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General Studies Science: Help & Review24 chapters | 338 lessons | 1 flashcard set

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Lesson Transcript

Instructor:
*Scott van Tonningen*

Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.

Curious about the different types of electrical power? In this lesson, we'll define DC and AC power, provide a few examples, and provide a quiz to check your understanding.

If you ever have a chance to visit a hydroelectric plant at one of our country's major dams, do it! One of the most impressive is the Robert Moses Niagara power plant in New York State. This power plant generates 2.4 million kilowatts of AC power. That's 2.4 billion watts, enough power to light 40 million 60-watt light bulbs!

On the other end of the spectrum, consider a standard digital watch that runs on 6 microwatts (millionths of a watt) of DC power. That's small enough that the tiny button battery in the watch can last 3-5 years under normal usage.

Each of these examples represents a type of electrical power. The dam produces three-phase, alternating current (AC) power and the button cell produces direct current (DC) power. Let's get into some definitions.

*P = VI*

**DC power** is the simplest. A constant voltage is available to push electric charge through the circuit. Depending on the electrical resistance encountered, a current is produced. The current can only flow in one direction. This combination of constant voltage and one-way current is converted at some point to other forms of energy (usually mechanical energy, heat, or both). DC power is simply the product of the voltage and current in watts (W) required to supply this converted energy. This is how the equation is written out:

**Understanding DC Power**

DC electrical power is very similar to the mechanical power generated while riding a bike from point A to point B. Let's say it is a distance of one mile. The power required depends on how much time it takes you to travel this distance. The faster you accomplish this task (speed), the more power is required.

Because of the hill and the friction provided by the road, it will take a certain amount of force on the pedals to achieve a certain speed. The force you apply to the pedals is like voltage. The speed achieved by the bike is like current. If you double the force on the pedals, you will double the speed at which the pedals rotate, and thus you will double the speed of the bike, assuming you don't change gears.

What about power? It turns out that you can measure power for the bike example in much the same way you do in DC circuits. If you multiply the force you apply to the pedals by the speed achieved by the bike, you can get a measure of how much energy per unit of time it takes to get from point A to point B at the speed you desire. That's what power is!

**Alternative Calculations for DC Power**

If you know two of the three variables in a DC circuit (voltage, current and resistance) you can always calculate DC power. The other two handy equations are:

Suppose a 12V battery is connected to a halogen headlight that has an internal resistance of 4 ohms. How much electrical power is delivered to the headlight?

DC power is used for lower voltage applications or when portability is important. Most applications that call for a battery are DC. The following table depicts some uses of DC power:

As you can see, a vehicle's headlights produce a voltage of 12 volts and a power of 40 watts. Not too extreme, right? Well, as you can see below that, the lunar rover on the moon has a motor that operates at 36 volts and has a power of 746 watts. There are many variations in the devices that we use.

Next time you see high voltage transmission lines, take a closer look. You will probably see a set of three main lines and then possibly a smaller wire either above or below the three. Often, you'll see two systems in parallel, three lines on each side of a tower. These three lines, plus the smaller ground wire, represent a type of AC power called **three-phase**, which simply means there are three independent AC power systems that operate in a balanced manner.

In the 1870s and 1880s, Thomas Edison and others advocated for DC power across the U.S. It wasn't until 1891 that the first practical three-phase AC system was demonstrated. It turns out that AC power is much easier to generate because it results from rotating machines, like water turbines, and it is easier to use in industrial applications, mainly motors. AC won out, and now we use three-phase to transmit electrical power around the globe.

Once the power gets closer to the users (like the houses in your neighborhood), it is usually separated into individual, **single-phase** AC lines, which means there is only one AC circuit, and the voltage is reduced to usable levels using transformers. In your neighborhood you might have single-phase power poles that look like the image on screen right now (unless your power is underground):

By the time the AC power gets to your house, you generally have 110-120V available plus a couple of 220V-240V circuits for things like ovens, dryers, and central air/heat.

AC power is the result of voltages and currents that are not constant; they cycle between positive and negative, completing the round trip 60 times per second (60Hz in the U.S.). A good illustration of this is to think about a pendulum moving back and forth. If we let the right side of the clock represent positive position for the pendulum and the left side represent negative, we obtain the approximate diagram of pendulum movement over time that you can see on your screen right now:

The pendulum movement is approximately a sine wave, and that is the same waveform both voltage and current have in AC circuits.

**Definition of AC Power**

**AC power** is still defined as a product of voltage and current, but it is a bit more complicated mathematically. Without going into too much detail, there is one fairly simple AC power calculation we can consider. Suppose you are interested in the power consumption of an electric heater. A heater is an example of a **resistive load** because it acts pretty much like a resistor (which converts voltage and current to heat). With a purely resistive load (lights are another good example), we can calculate power using an average voltage, average current, and the resistance as follows:

The average value for voltage and current is found by multiplying the peak value of the sine waves for voltage and current by 70.7%.

A high-voltage industrial heater is supplied by an AC voltage that has a peak value of 340V. The internal resistance of the heater is 8 ohms. What is the electrical power converted by the heater?

AC power is typically used in higher voltage applications or when portability is not an issue. The following table depicts some common uses of AC power:

As you can see, many of these examples use a lot of power, such as a clothes dryer. That's why we try to do as few loads as possible. It's also why your mom is always telling you not to leave the lights on when you leave a room. The amount of power produced by a toaster also explains why - if we're not paying attention - we end up with blackened cinder when we make toast in the morning.

Electrical power occurs whenever electrical energy is converted to other forms of energy, such as heat or mechanical energy. **DC power** is more common in low voltage applications where portability is important. It is calculated as the product of voltage and current, in watts, and there are two other handy forms of the equation.

The equation we use to calculate DC current flow is:

*I = P/V*

For a quick refresher, voltage is measure in volts (v), power is measured in watts (w), and current flow is measured in amps (a).

**AC power** takes a couple of forms and is also calculated as a product of voltage and current. **Three-phase** consists of three independent, balanced AC circuits; is easy to generate; and is used for transmission over long distances as well as in some industrial motors. **Single-phase** is the type of AC power used for high voltage (110V or 220V) applications in households.

If the load is purely **resistive**, then you can use the DC power equation for AC as long as you use average voltages and average current. The average value is 70.7% of the peak value of the voltage and current sine waves. To calculate the power consumed by an AC-operated device, you need to square the average voltage and divide by the resistance, which is measured in ohms.

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General Studies Science: Help & Review24 chapters | 338 lessons | 1 flashcard set

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