Open and Closed Circuits

One-sentence summary

When a circuit is closed, electric current flows and the components work; when a circuit is open, the path is broken and no current flows.

Why does it matter?

Almost every electronic device works on the idea of open and closed circuits. When you press a light switch, you are really opening or closing a circuit. Stopping a robot's motor, turning on an LED, sounding a buzzer; all of these come down to whether current flows or not.

If you understand this lesson, you can find out why a circuit is not working more easily. Usually the problem is simple: the path is broken somewhere, which means the circuit has been left open without you noticing. We will also meet one of the most dangerous mistakes, the short circuit.

Let us start with a definition. A circuit is a closed path that electric current can travel through. For current to flow, this path has to be unbroken from start to finish.

When does current flow?

Closed circuit: the path is complete

In a closed circuit the current's path is not broken anywhere. Current leaves one end of the battery, passes through the components, and returns to the other end. Because the path is complete, current flows and the LED lights up, the motor turns, or the buzzer sounds.

Think of it this way: for water to flow through a garden hose, both ends must be open and the inside must be unbroken. In electricity, the current also needs the path to form an unbroken loop.

Let us write a simple closed circuit as a text schematic. The components are a battery (3 V, that is 2×AA), a resistor and an LED.

   +  ----[ Resistor ]----▶|---- 
  (Battery 3 V)          (LED)   |
   -  ------------------------- 

Let us read the same path step by step:

1. Current leaves the + (plus) terminal of the battery.
2. It passes through the resistor. The resistor limits the current to protect the LED.
3. It passes through the LED, and the LED lights up. (The triangle in the LED symbol ▶| shows the direction of the current.)
4. Current returns to the − (minus) terminal of the battery.

Because the path is closed from start to finish, the LED lights up. This is called a closed circuit.

Open circuit: the path is broken

In an open circuit the path is broken somewhere. Maybe a wire has come loose, or a connection is not tight. Because the path is not complete, current cannot flow and the LED stays off.

   +  ----[ Resistor ]----  X   ---▶|---- 
  (Battery 3 V)         (break here)  (LED)  |
   -  --------------------------------- 

The X here is the point where the path is broken. Current cannot jump across that gap, so the LED does not light up. If a circuit is not working, the first question in your mind should be: "Is the path left open somewhere?"

Two everyday examples:

• Flashlight: When you switch it on, an inside switch closes, the circuit becomes closed and the bulb lights up. When you switch it off, the switch opens the circuit; the path breaks and the bulb goes dark.
• Fridge light: When you open the door, a small button on the frame is released, the circuit closes and the light turns on. When you close the door, the button is pressed, the circuit opens and the light goes off. This is why we imagine the light is always on inside the fridge, but it is not.

The role of the switch

A switch opens and closes a circuit

A switch is the component that lets us open and close a circuit on purpose. In the closed position it completes the path and lets current pass. In the open position it leaves a gap in the path and stops the current.

So a switch is really a "break" that you can control. When you want, it joins the path; when you want, it separates it.

Let us add a switch to our earlier circuit:

   +  ---[ Switch ]---[ Resistor ]---▶|--- 
  (Battery 3 V)                     (LED)  |
   -  ----------------------------------- 

Now the circuit can be in two different states:

Switch CLOSED  →  path complete →  current flows →  LED on
Switch OPEN    →  gap in path   →  no current    →  LED off

Notice this: in electronics, "switch closed" means the circuit is working. In everyday language the word "close" usually means to stop something, so this can be confusing at first. Here, remember: closed = path joined = current flows.

Switches in everyday life

• Wall lamp button: When you press the button, two metal parts touch each other, the circuit closes and the lamp lights up. (Careful: throughout this lesson we work only with low-voltage sources such as batteries, never with a wall socket or mains electricity.)
• Game controller button: Each button on a game controller is a small switch. When you press it, the circuit inside closes and a command is sent; when you release it, the circuit opens again.

Short circuit: the dangerous shortcut

What is a short circuit?

A short circuit is when current finds a direct path between the + and − terminals of the battery without passing through components such as the resistor and LED. In other words, the current takes a "shortcut."

Why do we not want this? Because components limit the current, and the resistor keeps it at a safe level. If the current skips these components and returns straight through a wire, the current becomes very large. When this happens:

• The battery heats up quickly.
• The wires can get hot.
• The battery drains, and with some battery types this can become dangerous.

   +  ------------┐
  (Battery 3 V)   │  ← no component, plain wire: SHORT CIRCUIT
   -  ------------┘

In this schematic the current returns directly from one terminal of the battery to the other without meeting any resistance. Never build a connection like this on purpose.

How do we avoid a short circuit?

• Always have a load in your circuit: if you are lighting an LED, use a suitable series resistor.
• Do not connect the + and − terminals of the battery directly with a bare wire.
• Make sure the bare ends of wires do not touch each other by accident.

As a small reminder, let us calculate the series resistor simply. We use Ohm's law from the previous lesson: R = V / I.

• Battery voltage: 3 V
• Voltage across the LED: about 2 V (so 3 − 2 = 1 V is left across the resistor)
• Current we want through the LED: about 10 mA = 0.01 A

R = V / I = (3 V − 2 V) / 0.01 A = 1 V / 0.01 A = 100 Ω

So a resistor of about 100 ohms will run this LED safely. If you do not have exactly 100 Ω, choosing a slightly larger value (for example 150 Ω or 220 Ω) is safer; the LED shines a little less brightly but is not damaged.

Mini activity

Suppose you have these components: a 3 V battery (a 2×AA holder), an LED, a resistor of about 100–220 Ω, a small switch (if you do not have one, you can push a wire end in and out by hand) and connecting wires. Work with an adult.

Steps:

1. Draw the components on paper and follow the current's path with a pencil. Does the path start at the battery and return to it?
2. Build the circuit: battery (+) → switch → resistor → LED → battery (−). Watch the direction of the LED; if you connect it backwards it may not light.
3. Look at the LED while the switch is open. Is it lit? (It should not be.)
4. Close the switch. The LED should light up.
5. Now open and close the switch again and watch when the circuit is "closed" and when it is "open."

Extra question: if you remove the LED, does the circuit become open or closed? (Guess for yourself; we will discuss it in the check questions.)

Common mistakes

Mixing up "closed" and "open"

In electronics, a closed circuit is a working circuit. "Closing" the switch lets the current pass. The everyday habit of "close = stop" works backwards here.

Forgetting the series resistor

If you connect the LED directly to the battery, the current becomes too high and the LED can be damaged. A suitable series resistor is always needed.

Connecting the LED backwards

An LED only lets current pass in one direction. If you connect it backwards, the LED will not light even if the circuit looks closed. If it is not lighting, check the direction of the LED first.

Joining the battery terminals directly

Touching one wire to both terminals of the battery at the same time creates a short circuit. The battery heats up and this can be dangerous. Never do this.

Safety note

• In this lesson use only low-voltage sources: batteries (for example 2×AA, 3–6 V), USB, micro:bit or Arduino. Never work with mains electricity, wall sockets or exposed wiring.
• When working with batteries, motors, soldering, cutting tools and hot surfaces, always have an adult with you.
• Watch the polarity (+ and − terminals), always use a suitable series resistor, and do not create a short circuit.
• Lithium batteries can be dangerous when short-circuited or when they heat up; do not join the terminals with a bare wire and do not leave a battery hot. If a battery gets warm, tell an adult and open the circuit immediately.
• If your circuit is not working, first cut the power (open the switch or remove the battery), then check the connections.

Lesson summary

• In a closed circuit the path is unbroken, current flows and the components work.
• In an open circuit the path is broken somewhere, so no current flows.
• A switch lets us open and close a circuit on purpose; in the closed position it completes the path.
• A short circuit is when current skips the components and returns directly, and it is dangerous.
• A safe circuit needs correct polarity, a suitable series resistor, and avoiding short circuits.

Check questions

1. For current to flow in a circuit, how must the path be: open or closed?
2. When the "switch is closed," does the LED light up or go dark? Why?
3. You connected an LED but it does not light. Write two possible reasons why the circuit might be left open.
4. What is a short circuit and why is it dangerous?
5. If you remove the LED from the circuit in the mini activity, does the circuit become open or closed?

Answers

1. The path must be closed. If the path forms an unbroken loop from start to finish, current can flow.
2. The LED lights up. In electronics the closed position of a switch completes the path, so current passes and the LED lights.
3. Examples: a wire end may have come loose, a connection may be loose, the switch may be left open, or the LED is connected backwards (in which case the path is not completed electrically). Writing any two of these is enough.
4. A short circuit is when the current finds a direct path between the + and − terminals of the battery without passing through components such as the resistor and LED. Because the current becomes very large, the battery and wires heat up; that is why it is dangerous.
5. The circuit becomes open. When you remove the LED, a gap is left in the path, the current cannot cross it, and the circuit is not completed.

Source and verification note

For “Open and Closed Circuits”, verification focuses on whether the relationship between When does current flow? and Open circuit: the path is broken remains consistent across examples. Activities are designed only for low-voltage direct-current circuits; mains electricity is not used. Component values must be rechecked for the actual circuit, and connections should be reviewed with an adult before power is applied.

Next lesson

Series and Parallel Connections: We will discover how connecting components one after another or side by side changes the current and the brightness.