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Electromagnets
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True or false? An iron bar placed inside the coil of an electromagnet becomes magnetic.
Check this out! A bunch of scrap metal, lifted into the air, with the help of a magnet! And now -- the magnet drops the scrap! But hang on a moment. A magnet can't be switched on and off, can it?
Hmmm. There are thick wires going to that giant magnet... How does this all fit together? Has magnetism something to do with electricity? Let's state an hypothesis: Electricity can affect magnets.
Let's follow Kim to the lab, and test it! Here are: A light bulb, a couple of batteries, some wires and -- a compass. That's it, now we have a circuit. Since the light is lit up, we know there's an electric current running through the circuit. Now, let's see if the current can influence something magnetic.
Bring in the compass! A compass: that's a small magnet that can move freely. What's going to happen now, if Kim moves the compass closer to the conductor? The compass needle changes direction! Try breaking the circuit, Kim!
Yes, now the compass needle is pointing North again. And if the current is back on... ... the needle will change direction again. The hypothesis seems to hold. There is a connection between electricity and magnetism.
Try moving the compass, to the other side of the conductor. The compass needle turns, and points the other way! Normally, a compass aligns itself to the magnetic field that surrounds the Earth, between the North and South poles. But here, around the conductor, the current causes a magnetic field, that goes around the conductor. That's why the needle turns around when the compass is placed on the other side of the conductor.
The magnetic field gets weaker the further away from the conductor it gets. ... wonder if we could get the magnetic field to reach further - if we could increase the strength of the magnetic field... If current can cause a magnetic field, then perhaps more current can cause a stronger magnetic field? Hypothesis: If we increase the voltage, so that the current increases, the magnetic field will reach further out from the conductor. Change the battery!
Twice the voltage, gives twice the current. And let's test again. Yes! Now the compass is affected further away than previously. Hypothesis verified: More current give a stronger magnetic field.
Hmmmm. Maybe... What if we... place several conductors next to each other? Can multiple magnetic fields, in effect, amplify each other?
Try rolling up the conductor, so the current passes through the same place several times, in the same direction! Yes, exactly, like that, so there are many turns, forming a coil. And now we measure. Yes! Now the magnetic field reaches even further!
This is called an electro-magnet. One cool thing with normal magnets - those that work without electricity - is that they can make an ordinary piece of iron magnetic too. Check this out. The magnet holds the iron bar. And as long as they are close to each other, the iron bar is also magnetic.
Hypothesis: If we place the iron bar inside the coil, then the iron bar will become magnetic.There. There. A bar of iron. Around the iron bar: A coil of copper wire, that conducts an electric current. And the test: Does the iron bar get magnetic?
Yes, indeed! And now curiosity sets in: Has the magnetic field around the coil become stronger from this? Eeeeeh. Yes, a lot stronger. Break the circuit now!
This is a major benefit with electromagnets, compared to ordinary magnets. They can be turned off! Electromagnets are used to open and close doors automatically... To lift metal objects, and drop them... And it's possible to make electromagnets very strong.