE SAME
V = V1 + V2 = i R1 + i R2 = i (R1 + R2)
V/i = R1+R2
Combination behaves like a resistor of resistance R1+R2
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Clicker
What happens to the brightness of a bulb in a simple circuit with an
ideal battery if a second identical bulb is added in series?
(A)  Gets brighter
(B)  Stays the same
(C)  Gets dimmer
(D)  Goes out
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Analyzing circuits, part 1
Voltage change across each circuit element:
Battery: +E from small plate (-) to large plate (+)
Resistor: decreases by iR in direction of current arrow
LOOP RULE: total voltage change around loop is ZERO
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If the potential at point P is 100 V, what is the potential at point Q?
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Energy delivered / lost by each circuit element
50 V battery is charging
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A real battery
The voltage difference between the
terminals decreases as the current
through the battery increases
Behaves like an ideal battery E in
series with internal resistor r
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Analyze a circuit with a real battery
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Analyze a circuit with a real battery
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