Now imagine a simple circuit that uses a light bulb (or LED) and a battery. The battery provides the energy to do work on the charges to move it from the negative to the positive terminal. Once the charge is at the high potential (the plus side of the battery), it’s like taking a chair lift to the top of a mountain… it is now ready to ski down the mountain with little to no effort. So once the charge is at the high potential terminal, it naturally flows through the wires to the low potential terminal. The ski lift is doing work to get you up the mountain against the nature of the gravitational field the same way the battery is doing work on the electric charge moving it from a low to high potential.
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The battery is the internal circuit, because energy is given to the charge. The external circuit is the part where the charge loses energy by moving along the wires and lighting up LEDs, making buzzers sound, turning motors, etc. Each element in the circuit takes energy from the charge (even the wire itself) and transforms it into something useful or not. Light, sound, motion are all useful forms of energy. The heat coming from an incandescent light bulb would be non-useful thermal energy.

As the charge moves through a device, it starts out at a higher energy than it leaves the device with, so there’s a voltage drop across that circuit element. The charge returns to the low potential side of the battery at zero volts and is ready to be pumped back up to the high voltage positive terminal.

Click here to go to your next lesson on reviewing the electric potential difference.

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