Series versus parallel circuits

The idea

In a series circuit every component carries the same current and one failed component breaks the whole circuit; a parallel circuit divides the current among components and lowers total resistance, letting each draw a different current.

Festival lights

A long string of small coloured bulbs is hung for a festival. One bulb inside the string burns out. The string may behave in one of two ways, depending on how the bulbs are joined.

If the bulbs are joined one after the other in a single path, what happens when one fuses?

• The whole string goes dark
• Only the fused bulb goes dark
• The other bulbs glow brighter

Fuse a bulb

Choose in series or in parallel, then make the middle bulb fuse.

Two different circuits

In series, the same current passes through all the bulbs, and one failure breaks the only path. In parallel, the current is divided among the bulbs, each draws the current it needs, and a failure in one branch leaves the others complete.

In a series circuit every component carries the same current and one failed component breaks the whole circuit. A parallel circuit divides the current among components and lowers the total resistance, letting each draw a different current.

Notes

A series circuit has the same current everywhere and one failed part breaks it all; a parallel circuit divides the current, lowers the total resistance, and lets each part draw a different current.

Check yourself

In a series circuit one bulb fuses. What happens to the other bulbs?

Why is it impracticable to connect an electric bulb and an electric heater in series?

Why does an electrician take a long time to find a dead bulb in a string of festival lights joined in series?

What does a parallel circuit do to the current through the gadgets?

• They glow as before. In a series circuit a failed component breaks the whole circuit, so none of the components works.
• None of them works, because the circuit is broken — correct. Yes!
• They glow brighter. The circuit is broken, so no current flows through any of them.
• The heater is bigger than the bulb. The problem is not size. In a series circuit the current is the same through both, while they need widely different currents.
• A bulb cannot carry any current. A bulb carries current. The difficulty is that in series the same current would flow through both.
• They need currents of widely different values, but in series the current is the same — correct. Yes!
• Each bulb has to be tested to find which one has fused — correct. Yes!
• Because the dead bulb makes the others brighter. When one bulb fails in a series string the circuit is broken and none works, so the dead one has to be found by testing each.
• Because the string has no wires. The string has wires. The bulbs are in series, so one fused bulb stops all, and each has to be tested.
• It makes the same current flow through each. That is what a series circuit does.
• It divides the current among them, so each can draw a different current — correct. Yes!
• It raises the total resistance. The total resistance in a parallel circuit is decreased.