Science Fenns K-5Grades K to 5
A flashlight cut open to show a battery, a bulb, a switch and a metal strip forming a complete loop.

Science / All lessons / Energy on the Move

Closed Circuits

Light and heat from a complete loop.

Lesson 21 of 40

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Before we start

Today’s story

Closed Circuits

Jade clicks on a flashlight in a dark tent. The beam shines. She clicks it off. The beam is gone. The switch did that by opening or closing a path.

A circuit is a path for electricity. In a closed circuit the path is a loop with no gaps. Electricity leaves the battery, goes through the wire and the bulb, and comes back. The bulb lights. It also gets warm. Electrical energy changes to light and heat.

Break the loop and the energy stops. A switch breaks the loop on purpose. An open circuit has a gap. Let us build and test. Here are the steps.

See it

Five steps. Every time.

    Hands on

    Light the Bulb

    30 minutes · Do this with a grown-up nearby

    You need

    • One D battery
    • One small flashlight bulb in a holder
    • Two wires with bare ends
    • A paper clip and two brass fasteners on cardboard for a switch
    • Tape
    • Notebook

    Do this

    1. Draw your circuit first.
    2. Attach one wire to each end of the battery with tape.
    3. Connect the wires to the bulb holder.
    4. Watch the bulb light.
    5. Make a switch with the paper clip and fasteners.
    6. Open and close the switch.
    7. Feel the bulb after one minute and record how warm it is.
    Write or draw: A drawing of the circuit with arrows showing the loop, and a sentence about the bulb with the switch open and closed.

    Watch for: Notice that the bulb lights only when the loop is closed. Notice the bulb warms.

    Safety: Use only a flashlight battery, never touch a wall outlet, and let the bulb cool before you touch it.

    Same every time

    Three rules

    Rule 1A closed circuit is a full loop.

    No gaps.

    Rule 2A switch opens or closes the loop.

    It is a gate.

    Rule 3Use batteries only.

    Never use wall outlets.

    Grandpa Fenn’s notebook

    Say it like a scientist

    circuit
    A path that electricity can follow.
    closed circuit
    A loop with no gaps so electricity can flow.
    switch
    A part that opens or closes a circuit.
    battery
    A part that stores and supplies electrical energy.

    Poppy’s quiz

    Six questions

    Tap an answer. Green means right. You will see why.

    Talk about it

    Say it out loud

    • How do you know the circuit is closed?
    • What switches do you use at home, and what do they turn on?
    Did you know

    The word circuit comes from a Latin word that means going around.

    Someone who does this

    An electrician installs and fixes circuits in homes and barns.

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