In this video, Barron Stone explains the rate at which energy is transferred in an electrical circuit, or electrical power and how it is measuring in units of Watts. Learn about the relationship that voltage and current have to electrical power and how to calculate the power consumption of your circuits.
- [Narrator] We build electronic circuits to do something. That something could be lighting an LED, turning on a motor, or playing music through a speaker. All of those actions require power and energy. Energy is the ability of an object to perform work, which is a physics term that means moving something from one place to another, and energy comes in many different forms including mechanical energy, chemical energy, thermal energy, and my personal favorite: electrical energy.
To use these different types of energy to do things, we need to get it from somewhere, but where does all that energy come from? As the brilliant scientist, Albert Einstein, is credited with saying, "Energy cannot be created or destroyed, but it can change from one form to another." For example, the chemicals inside of a battery react to convert stored chemical energy into electrical energy, and a light bulb connected to that battery can transform that electrical energy into two other forms of energy.
It transforms into thermal energy, in the form of heat, and electromagnetic energy in the form of light. One of the main goals when working with electronics is to use these transformations to do something useful. We can generalize the components that perform these transformations as either being a producer or a consumer of electric energy. Producers, like the battery, convert some form of energy into electricity, and consumers, like the light bulb, convert electric energy into another useful form.
Electric energy itself exists in one of two forms. Electric potential energy, more commonly known as voltage, is a static buildup and imbalance of electric charge, which has the potential to do work by moving electrons from one place to another, and when those electrons actually do move along a conductive path, that's a form of kinetic energy known as current. Energy is measured using a unit called Joules, which represents an amount or how much energy is transferred.
You'll encounter joules occasionally when working with electronics, but more often you'll deal with energy in terms of power. Electric power describes the rate at which electric energy is transferred or transformed. Power is measured in units of watts, with one watt representing the energy transfer rate of one joule per second. Working in terms of watts instead of joules is useful because watts give us information about how much and how fast energy is being transferred in an electric system.
The amount of power consumed by a circuit component is equal to the rate of the current flowing through that component times the voltage across it. This is a common equation that you'll typically see abbreviated as: P equals I times V. As I stand here on the patio... (water drips gently) watering a pinwheel, I'm converting mechanical energy from the water in this hose into another form of mechanical energy: a spinning pinwheel. You can relate this to how an electric motor converts electrical energy, from current passing through the motor, into mechanical energy to turn a shaft.
The rate at which energy is transferred from the hose to the pinwheel represents power, and the power consumed by the pinwheel will depend on the pressure and flow rate, representing current and voltage. If I open up the faucet to increase the voltage on the hose, (water splashes louder) that'll increase the current on the hose due to Ohm's Law. The relationship, that power equals voltage times current, means that by increasing the voltage and current on the hose, I'm increasing the amount of power, and so I'm transferring energy at a faster rate to the pinwheel, and I can see the effect of that because it spins more quickly.
For this simple circuit, which has two consumer components, a resistor and an LED, I can calculate the power consumed by each component individually, using P equals I V. The power consumed by the resistor equals the current through the resistor, which is 15 milliamps, times the voltage across it, which is seven volts. This results in a power consumption of 105 milliwatts. Similarly, the power consumed by the LED is equal to 15 milliamps times two volts, which is 30 milliwatts.
To calculate the total power consumed by this circuit, I can add together the power that's consumed by the individual components. So, 105 milliwatts plus 30 milliwatts gives this circuit a total power consumption of 135 milliwatts. If I don't need to know the consumption of the individual components and I only cared about the total power consumption of this system, then I can simply consider the circuit as a whole. There's 15 milliamps flowing through both components and a total of nine volts across them.
Using those numbers to calculate the total power gives me the same answer as when I added together the individual components: 135 milliwatts. To give you a practical frame of reference for working with power, here are a few examples of the power consumed by devices you may encounter in your everyday life. Ranging from a quarter of a watt, consumed by a microcontroller device, like an Arduino Uno, all the way up to household appliances, like a microwave oven that can consume over 1,000 watts.
- Comparing conductors and insulators
- Relating electric current to flowing water
- Exploring Ohm's law
- Measuring DC voltage
- Measuring direct current
- Calculating the power consumption of your circuits
- Choosing the right batteries to power your circuits