Transcript Ex. For the circuit shown find the a) current in each

AP Physics III.C
Electric Circuits
III.C.1 Current, Resistance,
Power
The shortcomings of
electrostatics
EMF – a gross misnomer. Simply
the potential difference of the
circuit
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
Ohm’s Law
An analogy to water
Resistance and Resistivity
Comparison to water again.
Resistivity is a property of a given
material. Resistance depends on
resistivity and geometry.
Concept check: a piece of copper wire has resistance R. In
terms of R, what is the resistance R’ of a second piece of
copper wire that is twice as long, and half the diameter of the
first?
Electric Power
A couple of derivations
Ex. An electric heater is operated by applying a potential difference
of 20.0 V to nichrome wire for a total resistance of 4.00 Ohms. Find
the current in the wire and the power dissipated by the heater.
How much does it cost to run the heater for 8 hours each day for
three days if the cost per kwh is $0.13?
III.C.2 Series, Parallel and
Combination 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
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. For the circuit below, find a) the equivalent resistance b)
the current in each resistor c) the potential difference across
each resistor d) the power dissipated by each resistor and e)
the power dissipated by the circuit.
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 potential
difference across each external resistor.
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.
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
Combination Circuits
Ex. For the circuit shown, find a) the current in the circuit b)
the potential difference across each resistor and c) the current
in each resistor.
Ammeters and voltmeters
Brightness (an intelligence component greatly
lacking in the students in this class)
For the bulbs shown: a) compare the brightness of each bulb
when the switch is closed b) and the brightness of B and C
when the switch is open. Is C more, less or have the same
brightness when the switch is open compared to when it is
closed?
Parallel Plate Capacitors
Charging a Capacitor
Capacitors in Circuits
Capacitors in Parallel
CP = C1 + C2 + C3 + . . .
So, adding capacitors in parallel
increases the equivalent
capacitance. Why?
Adding capacitors in series
reduces the equivalent
capacitance. Why?
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
RC-circuits
• Immediately after the switch of an RCcircuit is closed, the capacitor is treated as a
wire with no resistance
• After the switch has been closed a long
time, the capacitor is treated as broken wire
Ex. (Princeton Review) Determine the current through and
the voltage across each electrical device in the circuit when a)
just after the switch is closed and b) the switch has been
closed for a long period of time.
Ex. (Princeton Review) Determine the current through and
the voltage across each electrical device in the following
circuit when a) the switch has been closed and b) the switch
has been closed a long time.