When a battery, wires, a bulb and a switch are joined in one unbroken loop, electricity flows and the bulb lights up; this path for electricity is called an electric circuit.
Joining several bulbs one after another on a single path is a series connection; joining them side by side on separate branches is a parallel connection. In series, taking one bulb out turns them all off; in parallel, the others stay on.
Sending a current through a coil wound around a nail turns the nail into a magnet called an electromagnet. More turns make it stronger, and cutting the current takes the magnetism away.
Do experiments only with dry-cell batteries, and never play with the sockets at home.
The path for electricity: an electric circuit
For a bulb to light, the path from one end of the battery has to pass through the bulb and come back to the other end without a break. A switch is a gate that opens and closes one spot on this path, so while it is open the bulb stays dark no matter how many wires are connected. When several bulbs are joined, putting them one after another on a single path or side by side on branches changes how bright they are and what happens when one goes out, so when you look at a circuit, start by counting how many paths it has.
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Turn the switch on and the circuit closes, electricity flows and the bulb lights
💡 What is an electric circuit?
①The path that electricity flows along is called an electric circuit.
②The path has to run without a break from one end of the battery, through wires, a bulb and a switch, back to the other end.
③If the path is broken at even one spot, no electricity flows.
④A switch closes or opens the path to start or stop the flow.
Series and parallel connections
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Choose series or parallel and take out the middle bulb to see what happens to the others
In the first picture, turning the switch on closes the gap in the left-hand wire, small dots travel around the rectangle of wire, and the bulb lights up. In the second picture, choose series and three bulbs sit one after another on the top path, sharing one battery, so they all glow dimly; choose parallel and each branch between the top and bottom wires has its own bulb, so all three glow brightly. Press the remove-the-middle-bulb button: in series the path breaks and all three go dark together, while in parallel only the empty branch goes dark and the other two stay lit. In the third picture, raising the number of coil turns adds loops around the nail and lets more paper clips hang in a chain from its tip, up to five.
🔌 Series connection
①The bulbs are joined one after another on a single path.
②There is only one path, so taking one bulb out turns them all off.
③The bulbs share the same battery, so the more you add, the dimmer each one gets.
④Joining several batteries in series makes the bulbs brighter.
Electromagnets
When wires are drawn in, it is easy to think electricity must be flowing. But if the switch is open, the path is broken and the bulb is off. You might expect that adding more bulbs in series makes things brighter, but the bulbs share the same battery, so each one gets dimmer. A nail wound with a coil is not always a magnet either. It pulls on paper clips only while current flows, and when the current is cut, the clips drop off.
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The more turns in the coil, the stronger the electromagnet and the more clips it holds
5 turns
🧲 Electromagnets
①Wind enamel-coated wire around a nail many times and pass a current through it, and the nail becomes a magnet: an electromagnet.
②More turns in the coil, or a stronger current from more batteries in series, makes it stronger.
③Cutting the current takes the magnetism away.
④Reversing the current swaps the N and S poles.
⑤Electromagnets are used in speakers, motors and cranes that lift scrap iron.
Series: one path (one out, all off) / Parallel: branches (each works on its own)
Electrical devices at home are connected in parallel.
Strength of an electromagnet
More coil turns + more current → stronger
An electromagnet is a magnet only while current flows.
🎯 Key points
①An electric circuit is an unbroken path of battery, wires, bulb and switch.
②Series has a single path, so one bulb out turns all off, and more bulbs means dimmer bulbs.
③Parallel has branches, so one bulb out leaves the rest on, and brightness hardly changes.
④An electromagnet is a magnet made from a coil and a current, and it loses its magnetism when the current stops.
⑤The slider in the picture changes only the number of turns, but a stronger current also makes an electromagnet stronger.
Examples and Unit Test
Example 1
Two bulbs are joined in series. If one is taken out, what happens to the other?
1
In series, the bulbs sit one after another on a single path, so there is only one way for electricity to go.
2
Taking one out breaks the path, so no electricity flows and the other bulb goes off too.
▸ The other bulb goes off as well.
In the second picture, choose parallel and take out the middle bulb: the other two stay lit.
Example 2
An electromagnet is made by winding a coil around a nail and passing a current through it. How can it be made stronger, and how is it different from a permanent magnet?
1
Winding more turns or using a stronger current makes the electromagnet stronger.
2
Cutting the current takes its magnetism away. Being able to switch it on and off is what makes it different from a permanent magnet.
▸ Wind more turns or use a stronger current. / It loses its magnetism when the current is cut.
In the third picture, ten turns of coil hold a chain of five clips.
Unit-test style
Which statement about electric circuits is correct?
①In a parallel connection, taking out one bulb turns them all off.
②An electromagnet loses its magnetism when the current is cut.
③Electricity flows even if a circuit is broken in the middle.
④In a series connection, more bulbs make each one brighter.
⑤An electromagnet gets stronger with fewer turns of coil.
▸ ② An electromagnet loses its magnetism when the current is cut.
1
An electromagnet is a magnet only while current flows, so cutting the current takes the magnetism away.
2
In parallel, the other bulbs stay on (①), no electricity flows in a broken circuit (③), more bulbs in series make each one dimmer (④), and more turns make an electromagnet stronger (⑤).