Power
Watt is Power?
In general physics terms, power is defined as the rate at which energy is transferred over time.
Energy can never be created or destroyed, only transferred to another form. A lot of what we’re doing in electronics is converting different forms of energy to and from electric energy. This energy change comes in many, potentially harmful, forms – heat, radiation, sound, nuclear, etc. – , and more power means more energy. So, it’s important to have an idea of what kind of power you’re working with when playing with electronics. With great electrical power, comes great potential for damage and destruction.
Power Formula and Units
Electric power is the rate at which energy is transferred. Energy is measured in terms of joules (J). Since power is a measure of energy over a set amount of time, we can measure it in joules per second. The SI unit for joules per second is the watt abbreviated as W.
To calculate the power of any particular component in a circuit, multiply the voltage across the load by the current running through it. This means the the formula is P=IV. We can see this in the GIF below. Voltage stays the same but as current is decreased, power, represented by the water wheel, slows down.
Calculation Example
I = V/R = 9V/100Ω = .09 Amps
P =IV = (.09Amps)(9V) = .81 Watts
Producers and Consumers
Each component in a circuit either consumes or produces electric energy. A consumer transforms electric energy into another form. For example, when an LED lights up, electric energy is transformed into electromagnetic. For example, a light bulb consumes power. Electric power is produced when energy is transferred from some other form to electricity. A battery supplying power to a circuit is an example of a power producer.
Power Producers
Generators
Solar Cells
Batteries
Thermoelectric Generator
Power Consumers
Motors
LEDs
Inductors
Electronics
Power Rating
All electronic components transfer energy from one type to another. Some energy transfers are desired: LEDs emitting light, motors spinning, batteries charging. Other energy transfers are undesirable, but also unavoidable. These unwanted energy transfers are power losses, which usually show up in the form of heat. Too much power loss – too much heat on a component – can become very undesirable.
Even when energy transfers are the main goal of a component, there’ll still be losses to other forms of energy. LEDs and motors, for example, will still produce heat as a byproduct of their other energy transfers.
Most components have a rating for maximum power they can dissipate, and it’s important to keep them operating under that value.
Resistors are some of the more notorious culprits of power loss. When you apply some voltage across a resistor, you’re also going to induce current flow across it. More voltage means more current, which means more power.
Remember back to our power-calculation example, where we found that if 9V were dropped across a 100Ω resistor, that resistor would dissipate .81W which is a lot of power for most resistors. Most resistors are rated anywhere from ⅛W (0.125W) to ½W (0.5W). If you drop .81W across a standard ½W resistor, ready a fire extinguisher.
If you have seen resistors before, you have probably seen these. The top resistor is a 1/2W and the bottom is a 1/4W. This type of resistor is designed for the low voltages and currents of electronics which means they are not built to dissipate a lot of power.