What Are Amps, Watts, Volts and Ohms?

Electrical systems are more efficient when using a higher voltage to reduce current. SCIENCE PHOTO LIBRARY/Getty Images

In an electrical system, increasing either the current or the voltage will result in higher power. Let’s say you have a system with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation I = P/V, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.

You know that P = 100 W, and V = 6 V. So, you can rearrange the equation to solve for I and substitute in the numbers.

I = 100 W/6 V = 16.67 amps

What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?

I = 100 W/12 V = 8.33 amps

So, this latter system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let’s rearrange the first equation:

I = V/R can be restated as V = I*R

Now you can substitute the equation for V into the other equation:

P = V*I substituting for V we get P = I*R*I, or P = I2*R

What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So, using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages.

This improvement in efficiency is what drove the automobile industry to consider switching from 12-volt electrical systems to 42-volt systems in the 1990s. As more cars shipped with electric-powered amenities — video displays, seat heaters, “smart” climate control — they required thick bundles of wiring to supply enough current. Switching to a higher-voltage system would provide more power with thinner-gauge wiring.

The switch never happened, because carmakers were able to boost efficiencies with digital technology and more efficient electric pumps at 12 volts. But newer hybrid and fully electric (EV) cars and trucks have electrical systems that average 450 to 650 volts to run powerful electric motors.

Originally Published: Oct 31, 2000

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