3.8: Power Flashcards
- A series circuit contains a 2 Ohm, 4 ohm resistor and a 6 ohm resistor connected
across a 24 V supply. What is the power dissipated by the 4 ohm resistor.
(a) 12 Watts
(b) 14 Watts
(c) 16 Watts
(c) 16 Watts
Power = I * V where I is current and V is the voltage dropped across the resistor
Using ohms law, total current in circuit is I = V/R, I = 24/12, I = 2A in circuit
Voltage drop across resistor , V = I*R. V = 2A * 4ohm, V = 8
Power dissipated (by heat) across resistor is I * V, 2A * 8V
Power = 16 Watts
- A parallel circuit contains a 2 Ohm, 4 ohm resistor and a 6 ohm resistor connected
across a 24 V supply. What is the power dissipated by the 4 ohm resistor.
(a) 96 W
(b) 144W.
(c) 288 W.
(b) 144W.
- A force of 40N expends 800J in moving through a distance. Calculate the distance
traveled?
(a) 20 meters.
(b) 40 meters,
(c) 80 meters.
(a) 20 meters.
Work = (distance)(force)
distance = work / force
distance = 800 / 40, distance = 20
- A body has a mass of 2Kg is supported 10 meters above the earth surface.
What potential energy does it possess?
(a) 20.6 J
(b) 105.4 J
(c) 196.2J
- A body has a mass of 2Kg is supported 10 meters above the earth surface.
(c) 196.2J
Potential Energy = h * m * g
Where h is height in metres, m is mass in kg and g is gravitational constant (9.81 ms^-2)
Potential Energy = (10m)(2 kg)(10ms^-2)
approx 200J or 196.2J (for simplicity took g as 10 instead of 9.81
- An aircraft weighing 30 Metric Tons is travelling at 200Km/h.Calculate its Kinetic
Energy.
(a) 25 MJ
(b) 46MJ
(c) 96 MJ
(Question flagged as not appropriate just remember formula for kinetic energy which is)
Kinetic Energy = (1/2)(m)(v^2)
where m is mass in kg and v is velocity in m/s
- If a current has a negative polarity when Kirchoff’s current law is applied, which of
the following, statements is true of the current?
(a) It is from a battery
(b) It is from a generator
(c) It is entering a junction
(d) It is leaving a junction
(d) It is leaving a junction
- Three equal resistors are connected in parallel and each resistor has a value of
300 ohms. What is the equivalent resistance of the circuit?
(a) 100Ω
(b) 150 Ω
(c) 600 Ω
(d) 900 Ω
(a) 100Ω
1/Rt = 1/300 + 1/300 + 1/300
1/Rt = 3/300
3Rt = 300
Rt = 100
- What is the current in a circuit with 15 ohms of resistance that uses 135 watts of
power?
(a) 10 A
(b) 15 A
(c) 3 A
(d) 9 A
(c) 3 A
Power = I * V, V by ohms law is V = I * R
Power = I * I * R
135 = I^2 * 15
I^2 = 9. I = 3
- A 600W landing lamp is supplied with 30V. What value has the current?
(a) 20A
(b) 18A
(c) 9A
(a) 20A
Power = I * V
600W = I * 30V
I = 600/30, I = 20A
- A 560 Watt rated electric pump is connected to a 24 Volt battery, how much
current will it draw?
(a) 13A
(b) 23A
(c) 33A
(b) 23A
Power = I * V
560 = I * 24
I = 560/24, I = 23A
- Which resistor would dissipate most power in a parallel resistance circuit?
(a) the resistor with the lowest resistance
(b) the resistor with the highest resistance
(c) there would be the same power in all resistors
(a) the resistor with the lowest resistance
- A 28VDC motor consumes 250 mA of current. What is the normal power
consumption?
(a) 28 W
(b) 7 W
(c) 0.25 W
(b) 7 W
- What is the total power used by a 15-ohm resistor with 4 amps of current?
(a) 60 W
(b) 240 W
(c) 360 W
(d) 900 W
(b) 240 W
- Three parallel connected resistors, each of 100 ohms value, are connected across
a 20V supply. What is the total power consumed from the supply?
(a) 1.2 watts
(b) 1.33 watts
(c) 12 watts
(c) 12 watts
Calculate total resistance to be 33 ohms
Power = Current * Voltage
Ohm’s Law = Current = V/R
Power = R^2 / R, 20^2 / 33, Power = 12 watts
- A gold band on a resistor indicates a tolerance of ?
(a) 10%
(b) 5%
(c) 20%
(b) 5%
- The resistance of a substance is inversely proportional to its :
(a) Cross-sectional Area
(b) Length
(c) Permitivity
(a) Cross-sectional Area
- Doubling the length of a resistive substance will cause its total resistance to:
(a) Halve
(b) Remain the same.
(c) Double.
(c) Double.
- A silver band on a resistor indicates a tolerance of?
(a) 10%
(b) 5%
(c) 20%
(a) 10%
- Doubling the cross-sectional area of a resistive substance will cause its total
resistance to:
(a) Halve
(b) Remain the same.
(c) Double.
(a) Halve
- In Ohm’s Law, increasing the voltage across a resistor will cause the resultant
current to :
(a) Remain the same.
(b) Increrase
(c) Decrease
(b) Increrase
- What is the unit of Conductance called?
(a) Ohms
(b) Farads
(c) Mhos or Siemens
(c) Mhos or Siemens
- A resistor carrying no tolerance band, indicates a tolerance of:
(a) 10%
(b) 5%
(c) 20%
(c) 20%
- A 28VDC supply is used to supply ac 7 ohm landing light bulb. How much current
will it consume?
(a) 196 Amps
(b) 7 Amps
(c) 4 Amps
(c) 4 Amps
- A 12VDC battery on a light aircraft’s Flap Motor consumes 0.2 Amperes. What is
the resistance of the motor?
(a) 60 ohms
(b) 2.4 ohms
(c) 24 ohms
(a) 60 ohms
- A water pump operating on 28VDC consumes 196 Watts. What is the Pump
Motor’s resistance?
(a) 12 ohms
(b) 4 ohms
(c) 40 ohms
(b) 4 ohms
Could be (b) or (c) but not (a) stupid q
(a) 3.3 kΩ
- Six resistors each of 6 ohms would be
(a) 36 ohms in parallel
(b) 1 ohm in series
(c) 1 ohm in parallel
(c) 1 ohm in parallel
- A 500 ohm resistor carries a 3 amp current. What is the power dissipated?
(a) 5 kW
(b) 4.5 kW
(c) 500 W
(b) 4.5 kW
- what is the power consumed by a 20 ohm resistor in series with two parallel
connected 20 ohm resistors when placed across a 30V supply unit?
(a) 10 watts
(b) 400 watts
(c) 30 watts
(c) 30 watts
- What is the PD of a circuit which has a 40 mA current and a 1 k ohm resistance?
(a) 400 V
(b) 4 V
(c) 40 V
(c) 40 V
- In a circuit containing three resistors of equal value connected in parallel, one
resistor goes open circuit. The current in the other two resistors will
(a) increase
(b) decrease
(c) remain the same
(c) remain the same
(c) B and D
- In a Wheatstone Bridge, above, R1 = 300Ω R2 = 1.5 kΩ and R4= 500Ω. If the
bridge is balanced, what is the value of R3?
(a) 900 Ω
(b) 4.5 kΩ
(c) 100 Ω
(c) 100 Ω
- A potentiometer has which of the following properties?
(a) 3 terminals
(b) preset values
(c) wire wound only
(a) 3 terminals
- A 3 ohm resistor dissipates 27 Watts. How much current flows through it?
(a) 3 A
(b) 9 A
(c) 0.15 A
(a) 3 A
- Potentiometers are used as a
(a) variable voltage source
(b) variable resistor
(c) variable current source
(a) variable voltage source
- Ohm’s law states that:
(a) e.m.f. = current divided by resistance
(b) resistance = current divided by e.m.f.
(c) current = e.m.f. divided by resistance
(c) current = e.m.f. divided by resistance
- If the resistance of a resistor which is in series with two other resistors is doubled
(a) the current in that resistance is doubled
(b) the current in that resistance is halved
(c) the volts drop across that resistor increases
(c) the volts drop across that resistor increases
- A DC circuit containing a total resistance of 100 ohms has a current flow of 250
mA. The power dissipated in the circuit is
(a) 0.4 watts
(b) 6.25 watts
(c) 5 watts
(b) 6.25 watts
- Total resistance in parallel for R1 and R2 is
(a) RT = (R1 x R2) ÷ (R1 + R2)
(b) RT = 1/R1 + 1/R2
(c) 1/RT = 1/R1 + 1/R2
(a) RT = (R1 x R2) ÷ (R1 + R2)
- A 10 ohm resistor has a 14.14 V peak drop across it. What power is dissipated?
(a) 19.99 W
(b) 10 W
(c) 1.414 W
SKIP AS QUESTIONS UNDER REVIEW - 2 POSSIBLE ANSWERS DEPENDING ON
(b) 10 W
14.14V peak drop across it indicates AC and because doesn’t and when calculating power for AC, we use the RMS value of 0.707
14.14 * 0.707 = 9.996 approx 10
Power = V^2 / R, 10^2 / 10, power 10 watts
(a) a voltage dependant resistor
(b) a variable differential resistor
(c) a visual display rectifier
(a) a voltage dependant resistor
- In a balanced Wheatstone bridge, across the centre of the bridge there is
(a) no current flow
(b) no voltage present at either end
(c) current and voltage at maximum
(a) no current flow
- What is the power dissipated in a 500 ohm resistor with a 3A flow?
(a) 1.5 kW
(b) 4.5 kW
(c) 750 kW
(b) 4.5 kW
- A potentiometer varies
i) voltage
ii) current
iii) resistance
i) voltage
- A 47 kilohm resistor has the following colour code
(a) Yellow, Violet, Orange
(b) Orange, Violet, Red
(c) Red, Orange, Yellow
(a) Yellow, Violet, Orange
- When resistors are in parallel, the total resistance is
(a) smaller than the smallest resistor
(b) the sum of all the resistors
(c) larger than the smallest resistor
(a) smaller than the smallest resistor
- An increase in operating temperature in most electrical devices carrying current
results in
(a) an increase in resistance and a decrease in current
(b) a decrease in resistance and an increase in current
(c) no effect on the resistance and current
(a) an increase in resistance and a decrease in current
- If a number of resistors are connected in parallel, the total resistance is
(a) the same as the lowest
(b) smaller than the lowest
(c) greater than the lowest
(b) smaller than the lowest
- The total current flowing in a circuit of 200 lamps in parallel, each of a resistance
of 400 ohm and connected across an input of 100 volts is
(a) 50 amps
(b) 25 amps
(c) 40 amps
(a) 50 amps
Rule to Remember:
Where resistors are connected in parallel and of the same value, the following applies:
2 resistors in parallel: 1/2 * single resistance value = R total
3 resistors in parallel: 1/3 * single resistance value = R total
=> 200 resistors in parallel: 1/200 * 400 = 2 ohm, V/A = I, 100/2 = 50amps
- In the circuit shown the 24 volt battery has an internal resistance of 1 ohm and the
ammeter indicates a current of 12 amperes. The value of the load resistance is
(a) 1 ohm
(b) 2 ohms
(c) 6 ohms
(a) 1 ohm
- The formula for resistance in series is
(a) RT = R1 + R2 + R3 …..RN
(b) RT = R1 x R2 x R3 …..RN
(c) 1/RT = 1/( R1 x R2 x R3 …..RN)
(a) RT = R1 + R2 + R3 …..RN
- What is the combined value of resistances of 4 ohm and 6 ohm connected in
parallel?
(a) 24 ohm
(b) 2.4 ohm
(c) 0.24 ohm
(b) 2.4 ohm
- If the voltage across a resistor is doubled
(a) the current is doubled
(b) the resistance is halved
(c) the current is halved
(a) the current is doubled
- A 47K resistor has the following colour code
(a) Yellow, Violet, Orange
(b) Orange, Violet, Red
(c) Red, Orange, Yellow
(a) Yellow, Violet, Orange
- A 300 ohm resistor would have a colour code of
(a) orange, orange, brown
(b) orange, brown, black
(c) orange, black brown
(c) orange, black brown
- The resistance of a conductor with a negative temperature coefficient will:
(a) increase with increased temperature
(b) decrease with increased temperature
(c) fall if the temperature is increased
(b) decrease with increased temperature
- A horizontal force of 100 Newtons is applied to a body to move it to a uniform
velocity through a distance of 40 Meters in 40 Seconds. Calculate the power
consumed?
(a) 100 watts
(b) 4,000 watts
(c) 40, 000 watts
(a) 100 watts
- What is the force required to move a mass of 10kgs to an acceleration of 50
meters per second.
(a) 50 Newtons
(b) 500 Newtons
(c) 500 watts
(b) 500 Newtons
- Three parallel resistors, each of value 100 ohms are connected across a 20 volt
supply. The total power being consumed by the circuit is?
(a) 1.2 watts
(b) 1.33 watts
(c) 12 watts
(c) 12 watts
- Which resistor would dissipate most power in a series resistance circuit?
(a) the resistor with the lowest resistance
(b) the resistor with the highest resistance
(c) there would be the same power in all resistors
(b) the resistor with the highest resistance
For this question remember power is current times voltage.
In a series circuit, the resistors have the same current but the voltage drop across each resistor is different with the highest resistance resistor having the biggest voltage drop and thus most power
In a parallel circuit, the resistors have the same voltage but different currents with the lowest resistance resistor having the greatest current flowing through it and thus most power
Question Number. 1. A 10 V battery supplies a resistive load of 10 ohms for 1 minute. What is the power supplied?.
Option A. 100 W.
Option B. 10 VA.
Option C. 10 W.
Option C. 10 W.
Question Number. 2. A resistor dissipates 80 Watts. If it runs for 4 hours 15 minutes, how much energy is dissipated in total?.
Option A. 20.4 kJ.
Option B. 1.224 MJ.
Option C. 191.25 J.
Option B. 1.224 MJ.
Question Number. 3. The earth lead of a 24 V equipment is 0.5 ohm resistance and carries 80 A. How much power does it dissipate?.
Option A. 1.92 kW.
Option B. 1.152 kW.
Option C. 3.2 kW.
Option C. 3.2 kW.
Power = I^2 * R. Since it is an earth lead it will not have the full 24V on it, so disregard voltage
Question Number. 6. Power is the rate of doing work. It is measured in.
Option A. joules/second.
Option B. joules * seconds.
Option C. watts/second.
Option A. joules/second.
Question Number. 7. The power in a circuit when voltage and resistance is given can be found by.
Option A. V*R.
Option B. VVR.
Option C. V*V / R.
Option C. V*V / R.
Power = V^2/R
Question Number. 10. A 48-volt source is required to furnish 192 watts to a parallel circuit consisting of three resistors of equal value. What is the value of each resistor?.
Option A. 4 ohm.
Option B. 36 ohm.
Option C. 12 ohm.
Option B. 36 ohm
Power = V^2 / R, R = 48^2 / 192, R = 12
Note this is Rtotal, so 1/3 * x = 12, x (single resistor) —> x = 36 ohm
OR
Each resistor dissipates 192/3 = 64 Watts. Voltage is the same across all resistors in parallel = 48 Volts
Power = V^2 / R. R will then give single resistor value 36 ohm
Question Number. 11. A piece of equipment is rated at 750 watts and the applied voltage is 30 volts. The value of the protection fuse should be.
Option A. 20 amps.
Option B. 15 amps.
Option C. 30 amps.
Option C. 30 amps.
Power = amps * Volts
Find the amperage from that (25amps). Then choose the fuse of the nearest size up
Question Number. 12. How much power must a 24-volt generator furnish to a system which contains the following series loads? 5 ohm, 3 ohm and 12 ohm.
Option A. 402 watts.
Option B. 28.8 watts.
Option C. 450 watts.
Option B. 28.8 watts.
Question Number. 13. A direct current of 12 milliamperes flows through a circuit which has a resistance of 1000 ohms. The power dissipated by the circuit is.
Option A. 144 milliwatts.
Option B. 12 watts.
Option C. 12 milliwatts.
Power = I^2 * R
= (12x10^-3)^2 * (10^3)
= (144x10^-6) *(10^3)
= 144x10^-3 or 144milliWatts
Question Number. 14. In a power circuit, the purpose of an inductor is.
Option A. to dampen voltage surges.
Option B. to dampen current surges.
Option C. to dampen power surges.
Option C. to dampen power surges.
Question Number. 16. A single phase circuit has an input voltage of 100V. The current is 10A and the circuit has a power factor of 0.5. The true power is.
Option A. 50W.
Option B. 1000W.
Option C. 500W.
Option C. 500W.
Apparent Power = I*V
= 100 * 10 –> 1000 Watts
True Power = PF * Apparent Power
= 0.5 * 1000 —> 500 Watts
Question Number. 17. Power in a DC circuit is found by.
Option A. multiplying the voltage by itself and dividing by the current.
Option B. multiplying current by resistance.
Option C. multiplying the resistance by the current squared.
Option C. multiplying the resistance by the current squared.
Question Number. 21. Which requires the most electrical power during operation?.
Option A. A 12-volt motor requiring 8 amperes.
Option B. Four 30-watt lamps in a 12-volt parallel circuit.
Option C. Two lights requiring 3 amperes each in a 24volt parallel system.
Question Number. 21. Which requires the most electrical power during operation?.
Explanation. a is 128=96W, b is 304 = 120W, c is 3242=144W.
Question Number. 23. The power expended in a given circuit is.
Option A. inversely proportional to cube-root of the current.
Option B. proportional to the square-root of the voltage.
Option C. proportional to the square of the voltage.
Option C. proportional to the square of the voltage.
