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Subelement L03a
Ohm's Law and Power.
Section L03a
If you have a hand-held transceiver which puts out 500 milliwatts, how many watts would this be?
• 50
• 0.02
0.5
• 5

A thousand milliwatts is one watt. Converting from milliwatts to watts: from small units to larger units, requires fewer digits, decimal point moves to the left by three positions, a thousand times less.

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What is the word used to describe the rate at which electrical energy is used?
• Resistance
Power
• Current
• Voltage

The watt is the unit used to measure the rate of energy use.

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If you have light bulbs marked 40 watts, 60 watts and 100 watts, which one will use electrical energy the fastest?
The 100 watt bulb
• They will all be the same
• The 40 watt bulb
• The 60 watt bulb

How fast does each one make the electrical utility meter on the side of your house spin ? The device with the highest wattage spins it the fastest.

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What is the basic unit of electrical power?
• The ampere
• The volt
• The ohm
The watt

Power, expressed in watts = voltage, in volts, TIMES current, in amperes. P = E * I. Watts = volts * amperes. The watt describe how fast electrical energy is used.

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Power is expressed in:
• volts
• amperes
• ohms
watts

Power, expressed in watts = voltage, in volts, TIMES current, in amperes. P = E * I. Watts = volts * amperes. The watt describe how fast electrical energy is used.

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Which of the following two quantities should be multiplied together to find power?
• Voltage and inductance
• Resistance and capacitance
Voltage and current
• Inductance and capacitance

Power, expressed in watts = voltage, in volts, TIMES current, in amperes. P = E * I. Watts = volts * amperes. The watt describe how fast electrical energy is used.

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Which two electrical units multiplied together give the unit "watts"?
• Amperes and henrys
Volts and amperes

Power, expressed in watts = voltage, in volts, TIMES current, in amperes. P = E * I. Watts = volts * amperes. The watt describe how fast electrical energy is used.

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A resistor in a circuit becomes very hot and starts to burn. This is because the resistor is dissipating too much:
• voltage
• resistance
• current
power

Power is voltage times current, P = E * I. When current flows through a resistor, a 'voltage drop' ensues. Volts times amperes become watts. Power is dissipated as heat.

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High power resistors are usually large with heavy leads. The size aids the operation of the resistor by:
• making it shock proof
allowing heat to dissipate more readily
• allowing higher voltage to be handled
• increasing the effective resistance of the resistor

Resistors are rated for resistance in ohms and safe power dissipation in watts.

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The resistor that could dissipate the most heat would be marked:
• 2 ohms
• 0.5 watt
20 watts
• 100 ohms

Resistors are rated for resistance in ohms and safe power dissipation in watts.

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If a current of 2 amperes flows through a 50-ohm resistor, what is the voltage across the resistor?
• 25 volts
100 volts
• 48 volts
• 52 volts

Ohm's Law ( I = E / R ) becomes E = R*I when solving for E. Voltage = resistance times current. Volts = ohms * amperes. 50 ohms * 2 amperes = 100 volts.

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How is the current in a DC circuit calculated when the voltage and resistance are known?
• Current equals resistance multiplied by voltage
• Current equals resistance divided by voltage
• Current equals power divided by voltage
Current equals voltage divided by resistance

Ohm's Law is I = E / R, current is voltage divided by resistance. Amperes = volts / ohms.

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How is the resistance in a DC circuit calculated when the voltage and current are known?
• Resistance equals current divided by voltage
Resistance equals voltage divided by current
• Resistance equals current multiplied by voltage
• Resistance equals power divided by voltage

Ohm's Law ( I = E / R ) becomes R = E / I when solving for R. Resistance is voltage divided by current. Ohms = volts / amperes.

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How is the voltage in a DC circuit calculated when the current and resistance are known?
Voltage equals current multiplied by resistance
• Voltage equals current divided by resistance
• Voltage equals resistance divided by current
• Voltage equals power divided by current

Ohm's Law ( I = E / R ) becomes E = R*I when solving for E. Voltage is resistance times current. Volts = ohms * amperes.

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If a 12-volt battery supplies 0.25 ampere to a circuit, what is the circuit's resistance?
• 3 ohms
• 12 ohms
• 0.25 ohm
48 ohms

Ohm's Law ( I = E / R ) becomes R = E / I when solving for R. Resistance is voltage divided by current. Ohms = volts / amperes. 12 volts / 0.25 amperes = 48 ohms.

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Calculate the value of resistance necessary to drop 100 volts with current flow of 0.8 milliamperes:
• 1250 ohms
• 1.25 kilohms
125 kilohms
• 125 ohms

Ohm's Law ( I = E / R ) becomes R = E / I when solving for R. Resistance is voltage divided by current. Ohms = volts / amperes. 100 volts / 0.0008 amperes = 125 000 ohms = 125 kilohms. [ Note that volts divided by milliamperes is kilohm ]

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The voltage required to force a current of 4.4 amperes through a resistance of 50 ohms is:
• 22.0 volts
• 0.220 volt
220 volts
• 2220 volts

Ohm's Law ( I = E / R ) becomes E = R*I when solving for E. Voltage is resistance times current. Volts = ohms * amperes. 50 ohms * 4.4 amperes = 220 volts.

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A lamp has a resistance of 30 ohms and a 6 volt battery is connected. The current flow will be:
• 2 amperes
• 0.5 ampere
• 0.005 ampere
0.2 ampere

Ohm's Law ( I = E / R ). Current is voltage divided by resistance. Amperes = volts / ohms. 6 volts / 30 ohms = 0.2 amperes.

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What voltage would be needed to supply a current of 200 milliamperes, to operate an electric lamp which has a resistance of 25 ohms?
• 175 volts
• 225 volts
5 volts
• 8 volts

Ohm's Law ( I = E / R ) becomes E = R*I when solving for E. Voltage is resistance times current. Volts = ohms * amperes. 25 ohms * 0.200 amperes = 5 volts.

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The resistance of a circuit can be found by using one of the following:
• R = I/E
• R = E/R
• R = E x I
R = E/I

Ohm's Law ( I = E / R ) becomes R = E / I when solving for R. Resistance is voltage divided by current. Ohms = volts / amperes.

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If a 3 volt battery supplies 300 milliamperes to a circuit, the circuit resistance is:
• 9 ohms
• 5 ohms
• 3 ohms
10 ohms

Ohm's Law ( I = E / R ) becomes R = E / I when solving for R. Resistance is voltage divided by current. Ohms = volts / amperes. 3 volts / 0.300 amperes = 10 ohms.

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In a parallel circuit with a voltage source and several branch resistors, how is the total current related to the current in the branch resistors?
• It equals the average of the branch current through each resistor
• It decreases as more parallel resistors are added to the circuit
• It is the sum of each resistor's voltage drop multiplied by the total number of resistors
It equals the sum of the branch current through each resistor

Each resistor added in parallel to the source draws some current ( in accordance with Ohm's Law, I = E / R ). The total current that the source must supply becomes the SUM of all these individual currents. Just like in your house, the total current drawn from the utility company is the sum of all the devices turned-on.

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Three resistors, respectively rated at 10, 15 and 20 ohms are connected in parallel across a 6-volt battery. Which statement is true?
• The voltage drop across each resistance added together equals 6 volts
• The voltage drop across the 20 ohm resistance is greater than the voltage across the 10 ohm resistance
The current through the 10 ohms, 15 ohms and 20 ohms separate resistances, when added together, equals the total current drawn from the battery
• The current flowing through the 10 ohm resistance is less than that flowing through the 20 ohm resistance

key word: PARALLEL. In a parallel circuit, the total current is the sum of the currents. All resistors are subjected to the same voltage in a PARALLEL circuit. Ohm's Law tells us that the smaller resistor will draw more current than the others.

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Total resistance in a parallel circuit:
• depends upon the voltage drop across each branch
• could be equal to the resistance of one branch
• depends upon the applied voltage
is always less than the smallest resistance

key word: PARALLEL. In a parallel circuit, each added resistor adds to the current drawn from the source. If more and more current is drawn, the total resistance must be going down. In PARALLEL, the total resistance is less than the smallest.

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Two resistors are connected in parallel and are connected across a 40 volt battery. If each resistor is 1000 ohms, the total current is:
• 40 milliamperes
• 80 amperes
• 40 amperes
80 milliamperes

Ohm's Law ( I = E / R ). Each resistor draws this much current: 40 volts divided by 1000 ohms = 0.040 amperes = 40 milliamperes. In PARALLEL, total current is the sum of the currents. Method B: identical resistors in parallel, total resistance is value divided by number. In this case, 1000 / 2 = 500 ohms. 40 volts / 500 ohms = 0.08 amperes = 80 milliamperes.

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The total resistance of resistors connected in series is:
• equal to the lowest resistance present
greater than the resistance of any one resistor
• less than the resistance of any one resistor
• equal to the highest resistance present

key word: SERIES. In a series circuit, there is only one current. This current must wrestle with each resistor one after the other. In SERIES, total resistance is the sum of the resistances. The same current flows through all of them.

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Five 10 ohm resistors connected in series equals:
• 5 ohms
• 10 ohms
• 1 ohm
50 ohms

key word: SERIES. In SERIES, total resistance is the sum of the resistances.

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Which series combination of resistors would replace a single 120 ohm resistor?
• Six 22 ohm
• Two 62 ohm
• Five 100 ohm
Five 24 ohm

key word: SERIES. In SERIES, total resistance is the sum of the resistances. Five times twenty-four = 120.

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If ten resistors of equal value were wired in parallel, the total resistance would be:
• 10 + R
R / 10
• 10 / R
• 10 x R

key word: PARALLEL. In a parallel circuit with IDENTICAL resistors, total resistance is value divided by number. In this example, the value of one R divided by 10.

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The total resistance of four 68 ohm resistors wired in parallel is:
17 ohms
• 12 ohms
• 34 ohms
• 272 ohms

key word: PARALLEL. In a parallel circuit with IDENTICAL resistors, total resistance is value divided by number. In this example, 68 / 4 yields 17.

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Two resistors are in parallel. Resistor A carries twice the current of resistor B, which means that:
• the voltage across A is twice that across B
• B has half the resistance of A
A has half the resistance of B
• the voltage across B is twice that across A

key word: PARALLEL. All resistors in a parallel circuit are subjected to the same voltage. Per Ohm's Law ( I = E / R, current = voltage divided by resistance ), if resistor A draws twice the current of resistor B, it must have half the resistance of Resistor B.

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The total current in a parallel circuit is equal to the:
• source voltage divided by the sum of the resistive elements
• current in any one of the parallel branches
sum of the currents through all the parallel branches
• source voltage divided by the value of one of the resistive elements

key word: PARALLEL. In a parallel circuit, the total current is the sum of the currents. Each branch is subjected to the same voltage and draws a current in accordance with Ohm's Law ( I = E / R, current = voltage divided by resistance ).

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Why would a large size resistor be used instead of a smaller one of the same resistance?
• For a higher current gain
• For less impedance in the circuit
For greater power dissipation
• For better response time

Remember that power is voltage times current, P = E * I. A resistor dissipates power into heat. A resistor can only dissipate so much power without burning up: i.e., its power rating. Larger resistors can dissipate more heat.

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How many watts of electrical power are used by a 12 volt DC light bulb that draws 0.2 ampere?
• 60 watts
• 24 watts
• 6 watts
2.4 watts

P = E * I, power is voltage times current, watts = volts * amperes. 12 volts * 0.2 amperes = 2.4 watts [ VDC = volts in a Direct Current circuit ]

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The DC input power of a transmitter operating at 12 volts and drawing 500 milliamperes would be:
• 12 watts
6 watts
• 20 watts
• 500 watts

P = E * I, power is voltage times current, watts = volts * amperes. 12 volts * 0.5 amperes = 6 watts.

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When two 500 ohm 1 watt resistors are connected in series, the maximum total power that can be dissipated by the resistors is:
• 1 watt
• 1/2 watt
• 4 watts
2 watts

This is about POWER RATING, not resistance. Two identical resistors can safely dissipate TWICE as much power as only one. [ Yes, total resistance will be twice as much, but that is immaterial here ]

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When two 500 ohm 1 watt resistors are connected in parallel, they can dissipate a maximum total power of:
• 1 watt
• 4 watts
2 watts
• 1/2 watt

This is about POWER RATING, not resistance. Two identical resistors can safely dissipate TWICE as much power as only one. [ Yes, total resistance will be half, but that is immaterial here ]

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If the voltage applied to two resistors in series is doubled, how much will the total power change?
Increase four times
• Decrease to half
• Double
• No change

P = E * I, power is voltage times current, watts = volts * amperes. Given the proportional relation of current versus voltage stated by Ohm's Law, if you double voltage in a circuit, current will double. Power is voltage times current, if both double, power has quadrupled ( 4 times more ).

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Which combination of resistors could make up a 50 ohms dummy load capable of safely dissipating 5 watts?
Four 2-watt 200 ohms resistors in parallel
• Two 5-watt 100 ohms resistors in series
• Two 2-watt 25 ohms resistors in series
• Ten quarter-watt 500 ohms resistors in parallel

Four 200 ohms @ 2 watts in parallel = 50 ohms @ 8 watts. Two 25 ohms @ 2 watts in series = 50 ohms @ 4 watts. Ten 500 ohms @ 0.25 watts in parallel = 50 ohms @ 2.5 watts. Two 100 ohms @ 5 watts in series = 200 ohms @ 10 watts.

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A 12 volt light bulb is rated at a power of 30 watts. The current drawn would be:
• 12/30 amperes
30/12 amperes
• 18 amperes
• 360 amperes

The Power Law: P = E * I, power is voltage times current. Transformed to solve for I, it becomes I = P / E. In this example, I = 30 watts / 12 volts.

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If two 10 ohm resistors are connected in series with a 10 volt battery, the power consumption would be:
• 100 watts
5 watts
• 10 watts
• 20 watts

Two 10 ohm resistors in series present a total resistance of 20 ohms. Use Ohm's Law ( I = E / R ) to compute current as 10 volts divided by 20 ohms = 0.5 amperes. The Power Law: P = E * I, power is voltage times current. Power for this example becomes 10 volts times 0.5 amperes = 5 watts.

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One advantage of replacing a 50 ohm resistor with a parallel combination of two similarly rated 100 ohm resistors is that the parallel combination will have:
• lesser resistance and similar power rating
the same resistance but greater power rating
• the same resistance but lesser power rating
• greater resistance and similar power rating

This is about POWER RATING, not resistance. Two identical resistors can safely dissipate TWICE as much power as only one. Two resistors of 100 ohms in PARALLEL yield a total resistance of 50 ohms ( In a parallel circuit with IDENTICAL resistors, total resistance is value divided by number ).

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Resistor wattage ratings are:
• variable in steps of one hundred