Download Circuitry ~ Learning Guide Name: Instructions

BCLN PHYSICS 12 - Rev. Sept/2012
Circuitry ~ Learning Guide
Name: ________________
Instructions:
Using a pencil, answer the following questions. The Pre-Reading is marked, based on effort,
completeness, and neatness (not accuracy). The rest of the assignment is marked, based on effort,
completeness, neatness, and accuracy. Every time you see a bold word, make sure you refer back
to your "Submission Requirements." Do your best!
Pre-Unit Thoughts:
This unit looks at electric circuits and how various components (resistors, batteries, switches etc)
affect the circuit. The last unit examined Electrostatics. Both units involve charges. What makes
the charges in circuits different than the charges examined in ElectroSTATICS?
Electrical Energy and Power:
1. What condition is necessary for charges to flow?
2. Why are electrons, rather than protons, the principle carriers of charge in metal wires?
3. What, exactly, is an Ampere? How does this relate to charges?
4. How much energy is supplied to each coulomb of charge that flows through a 12V battery?
5. Voltage is often referred to as “Electric Pressure”. Does voltage flow through a circuit, or is
voltage established across a circuit?
BCLN PHYSICS 12 - Rev. Sept/2012
6.
If the voltage across a circuit is held constant while the resistance doubles, what changes
occur in the circuit.
7. The current through an ammeter is 5.0 A. In one day, how many electrons will pass through
the ammeter? (ans: 2.7 x 1024 electrons)
8. What, exactly, is Power in the context of an electrical circuit? Which of the following are units
of Power and which is a unit of Energy – a watt, a kilowatt, a kilowatt-hour?
9. A 2.2 kΩ resistor is rated at ‘½ W’. What is the highest voltage you could safely apply to the
resistor without risking damage to it from overheating? (ans: 33V)
BCLN PHYSICS 12 - Rev. Sept/2012
Resistance and Resistors:
1. All the resistors in the circuits below are identical at 2Ω each. The batteries are ideal with
voltages of 6 or 12 volts as shown below. All connecting wires have negligible resistance.
Rank the current passing through the upper right hand corner of each circuit from greatest
to least.
Greatest 1_______ 2________ 3________ 4________ 5________ 6________ Least
Or, the current is the same in each situation. __________
Please carefully explain your reasoning.
BCLN PHYSICS 12 - Rev. Sept/2012
2. Will water flow more easily through a wide or a narrow pipe? What about a long or a
short pipe? How does this relate to a wire’s resistance? Write the general relationship(s)
between a wire’s resistance, R, it’s length, l, and it’s cross-sectional area, A.
3. The circuit below is a combination of series and parallel components. Identify the
components that are in series with each other, and those that are in parallel.
4. If all of the resistors above are 20Ω resistors, what is the equivalent resistance? Show all work
and draw your final simplified circuit using the equivalent resistance. (ans: 26.7Ω )
BCLN PHYSICS 12 - Rev. Sept/2012
5. If another resistor, R 7 was placed in parallel with R 6 would the overall current increase or
decrease? Since current is a flow of charges (like bumper-to-bumper traffic) explain your
answer by describing how and why this flow changes the way it does.
6. Determine the equivalent resistance in the circuit below: (ans: 128.8 Ω)
BCLN PHYSICS 12 - Rev. Sept/2012
Kirchoff’s Laws and Circuit Analysis:
1. Kirchoff’s Voltage Law is often referred as “Conservation of Energy” in a circuit. Defend
this.
2. Kirchoff’s Current Law is often referred to as “Conservation of Charge”. Defend this.
3. In a circuit of two lamps in series, if the current through one lamp is 1A, what is the current
through the other lamp? Defend your answer.
4. The voltage provided by the battery for the circuit powering the two lamps above is 6V. If
one of the lamps has a voltage of 2V, what is the voltage across the other?
5. What is an Ammeter and a Voltmeter? How are they connected into a circuit? One of these
devices is said to have an infinite resistance (huge) while the other is said to have zero
resistance. Based on how they are connected into a circuit explain why each device needs this
resistance value so as not to alter the flow in the original circuit.
BCLN PHYSICS 12 - Rev. Sept/2012
6. Shown below is a DC circuit that contains two switches. Each switch is resistanceless
when closed. All of the connecting wires should be considered to have zero resistance.
All of the resistors shown are identical. The circuit contains an ideal ammeter and an
ideal voltmeter. The diagram shows the switches open.
Below the diagram are four different switch configurations for the circuit.
Rank these configurations in terms of the voltmeter reading.
Largest 1 ______ 2 ______ 3 ______ 4 ______ Smallest
Or, all configurations produce the same voltmeter reading. __________
Or, all configurations produce a zero voltmeter reading. __________
Please carefully explain your reasoning.
BCLN PHYSICS 12 - Rev. Sept/2012
7.
For the circuit above determine:
a. the equivalent resistance of this circuit (ans: 91.25 Ω)
b. Simplifying circuits by using equivalent resistances often makes calculations of current or
voltage easier. Let’s consider the following question: What is the current through the 54 Ω
resistor? BEFORE we answer this let’s simplify the circuit.
i. Redraw the circuit so that we are only left with two resistors; the 54 Ω and the equivalent
of the remaining resistors.
ii. What type of circuit are we left with? SERIES or PARALLEL?
iii. Solve the simplified circuit: What is the current through the 54 Ω resistor? (ans: 0.132 A)
iv. How much power is dissipated in the 54 Ω resistor? (ans: 0.934 W)
BCLN PHYSICS 12 - Rev. Sept/2012
8. Consider the circuit below:
a. What is the equivalent resistance of the above circuit? Total current leaving the battery? (ans:
10 Ω,1A )
b. What current exists at A? What is the voltage across the 3Ω resistor? Why is the current the
same as the total current? (ans: 1A, 3V)
c. What is the potential difference between the ends of the 8.0 Ω resistor? What current? (ans: 6
V, 0.75 A)
d. What is the current through the 24 Ω? The voltage? (ans:0.25A, 6V)
e. Using Kirchoff’s Voltage Law and Current Law determine the voltage and current through
the remaining 1 Ω resistor. (ans: 1 V, 1 A)
BCLN PHYSICS 12 - Rev. Sept/2012
Terminal Voltage:
1. Why does the value of the terminal voltage depend on the circuit while the batteries Emf
remains constant?
2. Power in a resistor is often dissipated as heat. Why does a battery heat up more when
connected to a circuit with a low resistance?
3. Consider the circuit below: The Emf of the battery is 6.0V while the terminal voltage
(across A and B) is 4.6V.
a. What is the total current I o ? (ans: 1.38 A)
b. What is the internal resistance r? (ans: 1.01 Ω )
BCLN PHYSICS 12 - Rev. Sept/2012
Transmission Lines:
1. Typical transmission lines that provide power to cities are depicted below:
The current trough transmission lines is said to be AC as opposed to DC. Discuss the
differences between these two terms by examining how the current moves in each.
2. The goal of transmitting power over large distances is to minimize the losses in the lines
themselves without spending too much money to achieve this goal. The diameter of these
wires is huge. Copper is expensive. Why are the wires so thick?
BCLN PHYSICS 12 - Rev. Sept/2012
3. At its most basic level transmission lines can be considered as a simple circuit depicted
below:
a. What type of circuit is depicted above? Series or Parallel?
b. What remains constant in this type of circuit?
c. R wire is the resistance found in the transmission lines. We want the wires to remain as
cool as possible since heat is considered a waste of energy (a loss). Discuss why it is
better for a Power Plant to transmit 100 MW of power at a high voltage and low current
(recall: P = VI) as opposed to a high current, low voltage.
BCLN PHYSICS 12 - Rev. Sept/2012
4. For the transmission lines depicted above in question #3: The Power Plant is delivering 5 x
107 W of power to the lines using 2.5 x 105 V. Show all equations first.
a. How much current is travelling through the lines? (ans: 200A)
b. The resistance of the lines themselves is 2 Ohms. How must power is dissipated (used
up) by the lines? This is considered to be the “Power Lost”. (ans: 80 000 W)
c. Therefore, how much of the original power ends up at the city? You’ll recall that
efficiency is a ratio of output/input using either energy or power. What is the efficiency
of the lines? (ans: 4.992 x 107 W, 99.8%)
d. When determining the Power Lost by the lines one almost always uses the equation P =
I2R (as opposed to P = VI or P = V2/R). Why is this the most appropriate choice of
equation? Why do the other two not work directly? What would we nee to know in
order to effectively use the other two equations?
BCLN PHYSICS 12 - Rev. Sept/2012
5. A power station delivers 455 kW of power at an input voltage of 4.00 x 104 V to a factory
through lines whose total resistance is 5.00 Ω. If the input voltage was reduced to 1.00 x
104 V, by what factor would the power wasted in the lines be multiplied? Show ALL
work! (ans: 16x)