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Energy Transfers in Circuits: Equations & Examples

Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

In a circuit, energy is always being transformed from electrical energy into other forms, like light and thermal energy. In this lesson, learn how energy is transformed and how to calculate exactly how MUCH energy is transformed in a circuit.

What Makes a Circuit Work?

When you flip on the light switch, you trust that, as long as the electricity is flowing, the light will come on. This takes energy! Have you ever thought about what is actually going on inside that bulb to make it light up? Inside all matter, including the bulb filament and connecting wires, there are tiny particles called electrons. Electrons are negatively charged, and in certain materials, like electrically conductive metals, some of these electrons are free to move from atom to atom.

In a light bulb, electric energy is transformed into other forms of energy like light and thermal energy
light bulb

Normally, these electrons are always bouncing around and bumping off of each other within the wires but not going anywhere. However, when you flip the light switch, you complete the circuit and connect all those electrons to an energy source which creates a potential difference, or voltage between the two ends of the filament in the light bulb. Now, while the electrons are still bouncing around and bumping into each other, they all slowly move forward at the same time, too.

Imagine that you are in a large crowd and everyone is trying to get out of the door at the same time. The people in the crowd are all slowly moving toward the door, but at the same time they bump into each other and don't always move in a straight line. This is exactly what the electrons are doing!

The flow of charges in a circuit is known as current, and current is never used up in a circuit. If a certain number of electrons are pushed into the light bulb filament, then the same number must come out the other side. Electrons are not destroyed in the bulb!

However, something DOES happen to them. It is difficult for the electrons to move through the filament because the filament has some resistance to the flow of electrons. As they push through, some of their energy is transformed into other forms of energy, like light and thermal energy. The electrons that come out the other side are bouncing around a little bit more slowly than they did before because some of their energy has been used to light up your house!

Energy, Charge, and Voltage

When a voltage is applied to a charged object (even a tiny one like an electron), it gives it some electrical potential energy. This is what causes electrons to move in a circuit as the potential energy is converted into kinetic energy. The amount of energy depends on both the charge and the voltage applied.


energy, voltage, and charge


A single electron has a charge of 1.602x10-19, so if a potential difference of 120 V is applied to a light bulb filament, each electron in the filament will acquire 1.922x10-17 Joules (J) of energy.


energy of an electron in a circuit


While that may not seem like a lot, remember that electrons are really tiny and there are A LOT of them in the filament, so the total amount of energy is much, much higher.

Energy and Power

You can also determine the amount of electrical energy that is being transformed into light and thermal energy by your light bulb if you know how much power it is using. Power is the rate at which energy is transformed, so as the power used increases, the amount of energy transformed increases as well. Power is usually measured in units of Watts (W), and each Watt represents one Joule of energy being used each second.

The total amount of energy transformed is equal to the power multiplied by the time.


energy power and time


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