What Is a Series Circuit? (page 2)
You should notice that the lights in the circuit you made with two lights and one battery are not shining as bright as when there was just one light. When you added a second battery, the two lights should shine as brightly as one did with one battery.
The green plastic covering the wires acts as insulator. An insulator prevents an electrical current from flowing. The green plastic covering on the Christmas tree lights insulates your Christmas decorations and you from the flowing electrical current. Remember that a circuit is a circle of electrical current. When you first made the Christmas bulb light up, electrical current flowed from negative pole of the battery, through the bulb, and through the battery back to the positive pole of the battery. The electrical current encountered some resistance in the thin wire making up the light bulb. Some of the electrical energy was transformed into light, and a bit of heat. When the second bulb was added, all the electrical energy had to be shared equally between the two bulbs. Since there was more resistance, but the same amount of power, the bulbs did not glow as brightly. When you added another battery, which added more voltage, the bulbs glowed brightly again.
You might try doubling the power with one light bulb. It is also interesting to compare how brightly some color lights glow compared to other colors. Do you see a pattern? If this intrigues you, investigate why red lights are used in photo developing rooms. Lastly, once you understand the series circuit, you should investigate parallel circuits.
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