Electrical Power

Now that we’ve discussed how circuits are set up, let’s look at how to calculate the electrical power that flows through them.

Power

Power is the rate at which electrical energy is used in a circuit. It is measured in Watts (W). A wattmeter is the meter that measures a circuit’s power.

Power= Voltage $\ast$ Current

$P=V\ast I$

Let’s look at an example together.

If a circuit has a voltage of $12V$ and a current of $2A$, what is the power?

$P=V\ast I$

$P=12\ast 2$

$P=24$ watts.

Combining Ohm’s law and power

Since Ohm’s Law and the power formula share the same variables, you can combine them to find power using any two known values, not just voltage and current.

If you know current and resistance, substitute $V=I\ast R$ into $P=V\ast I$: $P=I^2\ast R$

If you know voltage and resistance, substitute $I=\frac{V}{R}$ into $P=V\ast I$: $P=\frac{V^2}{R}$

For example, if a circuit has a current of $3A$ and a resistance of $4\Omega$, the power is:

$P=I^2\ast R$

$P=3^2\times 4$

$P=9\ast 4$

$P=36$ watts

This means that no matter which two values you’re given, you can always solve for power.

Exam tip: The ASVAB doesn’t give you a calculator, and power questions typically use small whole numbers. Try squaring or multiplying the given values mentally before you look at the answer choices.

AC vs. DC

In electricity, we have two types of current: direct and alternating. Direct current (DC) flows in one direction only, while alternating current (AC) periodically reverses direction.

AC is used to transmit power over long distances because its voltage can be easily increased or decreased using transformers. Transmitting power at high voltage is more efficient, as less energy is lost as heat due to the resistance of the transmission lines.

We can use the power formula to see exactly why this works. Since $P=V\ast I$, delivering the same amount of power at a higher voltage means the current can be lower. And since the energy lost to resistance in a wire follows $P=I^2\ast R$, even a small drop in current makes a big difference. Cutting the current in half, for example, cuts the transmission loss to a quarter of what it was. The voltage can then be reduced to safer levels for use in homes and businesses.

Even though the electricity from the outlet comes in the form of AC power, most electronic devices operate on internal DC power. This is because DC power provides a stable, one-directional flow of current that these devices need to function. Electronic devices convert AC to DC through the use of power adapters or built-in power supply units.

Diodes are the component that makes this conversion possible. A diode allows current to flow in only one direction and blocks it in the other. By arranging several diodes together in a specific pattern, a power supply can convert the back-and-forth flow of AC into the one-directional flow of DC. We’ll cover diodes and how they work in more detail in the Semiconductors chapter.

Capacitors

A capacitor is a device that stores electrical energy in an electrostatic field by collecting electrical charge. It is the electrical equivalent of a spring, storing energy that can be released later. A basic capacitor is two conductive plates separated by an insulating material called a dielectric.

Capacitance is measured in Farads (F).

Capacitors are often used alongside diodes in power supplies. After a diode converts AC to DC, the output isn’t perfectly smooth; it still has ripples. A capacitor smooths out these ripples by storing energy during the peaks and releasing it during the dips, delivering a steadier flow of DC power.

Frequency

Frequency is the number of cycles the alternating current completes per second, measured in Hertz (Hz). The standard frequency for the American electrical grid is $60$Hz, while most of Europe, Australia, and much of Asia use $50$Hz.

The entire grid must be standardized to the same frequency because all electrical generators must be synchronized to spin at the same rate. By spinning, generators produce electricity, and the speed at which they spin determines the frequency of the electricity.

Generators are constantly being adjusted to keep their spin synchronized while meeting the electricity demand.