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AP Physics III.D
Electric Circuits
20.1 EMF and Current
The shortcomings of
electrostatics
The amazing battery
Current
Ex. The amount of charge that passes through the filament of a
certain light bulb is 1.67 C in 2.00 s. Find the a) current and b)
the number of electrons that pass through the filament in 1.00 s.
Conventional current
• Hypothetical flow of positive charge
• Consistent with positive test charge and
electric fields
• Always goes from higher potential to lower
potential
20.2 Ohm’s Law
An analogy to water
Ohm’s (Georg – a former high
school teacher we might note)
Law – the ratio of potential
difference to current
20.3 Resistance and Resistivity
Comparison to water again.
Resistivity is a property of a given
material. Resistance depends on
resistivity and geometry.
Ex. Calculate the resistance per unit length of nichrome wire with
radius of 0.321 mm. If a potential difference of 10.0 V is maintained
across 1.00 m of a length of nichrome wire, what is the current
in the wire?
p. 619: 1-3, 6-7, 9
2. a) 0.036 C b) 2.2 EE 17
6. 1.3 EE 6 J
9. b) recall Q = mcΔT
20.4 Electric Power
A couple of derivations
Ex. An electric heater is operated by applying a potential difference
of 50.0 V to nichrome wire for a total resistance of 8.00 Ohms. Find
the current in the wire and the power dissipated by the heater.
How much does it cost to run the heater for 24 hours if the cost
per kwh is $0.12?
20.6 Series Circuits
Series circuit – all devices are
connected in such a way that there is
the same current through each device.
In series wiring the potential
difference is divided (potential drop)
among the resistors.
Equivalent resistance for a series
circuit (note the largest resistor has
the greatest effect on the equivalent
resistance for a series circuit)
Note: the power in a series circuit
can be found from the sum of the
powers delivered to the individual
resistors, or the power delivered to
the equivalent resistance.
Ex. (this one is in the book) For the circuit shown find the a) current b) power dissipated by each resistor c) the total power delivered to the resistors by the battery.
p. 620: 21-24, 39-46
22. a) 4.4 Ohms b) 2.7 A
24. $5.0 million per day
40. 9.0 V
42. 4.0 EE 1 Ohms
44. a) 1.6 A
b) 14 V, 8.0 V, 1.6 V
c) 22 W, 14 W, 2.6 W
46. 140 W
20.7 Parallel Circuits
Parallel circuit – a circuit wired in
such a way that the same voltage
is applied across each resistor.
Pondering the equivalent
resistance of a parallel circuit.
So show me the formula.
For parallel circuits, the smallest
resistor has the greatest effect on
the equivalent resistance. This
explains a short circuit.
Ex. For the circuit shown find the a) current in each resistor and
b) the total power dissipated by the three resistors.
Ex. Three resistors with resistances of 20.0 Ohms, 40.0 Ohms
and an unknown resistance are connected in parallel to a
battery with a potential difference of 24.0 V. The current in
the battery is 3.00 A. Find a) the equivalent resistance b) the
unknown resistance and c) the current in the unknown
resistor.
Ex. For the circuit shown find a) the potential difference
across the battery b) the current in the 12 Ohm resistor c) the
current in the unknown resistor d) the equivalent resistance
and e) the power dissipated by each resistor.
Summary of Series and Parallel
Circuits
• Current
• Resistance
• Potential difference
p. 621: 49-56
50. 290 Ohms, 140 Ohms
52. R/9
54. a) yours
b) 7.06 A
56. 5.24 Ohms, 0.76 Ohms
c) 2350 W
20.8 Combination Circuits
Ex. What is the current in the 5.00 EE 2 Ohm resistor for the
circuit shown?
Brightness (an intelligence component greatly
lacking in the students in this class)
80B2, 82B4, Rev. 05B6
80B2
a) Use VD and I in
device to find n
b) Use total I, 12 V and
Rp
c) 48 W
82B4
a) You draw
b) 600 Ohms
c) 27 J
05B6
a) Use ideal gas law, V
of gas, solve for H
b) You draw
c) Use slope of graph
and Part a to find n
20.9 Internal Resistance
The internal resistance r is
connected in series with the
external resistance R. r causes the
potential difference between the
terminals to drop below the
maximum EMF. This actual
voltage is the terminal voltage.
Ex. For the circuit shown find the a) current drawn from the
battery b) the terminal voltage of the battery and c) the current
in the 6.0 Ohm resistor.
Capacitors in Circuits 20.9
Capacitors in Parallel
Stored energy for capacitors in
parallel
The net effect for capacitors in
parallel is an increase in capacitance
(Why?)
Capacitors in Series
Summary for capacitors in circuits
• Parallel – potential difference is the same
across each capacitor, the charge on each
capacitor may differ
• Series – the charge on each capacitor is the
same, but there is a voltage drop across each
capacitor
87B4, 88B3, 90B3
• 87B4 (Easy!)
a) 12 Ohms
b) 4.8 V
c) 1.2 V
d) 0.36 W
e) 24 C
88B3
a) 8 A
b) 2 A
c) 20 V
d) 60 micro C
e) 6.0 EE -4 J
90B3
a) 6 Ohms b) 2 A
c) 22 V
d) 12 W
e) Yours f) 44 W
91B4, 97B4.a-b, 02B3
91B4
a) 4.5 V
b) 0.5 Ohms
c) 1.0 Ohms
d) 9A
e) You draw
97B4.a-b
a) i. 6.4 W ii. 58 W
b) You draw
02B3
a) i. 0.25 A ii.0.33 A
b) i. yours, 0.14 A
ii. Yours, yours too
c) Ufiggeritout
d) i. 70 W ii. 17 W