What Is a Series Circuit?

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Updated on Nov 15, 2013

Electricity is both commonplace and mysterious.We rely on it every day to run our lights and appliances, but how does it work?

An electrical current occurs when electrical charges are in motion.You can think of an electrical current the same way you think of a current in the water.Some water currents flow slowly,while other water currents, like mountain streams,flow quickly.How fast a current flows depends partially on the water pressure.If the pressure is high, the current flows very quickly.The term used for electrical pressure is called voltage.Another factor that determines how fast water flows is the size of pipe.If the pipe narrows, the water’s overall flow rate slows down.In the world of electricity, factors that slow electrical flow down are called resistors.The thin coiled wire inside a light bulb is a resistor.When electrical current flows through a light bulb, it slows down, and some of the electrical energy is changed into light.This resistance generated light is used to illuminate the rooms of our house, and to make our holidays festive.

Speaking of holidays, Christmas tree lights are a great source of cheap light bulbs and wire for hands-on explorations of electricity.If you get a pair of strippers, and couple batteries, you can build your own circuit, which is a controlled circular path of electricity.The first type of circuit you should build is a series circuit. In a series circuit, the electric charge flows in only one circular path.Things get interesting when you add resistors, and additional sources of power.


What is a series circuit?


  • Strand of Small Christmas Tree Lights (it would be great to recycle an old set)
  • Copper Wire Strippers (available at hardware stores)
  • Two Name brand AA batteries (low cost versions can overheat)
  • Electrical tape
  • Clean, dry, non-metallic table to work on.


  1. Make sure your hands are dry.
  2. Using the cutting part of strippers, cut the light strand at a point halfway from the next light on each side of the desired light bulb.
  3. Cut out another bulb in the same way.
  4. Using the strippers, strip the green plastic off the last 1/4 inch or so of wire on either end of the cut light bulbs. What purpose does the green plastic serve?
  5. Use the electrical tape to secure the stripped wire on either side of the battery.The bulb should light up. If it doesn’t, make sure the stripped wire is making full contact with the battery.If it still doesn’t work, try another bulb.Once you have made the bulb light up, you have created a working circuit. Which way are the electrical charges moving?
  6. Now it is time to make a series circuit.Remove the tape the wire from the positive pole of the battery.Why does the light go out?
  7. Twist one strand of the other bulb with wires onto one strand of the original bulb.Secure the other end back on to the positive pole of the battery.What happens?
  8. Remove the tape the wire from the positive pole of the battery again.
  9. Position another battery onto the first the first battery, so that that positive pole of one is touching the negative one of the other.
  10. Recreate your circuit.What do you notice about the brightness of the lights?


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 positivepole 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.

Going Further

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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