What Are Electric Power Sources?

Every electric circuit starts with a power source. Electric power sources supply energy to electric systems by moving the electrons in a circuit and thereby creating electric current.

The most common power sources are batteries and grid (mains) electricity.

Batteries produce a direct current (DC) whereas the power grid produces an alternating current (AC).

Many systems also use power supplies or AC adapters that convert one form of electric power (usually grid electricity) into a different form that is more useable for a specific device.

Batteries are a common source of electric power.

For example, electronic devices like laptops use DC (direct current) power so an AC adaptor to convert AC grid electricity into a DC (direct current) signal that can be used by the device.

Power sources do two important things:

  1. They supply energy to the circuit in the form of an electric potential difference, i.e. voltage.
  2. They provide a source and sink for electrons in a circuit.

As a simple analogy, you can think of a power source as the heart of a circuit; just as our heart circulates blood to enable our bodies to function, electric power sources pump or circulate electrons, enabling electric circuits to function.

You can think of a power source as a ‘pump’ that keeps electrons flowing in a circuit. Without a power source, a circuit will quickly lose energy due to the electrical resistance of its components.

Power sources are known as active components because they supply energy to the electric circuit.

Power sources supply electric power by pushing and pulling the electrons in a circuit. Without a power source, circuits quickly stop working due to energy losses. Think about the battery in your phone or tablet. When the battery runs out of charge, it stops functioning as a power source and your device quickly shuts down. Power sources are really important because every circuit and component relies on them in order to function. We start our discussion on circuits with power sources because they are the beating heart of every circuit.

DC vs. AC Power Sources

A power source is a device or machine that supplies electric power to a system. You can think of a power source as something that provides energy to electrons in a circuit.

There are two common forms of power sources, named after the type of electric current that they produce; direct current (DC) and alternating current (AC).

Rope Analogy for DC and AC

We will use an analogy to help develop our understanding of DC and AC power.

Think of a rope that is wrapped around two wheels that are separated by a small distance, like a pulley clothesline.

In this analogy, you are the power supply; by providing the energy that moves the rope, you are exerting a mechanical energy that can be used to drive a machine.

An image of a rope wrapped around two wheels.

Imagine standing under the rope and trying to power a machine by moving the rope. You have two choices:

  1. You can push the rope consistently in one direction. This is an analogy for DC power.
  2. You can alternately push the rope in one direction and then pull the rope in the other direction. This is an analogy for AC power.

In either case, the motion of the rope and wheels can be used to power a machine. A simple way to do this would be to insert a shaft into the hub of one wheel and then connect this shaft to a machine with a belt.

1) One-directional power (DC power analogy)

In this example, you would stand near the bottom rope and push it consistently in one direction.

As you push the rope, you drive power through the machine in one direction, turning each of the wheels.

This is similar to how windmills work. You could use this machine to perform a rotary action like grinding flour, or turn a screw that connects to an olive or grape press.

This is an analogy for Direct Current.

Pushing the rope in one direction; an analogy for direct current (DC).

Pushing the rope causes the wheels to turn. The wheels always turn in the same direction.

Instead of pushing a rope, a DC power source pushes electrons.

Instead of a mechanical system, a DC system uses electric or electronic components. A DC system can also convert electrical power into mechanical power. It can power a motor that can be used with gearboxes and pulleys as well.


2) Bi-directional power (AC power analogy)

In this (second) example, you push the rope in one direction for a specific period of time, and then reverse the direction so that you pull it the other way for the same amount of time.

Pushing the rope in the forward direction.

Phase One: Pushing the rope in the forward direction.

You start by pushing the rope in the same direction as you did in the first example.

Pulling the rope in the reverse direction; an analogy for alternating current (AC).

Phase Two: Pulling the rope in the reverse direction.

After some time, you reverse the direction of the rope. You power the machine in the opposite direction for the same amount of time as you did in ‘Phase one’.

Just as with the first example, the back-and-forth motion can be used to power a machine; it would simply need to be designed a little differently than a machine made for one-directional power.

For example, in the first example we mentioned that the motion could be used to power a screw that could be used for a grape or olive press. In that example, the press would need to be reset after each use. Using a back and forth motion, we could make a press that first presses down during the forward motion and then pulls itself back up during the reverse motion.

Another example is a windshield wiper or an arm that sweeps side to side.

This back and forth action is an analogy for AC power.

AC power sources push and then pull electrons in a circuit at a precisely timed frequency. Just like with DC power, engineers have figured out many clever ways to use AC power. In fact, AC power is the standard for large scale power generation. AC power isalso commonly converted into DC power.

In the case of solar panels (photovoltaic cells), DC power is converted to AC power so that it can be used by the power grid.

Resistance in Electric Circuits

What would happen if you stopped pushing or pulling the rope? Thanks to friction, the machine would quickly come to a stop. The same thing happens in a circuit that is disconnected from a power source.

In the case of circuits, the equivalent of ‘friction’ is something called electric resistance. Every electric component has some amount of electric resistance. Even conductors like wires have some resistance to the movement of electrons. That’s because conductors don’t conduct electricity perfectly, and they lose some energy as heat as a result. The energy loss quickly causes all the electrons in the circuit to stop moving when disconnected from the power source, even if the circuit remains closed.

In AC circuits, resistance is called impedance. That’s because the total ‘resistance’ to current flow in an AC circuit doesn’t just come from electric resistance. Capacitance and inductance also contribute to the overall opposition to current flow in an AC circuit. The total opposition to current flow, caused by resistance, capacitance and inductance is called impedance.

Power Sources Supply Energy

The main job of a power source is to supple electrical energy to a circuit. This is accomplished in different ways depending on the type of power source.


Batteries provide a direct current (DC) and convert chemical energy into electrical energy. Electrons leave the negative terminal of the battery, which is called the anode. The electrons travel through the circuit, and then re-enter the battery at the positive terminal, also called the cathode.

Within the battery, two chemical reactions take place such that electrons are produced in the anode and accepted in the cathode. The electrons accumulate in the anode, and experience a repelling force from the chemical reaction in the anode. Simultaneously, a chemical reaction in the cathode produces positively charged ions that attract electrons in the circuit.

Eventually, the chemical reactions within the battery slow down and stop completely. Some batteries can be recharged, which basically works by reversing the chemical reactions in the anode and cathode. Instead of producing electricity during the chemical reaction, electricity is used to restore the chemicals to their original state. Disposable batteries become permanently depleted, and the chemical reactions that provide electrical energy cannot be reversed.

AC Generators

AC generators are the most common source of alternating current electricity. Generators convert the mechanical energy of a spinning shaft (commonly from a steam turbine) into electrical energy via the principle of magnetic induction.

Image of an AC generator.

AC generators are composed of a coil of wire that is connected to a spinning shaft and a set of magnets. As the coil is rotated, the electrons inside experience a force due to the magnet. As the coil passes the positive pole of a magnet, the electrons are pushed in one direction, and as the coil passes the negative pole, the electrons are pulled back in the other direction.

This is how AC generators produce an alternating current.

Voltage and Current Sources

Voltage Sources

A voltage source is a device that can maintain a fixed voltage no matter the load (i.e. resistance) on the circuit. Just like other components, voltage sources are theoretical, ideal versions of a real life system. In the real world, no voltage source is perfectly ideal; for one, voltages supplied always vary with the load, even if just a little. When you tap into the power grid by plugging a device into a wall socket, you aren’t always getting the same exact voltage. The power supplied by electric companies typically has a tolerance of about +-5%. In the US and Canada, the rated voltage is 120 +-6 V, meaning that the actual voltage supplied could be as little as 114V, and as high as 126V. All of our electrical devices need to be designed with this voltage supply variation in mind.

Very often, you will hear the term ‘voltage source’ intermingled with the term ‘power source’. That’s because most power sources in the world function similarly to ideal voltage sources. Batteries and generators are both modeled after ideal voltage sources. So remember: when we use the term ‘voltage source’, we are talking about a source that tries to supply constant voltage, but will do so imperfectly in real life.

Current Sources

Current sources are the dual of voltage sources. An ideal current source always supplies the same current no matter the circuit or load (i.e. resistance). Just like with voltage sources, no ideal current source exists in real life. However, some examples of pretty good current sources do exist. Current sources can be made from transistors or operational amplifiers. We’ll tackle this subject in our Module on Semiconductors.

Electric Power

This wouldn’t be much of a lesson on power sources if we didn’t at least introduce the concept of electric power.

Electric power is measured in the SI unit of the watt, which is the amount of energy supplied (in Joules) per second.

In other words, power is the rate at which energy is supplied to the circuit. In order to get the total energy supplied, you multiply the power by the amount of time. That’s why when we get our energy bill, the measurement of energy is given in ‘watt-hour’, or ‘kilowatt-hour’. You have to multiply the power by the time in order to see how much energy was actually used.

Within a circuit, power is also equal to the voltage times the current. Voltage is the amount of energy needed to separate charge, and current is the flow of electric charge through the circuit (per second).

Power = Voltage \times Current => P = VI

We cover electric power in much more detail later in this module.

Electrical Loads

An electrical load is a component that consumes electric power. For instance, any time you plug a device into a wall socket, your device functions as a load because it consumes power supplied by the power grid (aka mains). In a battery powered device, the battery supplies the energy while the rest of the device functions as the load.

Electrical loads are often thought about as the resistance (or impedance) offered by the load. In a circuit with a voltage source, the resistance of the load determines the current in the circuit.

Relating the concept of an electrical load back to active vs. passive components, we can see that loads are passive because they do not supply energy to the circuit. In contrast, power sources are active components because they do supply energy to the circuit.

Now that we have an understanding of how a circuit gets its power, let’s learn more about circuits with Lesson 3: Electric Current!