The Foundation of Electricity
Like most things in life, even something as complicated as electricity has simple terminology and parts that it can be broken down into. Understanding these properties and how they are related will give you a deeper understanding of what is going on in a circuit. Ohm's law is the most basic of formulas and can be applied to both simple and complex circuits. It works with AC and DC voltages and with a little algebra it easily demonstrates the relationship between the different properties of electricity.
The parts of the equation
Knowing what the parts are on a stand-alone basis will help you understand when it all comes together. It all may sound like gibberish, or may just look like a mess of numbers. I promise they all interconnect in a brilliant and simple way.
Voltage (V)
Voltage is a difference of electrical potential between two points in a circuit. It is what pushes electrical current through a conductor. If you compared water to electricity, this would be similar to water pressure in a pipe.
If you live in the USA, your normal plugs will be 120V. Your car battery is likely 12V. The smoke detector battery is 9V. AA and AAA batteries are 1.5V.
Current (I)
Current is the flow of electrical charge through a circuit. It is measured in amperes (A) or amps for short. It is the amount of electrons flowing through the circuit. Following the water analogy, this would be the volume of water flowing through a pipe.
Most plugs in your house can handle either 15 or 20 amps at most. The device you're reading this on likely uses less than an amp to charge or power it. Modern appliances are pretty energy efficient. The highest amp draws in your house would be your air conditioner, electric stove, electric dryer, or anything else with a big motor or heater. Pools are in this category of high amp draw. These devices can range from 15-50+ amps a piece. Older homes have a breaker panel that is only rated up to 100 amps. Newer and larger homes are in the range of 200-400 amp
You may also notice these devices tend to generate a lot of heat. Another property of amps is they are one cause of heat. Since amps are directly tied to the amount of electrons flowing, the more electrons that flow, the more heat along the wires. This is true even when there is very little resistance on a wire.
Resistance (R)
Resistance is how much a material opposes the flow of current. The higher the resistance, the less current will flow. In the water analogy, this would be rocks or debris in the pipe making the water harder to flow. It is measured in ohms (Ω). If you're unfamiliar with the Ω symbol, it is the last letter of the Greek alphabet, called omega. Higher resistances end up causing more heat. The reason for this heat is the energy that would normally be pushed through the wire is having a harder time and ends up being dissipated as heat. This can be either good if you want it, or bad if you don't.
A hairdryer has high-resistance wires inside. These wires heat up as a result causing the air to get warm and allowing hair to dry. Heated blankets also have high-resistance wires inside that cause them to heat up on a cold winter's day. Your skin has a higher resistance than you think. Try touching a 9V battery to your skin, you won't feel anything. If you try to lick it on the other hand, your tongue will go numb. Your saliva and tongue have a much lower resistance, allowing even something that low of a voltage to shock you.
Bringing them Together
Ohms Law: V = IR
Voltage = Current x Resistance
- Using algebra you could rearrange this to be
- Current = Voltage/Resistance
- Resistance = Voltage/Current
Having this formula in mind, as long as you know 2 values you can solve for the missing one. Using a multimeter you can find these values even if all 3 are unknown. For example, you run into a heater with its label missing plugged into an outlet type you've never seen. With your meter, you can test the outlet and see it's 208 volts. Using the Ohm setting with the device unplugged you can see it has a resistance of 50 Ω. 208/50 = 4.16 A. Your meter may show between 4.1 and 4.2, which is fine.
This is good for many things, however, there is an additional law we must cover that you may find yourself using even more. This is especially true when you would like to understand why different equipment has higher voltages.
Watt's Law: P = IV
Power = Current x Voltage
Power in cars is something you are likely familiar with. It is measured in horsepower (hp). In electrical systems, it is measured in watts (W). 746 W is equal to 1 hp. Extending the example from above we can get its power measurement. 4.16A x 208V = 865.28 W. Let's call it 865W. That would be 1.16 hp. We usually only see horsepower on motors, not on heaters. This was to show how you could measure in hp.
Moving on from that point comes a more practical application. As mentioned earlier, high amp loads lead to more heat. more amps and more heat lead to larger wire sizes. A wire is usually copper and starts to get really expensive as you increase the size. Most wires of any size will be rated up to 1000V. The problem is the ampacity or amount of amps allowed changes drastically depending on wire size.
By using Watt's law, we can see that if the power consumption is constant, current and voltage have an inverse relationship. As one goes up the other goes down and vice versa. If we have a 4800W motor and use 120V power it would be 40A. If we used 480 volts on the other hand it would be 10A using the same formula. The power consumption is identical so your bill is the same from the power company, but the cost of wires to run each are drastically different.
With the 120V power, you would need 6 gauge wire. At the time of writing this, it costs $115 for 100 feet. Using 12 gauge for the 480V priced at $60 for 100 feet. Now each motor will need at least 3 of these wires. So $345 for 120V and $180 for 480V. This is the price for just one motor. A factory will have hundreds or even thousands of motors.
Finally
The above examples were oversimplified with nice round numbers. The actual calculations can be a bit more complex, especially once you get into things like 3 phase power. That is beyond the scope of this introductory article. The 2 laws stated above apply though and will prove to be indispensable to you in your career. They govern all things electrical and electronic.
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