Electric Circuits
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Transcript of Electric Circuits
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Electric CircuitsChapter 18
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18.1 Schematic Diagrams and Circuits
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What do you think?
• Scientists often use symbols to represent electrical components, such as batteries, bulbs, and wires. On the next slide, you will see the symbols for eight common electrical components that you have seen and discussed previously. • Predict the component shown by looking at
each symbol. • Briefly explain why you think each symbol
represents that particular electrical component.
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What do you think?
1
2
3
4
5
6
7
8
+Schematic Diagrams
Schematic diagrams use symbols to represent components.
They show how the parts in an electrical device are arranged.
+Electric Circuits
A path through which charges flowCan have one or more complete paths
LoadAn element or group of elements in a
circuit that dissipates energyEx: A simple circuit consists of a source of
potential differences and electrical energy, such as a battery, and a load, such as a bulb or a group of bulbs
+Electric Circuits
An electric circuit is a set of components providing a complete, closed-loop path for the movement of electrons. Called a closed circuit
If the path is broken, the electrons do not flow. Called an open circuit
+Inside a Light bulb
A complete conducting path is established inside the light bulb. The tip of the bulb (a) is
connected to one side of the filament (see the black line).
The threads on the side of the bulb (c) are connected to the other side of the filament (see the white line).
+Short Circuits
A short circuit bypasses the light bulb or other load. It is a closed circuit.Electrons flow directly from - to + without
passing through the bulb.The current is large and the wire becomes
hot.Short circuits in homes can cause fires.
Fuses or circuit breakers are designed to turn off the electron flow if short circuits occur.
+Potential Difference in Circuits
A device that increases the PE of the electrons, such as a battery, is a source of emf (electromotive force).Not really a force, but a PE difference
Energy is conserved in electric circuits.The potential difference (DV) for the battery
equals the energy converted into heat as the electrons move through the bulb.Electrons gain energy (battery) and lose
energy (bulb) as they make a complete trip.
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Now what do you think?
• Draw schematic diagrams showing each of the following circuits:• An open circuit including a battery, open
switch, and bulb• A closed circuit including a battery, closed
switch, and resistor• A short circuit including a battery, bulb,
and closed switch
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18.2 Resistors in Series or Parallel
+What do you think?
• Figure (a) shows a single bulb and battery as seen before. Figures (b) and (c) each show two bulbs connected to the battery. The batteries and bulbs are all identical. Answer the three questions on the next slide and explain your reasoning.
+What do you think?
• How will the brightness of (b) and (c) compare to each other and how does each compare to (a)? Explain.
• How will the brightness of (d) and (e) compare to each other and how does each compare to (a)? Explain.
• Compare the total current leaving the battery in each of the three circuits. Explain.
+Resistors in Series
Series describes components of a circuit that provide a single path for the current. The same electrons must pass through both
light bulbs so the current in each is the same.
+Resistors in Series
Light bulb filaments are resistorsWhen many resistors are connected in
series, the current in each resistor are the same
+Resistors in Series
Vbattery= V1 + V2
Conservation of energy
Vbattery= IR1 + IR2
Ohm’s law
Vbattery= I(R1 + R2)
Vbattery= IRequivalent
Requivalent = R1 + R2
+Equivalent Resistance
Solving problems with series resistors: Find the equivalent resistance. Use Req with Ohm’s law to find V or I.
Use I and R1, R2, etc. to find V1, V2, etc.
+Equivalent Resistance
The potential difference across the batter, V, must equal the potential difference across the load.
+Classroom Practice Problems
A 6.00 V lantern battery is connected to each of the following bulb combinations. Find the equivalent resistance and current in each circuit.One bulb with a resistance of 7.50 Two bulbs in series, each with a
resistance of 7.50 Four bulbs in series, each with a
resistance of 7.50
+Classroom Practice Problems
1. Start by drawing a picture
2. Take your inventory
3. Decide which equations to use
4. Solve
Answers: 7.5 , 0.800 A15 , 0.400 A30 , 0.200 A
+Resistors in Parallel
Parallel describes components providing separate conducting paths with common connecting points. The potential difference is the same for
parallel components. Electrons lose the same amount of energy
with either path.
+Resistors in Parallel
Ibattery = I1 + I2 Conservation of charge
Ohm’s law
Vbattery= V1 = V2 Potential energy loss is the same across all parallel resistors.
Because Vbattery= V1 = V2, the equation above reduces as follows:
1 2
11 1
eqR R R
1 2
1 2
battery
eq
V V V
R R R
1 2eq
V VV
VR VR VR
+Equivalent Resistance
Solving problems with parallel resistors: Find the equivalent resistance. Use Req with Ohm’s law to find V or Itotal.
Use V to find I1, I2, etc.
+Equivalent Resistance
The sum of currents in parallel resistors equals the total current
The Req for a parallel arrangement of resistors must always be the smallest resistance in the group of resistors
+Classroom Practice Problems
A 9.0V battery is connected to 4 resistors as shown below. Find the equivalent resistance for the circuit and the total current in the circuit.
24
57
9V
+Classroom Practice Problems
Given:
V= 9V R1= 2 R2= 4
R3= 5 R4= 7 Req= ?? I= ??
Equations:
V=IReqe 1 2 3
1 1 1 1....
R q R R R
+Classroom Practice Problems
1 1 1 1 1
2 4 5 7eqR
0.5 0.25 0.2 0.14 1.09
1 1 1 1 1
1 10.915
1.09eqR
99.84
0.915eq
V VI A
R
+Classroom Practice Problems
Find the equivalent resistance, the total current drawn by the circuit, and the current in each resistor for a 9.00 V battery connected to:One 30.0 resistorThree 30.0 resistors connected in
parallel
Answers:30.0 , 0.300 A, 0.300 A10.0 , 0.900 A, 0.300 A
+Summary
+Wiring Lights
The series circuit shows a bulb burned out. What will happen to the other bulbs? Would this also happen in the parallel
circuit?
Assuming the bulbs are identical: Which circuit will draw more current? In which circuit are the bulbs brighter?
+Now what do you think?
• How will the brightness of (b) and (c) compare to each other and how does each compare to (a)? Explain.
• How will the brightness of (d) and (e) compare to each other and how does each compare to (a)? Explain.
• Compare the total current leaving the battery in each of the three circuits. Explain.
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18.3 Complex Resistor Combinations
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What do you think?
• Household circuits typically have many outlets and permanent fixtures such as hanging light fixtures on each circuit. • Are these wired in series or in parallel?• Why do you believe one of these methods
has an advantage over the other method?• What disadvantages would the other
method of wiring have for household circuits?
+Complex Resistor Calculations
For complex resistors, you need to follow a few steps to be successful:1. Combine series in parallel2. Combine parallel sets3. Combine series set 4. Finish problem from there
+Complex Resistor Calculations
To find the equivalent resistance for the circuit shown above, follow the steps shown to the right:
1. 6+2= 8
2.
3.
4. 3 + 9 + 2.70 + 1= 12.7
1 10.12 0.25 0.37
8 4
12.70
0.37
+Complex Resistor Calculations
To find the equivalent resistance for the circuit shown above, follow the steps shown to the right:
Req for 6.0 and 2.0 Answer: 8.0
Req for 8.0 and 4.0 Answer: 2.7
Req for 3.0 and 6.0 and 2.7 and 1.0 Answer: 12.7
So, the resistance of all 6 resistors is equivalent to a single 12.7 resistor.
+Complex Resistor Calculations
For the 2.0 resistor, find the current and the potential difference. To solve this problem, use
the step-by-step approach shown.
Find the total current in the equivalent circuit. Answer: 0.71 A This is the current through
the 1.0 , 6.0 (on the left), and 3.0 loads
Find the total potential drop across the parallel combination of three resistors. Answer: 1.9 V Continued on the next slide
+Complex Resistor Calculations
Find the current through the combined 6.0 and 2.0 resistor. Answer: 0.24 A
Find the potential difference across the 2.0 resistor. Answer: 0.48 V
+Classroom Practice Problems
For the circuit shown, find the: Equivalent resistance Current through the 3.0 resistor Potential difference across the 6.0 resistor
Answers: 6.6 , 1.8 A, 6.5 V
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Now what do you think?
• Household circuits typically have many outlets and permanent fixtures such as hanging light fixtures on each circuit. • Are these wired in series or in parallel?• Why do you believe one of these methods
has an advantage over the other method?• What disadvantages would the other
method of wiring have for household circuits?