Electricity: Magnetic and Heating Effects · Lesson 2 of 7
Making an Electromagnet
“Build the idea of an electromagnet step by step, from a current-carrying coil to a stronger iron-core magnet.”
• Explain how a current-carrying coil can behave like a magnet. • Describe how an iron core strengthens the magnetic effect of a coil. • Define an electromagnet and distinguish it from a permanent magnet. • Interpret compass and paper-clip observations made with a coil. • Explain why an electromagnet is called a temporary magnet.
From a Straight Wire to a Coil
A straight current-carrying wire produces a magnetic field. If the wire is wound into many turns to form a coil, the magnetic effects of the turns act together. This makes a coil a useful way to produce a noticeable magnetic effect in a small region.
A simple investigation begins by winding insulated wire tightly around an iron nail and connecting the ends of the wire to a cell for only a short time. When the circuit is complete, the nail can attract iron paper clips. When the connection is removed, the clips fall away.
A Coil Behaves Like a Magnet
A more careful test uses a cylindrical coil and magnetic compasses placed near its two ends. When the coil is disconnected, the compass needles keep their usual direction. When current flows through the coil, the needles deflect. This shows that the current-carrying coil is producing a magnetic field.
The coil does not need an iron nail in order to show a magnetic effect. Even the coil by itself can deflect a compass when current flows. The iron core is added because it makes that magnetic effect stronger.
A current-carrying coil that behaves like a magnet is called an electromagnet. In many practical electromagnets, an iron core is placed inside the coil to make the magnetic effect stronger.
Why the Iron Core Matters
When an iron nail is inserted into the coil and the experiment is repeated, the compass shows a greater deflection and the ends of the nail can attract iron or steel paper clips. The comparison is important: the coil already behaves like a magnet, but the iron core makes the electromagnet stronger.
| Setup | What is observed | What it tells us |
|---|---|---|
| Coil, no current | Compass shows no extra deflection caused by the coil | No magnetic field is being produced by current |
| Coil with current | Compass deflects | A current-carrying coil behaves like a magnet |
| Coil with current and iron core | Greater compass deflection; clips are attracted | The iron core strengthens the electromagnet |
| Current switched OFF | Extra magnetic effect disappears | The electromagnet is temporary |
In the source activity, the coil is connected to the cell for only a few seconds. Leaving it connected unnecessarily can weaken the cell and the wire may become warm.
Problem
Two identical coils are connected to identical cells. Coil A has no iron core. Coil B has an iron nail inside it. Which observation would show that the iron core strengthens the electromagnet?
- 1.Place a compass in the same position near each coil while current flows.
- 2.Compare the amount of compass deflection.
- 3.A larger deflection near Coil B, together with stronger attraction of iron clips, supports the conclusion that the iron core strengthens the magnetic effect.
Quiz
What is an electromagnet?
What happens when an iron core is inserted into a current-carrying coil?
Which observation shows that a coil without an iron core can still act magnetically?
Why is an electromagnet described as temporary in this chapter?
When current through the coil is stopped, what should happen to iron clips held by the electromagnet?
Practice Problems
- Explain how the paper-clip experiment demonstrates that electric current can make a temporary magnet.
- Describe the purpose of placing compasses near the ends of a cylindrical coil.
- Compare a current-carrying coil with and without an iron core.
- A student removes the iron nail but keeps current flowing through the coil. Predict whether a nearby compass will still deflect and explain your reasoning.
- Write a short explanation of why an electromagnet can be switched ON and OFF.
Key Takeaways
• A current-carrying coil behaves like a magnet. • A coil can show a magnetic effect even without an iron core. • An iron core makes the electromagnet stronger. • An electromagnet depends on electric current, so its magnetic effect can be switched ON and OFF.