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UExcel Physics: Study Guide & Test Prep18 chapters | 201 lessons | 13 flashcard sets

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Lesson Transcript

Instructor:
*David Wood*

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this lesson, you will be able to explain the difference between AC and DC circuits and calculate AC current, AC voltage and AC power using both rms values and peak values. A short quiz will follow.

When you study physics, you spend most of the time studying direct current (or DC). It's simpler, makes it easier to understand the basic concepts, and it's better for visualization. But take a look around you. Look at the wall sockets nearby and the light bulbs above your head. Both of these things, and indeed most things around you, are powered by alternating current (or AC). The band AC-DC might have been partial to both, but nearly everything you experience in life will be AC-powered.

**Alternating current** is where current switches direction super-fast instead of flowing just one way around a circuit - one way then the opposite way, over and over. The most common rate at which this happens is 60 Hz, or 60 switches per second. This creates a current that varies sinusoidally, which means that it varies in the form of a sine curve, like this one:

Since the current is switching, so is the voltage. It also follows a sine curve. Because of this, we tend to express current and voltage as special averages called **rms** (or root mean squared). An AC circuit will have an rms current and an rms voltage. And those values are defined by the following equations:

You can see that V-zero is the peak or maximum voltage, and I-zero is the peak or maximum current. Those are the tops and bottoms of the sine curve.

But this lesson is titled 'AC Power,' so how do we calculate the power used by an AC circuit? As discussed in another video lesson, **power** is the energy used per second, measured in watts (or Joules per second). And in a circuit, you can calculate it by multiplying the current by the voltage. We can do exactly the same thing for an AC circuit; we just use the rms current and rms voltages. So below is our basic equation for power in an AC circuit: the rms voltage, measured in volts, multiplied by the rms current, measured in amps.

But what if you don't know the rms voltage or rms current? What if, instead, you know the peak voltage V-zero and peak current I-zero? Well, then we would need to use the previous equations for rms voltage and rms current. But to avoid using more than one equation, we can substitute these equations into the power equation, like this:

This then simplifies to tell us that the power used in an AC circuit is equal to the peak current, multiplied by the peak voltage, divided by two.

Okay, let's try an example! You're doing some tests on an energy-saving light bulb. You find that the maximum voltage it ever uses is 240 volts, and the maximum current that flows through it is 0.12 amps. For a regular light bulb, you look up some values and find that the rms voltage is 120 volts, and the rms current is 0.5 amps. What is the difference in power used by the two light bulbs?

Okay, so we need to figure out how much power is used by each light bulb and then compare them. For the first, we're given maximum values, and for the second, we're given rms values. So, we need to use a different equation for each and then compare the two power values.

For the energy-saving light bulb, we know that V-zero is 240 volts, and I-zero is 0.12 amps. So, we can figure out the power using this equation: (240 * 0.12) / 2 = 14.4 Watts.

For the regular light bulb, we know that V-rms is 120 volts, and I-rms is 0.5 amps. So, all we have to do here is use this equation, and multiply the two together: 120 * 0.5 = 60 Watts.

Last of all, to find the difference between them, subtract the smaller number from the larger one: 60 - 14.4 = 45.6 Watts. So, there is a difference in power used of 45.6 watts between the two light bulbs. And that's it - we're done!

Almost every electrical device we use in everyday life is powered by alternating current. **Alternating current** (or AC) is where current switches direction super-fast instead of flowing just one way around a circuit - one way then the opposite way, over and over. This creates a current that varies sinusoidally, which means that it varies in the form of a sine curve, like this one:

Since the current is switching, so is the voltage and so is the power used. They all follow a sine curve. Because of this, we tend to express current and voltage as special averages called **rms** (or **root mean squared**). An AC circuit will have an rms current and an rms voltage, and those values are defined by the following equations, where V-zero is the peak or maximum voltage, and I-zero is the peak or maximum current. Those are the tops and bottoms of the sine curve.

As discussed in another lesson, **power** is the energy used per second, measured in watts (or Joules per second). In an AC circuit there are two main equations you can use to calculate power: the top one, where you multiply the rms-voltage by the rms-current; or the bottom one, where you multiply the peak voltage by the peak current and then divide by two. Based on what you're given in a question, you can figure out which of the two equations to use.

Upon completing this lesson, you should be able to:

- Define alternating current (AC), root mean squared and power
- Identify the sine curve of AC, voltage and power
- Explain how to use the two main equations to calculate power in an AC circuit

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10 in chapter 13 of the course:

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UExcel Physics: Study Guide & Test Prep18 chapters | 201 lessons | 13 flashcard sets

- Go to Vectors

- Go to Kinematics

- Insulators and Conductors: Examples, Definitions & Qualities 6:38
- What is Electric Current? - Definition, Unit & Types 7:59
- Electrical Resistance: Definition, Unit & Variables 7:52
- Ohm's Law: Definition & Relationship Between Voltage, Current & Resistance 7:17
- What is Electric Power? 6:41
- Applying Kirchhoff's Rules: Examples & Problems 7:29
- Resistor-Capacitor (RC) Circuits: Definition & Explanation 6:28
- Impedance in Alternating Current Circuits 5:53
- R-L-C Series Circuits 8:01
- Alternating Current Power Calculations 5:09
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