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14.5 Alternating current versus direct current  (Page 4/10)

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Phet explorations: generator

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Generator

Section summary

  • Direct current (DC) is the flow of electric current in only one direction. It refers to systems where the source voltage is constant.
  • The voltage source of an alternating current (AC) system puts out V = V 0 sin 2 π ft size 12{V = V rSub { size 8{0} } "sin2"π ital "ft"} {} , where V size 12{V} {} is the voltage at time t size 12{t} {} , V 0 size 12{V rSub { size 8{0} } } {} is the peak voltage, and f size 12{f} {} is the frequency in hertz.
  • In a simple circuit, I = V/R size 12{I = ital "V/R"} {} and AC current is I = I 0 sin 2 π ft size 12{I = I rSub { size 8{0} } "sin2"π ital "ft"} {} , where I size 12{I} {} is the current at time t size 12{t} {} , and I 0 = V 0 /R size 12{I rSub { size 8{0} } = V rSub { size 8{0} } ital "/R"} {} is the peak current.
  • The average AC power is P ave = 1 2 I 0 V 0 size 12{P rSub { size 8{"ave"} } = { {1} over {2} } I rSub { size 8{0} } V rSub { size 8{0} } } {} .
  • Average (rms) current I rms size 12{I rSub { size 8{"rms"} } } {} and average (rms) voltage V rms size 12{V rSub { size 8{"rms"} } } {} are I rms = I 0 2 size 12{I rSub { size 8{"rms"} } = { {I rSub { size 8{0} } } over { sqrt {2} } } } {} and V rms = V 0 2 size 12{V rSub { size 8{"rms"} } = { {V rSub { size 8{0} } } over { sqrt {2} } } } {} , where rms stands for root mean square.
  • Thus, P ave = I rms V rms size 12{P rSub { size 8{"ave"} } = I rSub { size 8{"rms"} } V rSub { size 8{"rms"} } } {} .
  • Ohm’s law for AC is I rms = V rms R size 12{I rSub { size 8{"rms"} } = { {V rSub { size 8{"rms"} } } over {R} } } {} .
  • Expressions for the average power of an AC circuit are P ave = I rms V rms , P ave = V rms 2 R , and P ave = I rms 2 R , analogous to the expressions for DC circuits.

Conceptual questions

Give an example of a use of AC power other than in the household. Similarly, give an example of a use of DC power other than that supplied by batteries.

Why do voltage, current, and power go through zero 120 times per second for 60-Hz AC electricity?

You are riding in a train, gazing into the distance through its window. As close objects streak by, you notice that the nearby fluorescent lights make dashed streaks. Explain.

Problem exercises

(a) What is the hot resistance of a 25-W light bulb that runs on 120-V AC? (b) If the bulb’s operating temperature is 2700º C , what is its resistance at 2600º C ?

Certain heavy industrial equipment uses AC power that has a peak voltage of 679 V. What is the rms voltage?

480 V

A certain circuit breaker trips when the rms current is 15.0 A. What is the corresponding peak current?

Military aircraft use 400-Hz AC power, because it is possible to design lighter-weight equipment at this higher frequency. What is the time for one complete cycle of this power?

2.50 ms

A North American tourist takes his 25.0-W, 120-V AC razor to Europe, finds a special adapter, and plugs it into 240 V AC. Assuming constant resistance, what power does the razor consume as it is ruined?

In this problem, you will verify statements made at the end of the power losses for [link] . (a) What current is needed to transmit 100 MW of power at a voltage of 25.0 kV? (b) Find the power loss in a 1 . 00 - Ω size 12{1 "." "00"- %OMEGA } {} transmission line. (c) What percent loss does this represent?

(a) 4.00 kA

(b) 16.0 MW

(c) 16.0%

A small office-building air conditioner operates on 408-V AC and consumes 50.0 kW. (a) What is its effective resistance? (b) What is the cost of running the air conditioner during a hot summer month when it is on 8.00 h per day for 30 days and electricity costs 9.00 cents /kW ⋅ h size 12{9 "." "00"" cents/kw" cdot h} {} ?

What is the peak power consumption of a 120-V AC microwave oven that draws 10.0 A?

2.40 kW

What is the peak current through a 500-W room heater that operates on 120-V AC power?

Two different electrical devices have the same power consumption, but one is meant to be operated on 120-V AC and the other on 240-V AC. (a) What is the ratio of their resistances? (b) What is the ratio of their currents? (c) Assuming its resistance is unaffected, by what factor will the power increase if a 120-V AC device is connected to 240-V AC?

(a) 4.0

(b) 0.50

(c) 4.0

Nichrome wire is used in some radiative heaters. (a) Find the resistance needed if the average power output is to be 1.00 kW utilizing 120-V AC. (b) What length of Nichrome wire, having a cross-sectional area of 5.00 mm 2 size 12{5 "." "00"" mm" rSup { size 8{2} } } {} , is needed if the operating temperature is 500º C size 12{"500"°C} {} ? (c) What power will it draw when first switched on?

Find the time after t = 0 size 12{t=0} {} when the instantaneous voltage of 60-Hz AC first reaches the following values: (a) V 0 / 2 size 12{V rSub { size 8{0} } /2} {} (b) V 0 size 12{V rSub { size 8{0} } } {} (c) 0.

(a) 1.39 ms

(b) 4.17 ms

(c) 8.33 ms

(a) At what two times in the first period following t = 0 size 12{t=0} {} does the instantaneous voltage in 60-Hz AC equal V rms size 12{V rSub { size 8{"rms"} } } {} ? (b) − V rms size 12{-V rSub { size 8{"rms"} } } {} ?
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Source:  OpenStax, Physics 101. OpenStax CNX. Jan 07, 2013 Download for free at http://legacy.cnx.org/content/col11479/1.1
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