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AP Physics 1: Exam Prep12 chapters | 136 lessons

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Lesson Transcript

Instructor:
*Yuanxin (Amy) Yang Alcocer*

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this video lesson, you will learn how to find the power in an electric circuit using either the voltage or the resistance in the circuit. See how this power translates into the energy we see at work in light bulbs and other electrical products.

We are surrounded by electric circuits. We define an **electric circuit** as a closed loop through which electricity freely flows. The lights in your room are all powered by electric circuits. Your television, the computer you are using to read this lesson, and even your cell phone are all powered by electric circuits. Yes, electric circuits are very common nowadays in our electricity-reliant society.

When you turn these electric circuits on, you can see their energy and power at work. When you flip the switch for the lights in your room, you see your lights get bright and begin to emit light. When you turn on your television, you see the screen on your television turn on, and you can see various television programs brought to you by your local television providers. When you turn on your computer, you see your monitor turn on, and then, you can use all kinds of computer programs on it. When you turn on your cell phone, you are able to make phone calls and write texts to your friends and family

Where does this power and energy come from? It comes either from batteries or from your electricity provider. Since we are dealing with electric circuits, we are also dealing with electric power. We define **electric power** as the rate at which a circuit uses electrical energy. Electrical energy is the charge that comes from a battery or from the power plant. There are two ways to calculate this power. We can use either the voltage or the resistance of a circuit.

To use the voltage of the circuit, we can use this formula to calculate the power:

*P* = *V* * *I*

We have that power is equal to the voltage of the circuit times the current I of the circuit. When our unit for voltage is volts (*V*) and the unit for current is amperes, or amps for short (*A*), then we have to multiply them together to get the watt (*W*), the standard unit of power. So, say we have two 2.5-volt batteries connected together to make 5 volts. If we send a current of 0.5 amperes through it, we get a power of:

*P* = 5 *V* * 0.5 *A* = 2.5 *W*

You can power a clock with 2.5 watts of power.

Another way you can calculate power is by using the amount of resistance in a circuit. You can actually convert the power formula with voltage into the power formula with resistance by using Ohm's Law, which tells you *V* = *I* * *R* (voltage equals current times resistance).

*P* = *V* * *I*

*P* = (* I* * *R* ) * *I*

*P* = *I*^2 * *R*

Or,

*P* = *V* * *I*

*P* = *V* * (*V* / *R* )

*P* = *V*^2 / *R*

If you are given the current and resistance, you can use *P* = *I*^2 * *R* to find your power. If you are given the voltage and the resistance, then you can use *P* = *V*^2 / *R* to find your power.

For example, say you are given that a certain electric circuit has a resistance of 1500 ohms with a current running through it of 5 amps. The power used by this circuit is then:

*P* = *I*^2 * *R* = ( 5*A* )^2 * 1500 ohms = 25 * 1500 = 37,500 *W* or 37.5 *kW*

This is almost enough to run a curling iron.

Now, say that you are given another electric circuit that is run with 120 Volts and a resistance of 192 ohms. You use *P* = *V*^2 / *R* to find the power this circuit uses. You get:

*P* = ( 120*V*)^2 / 192 Ohms = 14400 / 192 = 75 *W*

This is enough to power a sewing machine.

Let's review what we've learned. An **electric circuit** is a closed loop through which electricity freely flows. **Electric power** is the rate at which a circuit uses electrical energy. The formula to find the power in a circuit is:

*P* = *V* * *I*

We use Ohm's Law (*V* = *I* * *R*) to derive two other power formulas that use the resistance of a circuit.

*P* = *V* * *I*

*P* = ( *I* * *R* ) * *I*

*P* = *I*^2 * *R*

Or,

*P* = *V* * *I*

*P* = *V* * ( *V* / *R* )

*P* = *V*^2 / *R*

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AP Physics 1: Exam Prep12 chapters | 136 lessons

- 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
- Calculating Energy & Power in Electric Circuits 4:58
- Series Circuits: Definition & Concepts 9:01
- Parallel Circuits: Definition & Concepts 6:43
- Electric Circuit Diagrams: Applications & Examples
- Finding the Equivalent Resistance: Series, Parallel & Combination Circuits 6:35
- Applying Kirchhoff's Rules: Examples & Problems 7:29
- Go to AP Physics 1: Direct Current Circuits

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