Chapter 7: Magnetism and electromagnetism
The chapter problem: the scrapyard crane
A scrapyard uses a crane to move scrap metal. The crane has a large electromagnet instead of a hook. The electromagnet is a coil of wire wound on an iron core.
These are the facts to use all the way through the chapter.
- The crane lifts steel scrap that is mixed with aluminium and copper.
- The electromagnet coil has 600 turns wound on an iron core.
- The current through the coil is 4.0 A when the magnet is lifting.
- The crane can pick up the steel and then release it.
- A small loudspeaker in the office makes announcements to the yard.
Later in the chapter, the yard is powered from a 12 000 V supply. A transformer steps this down to 240 V. The primary coil has 4000 turns, and the yard uses 24 kW of power.
By the end of the chapter you will explain why the crane uses an electromagnet, why the magnet attracts steel but not aluminium, and how the current is switched off to release the load. You will describe the magnetic field of the coil. At Higher tier you will also work out the force on a current in a magnetic field, the turns of a transformer, and how a loudspeaker makes sound.
First contact: no working yet. Predict. A steel can and an aluminium can are held near the crane magnet. Will the magnet pull both cans equally, pull only the steel can, or pull only the aluminium can? Write your prediction down.
If you are not sure where to start
Think about which metals a magnet can attract. A magnet is a non-contact force, so it can pull on a metal before the two touch. Not every metal behaves in the same way.
By the end of this chapter
You will be able to:
- describe the attraction and repulsion between the poles of two magnets;
- tell a permanent magnet from an induced magnet;
- describe the magnetic field of a magnet, including its direction and where it is strongest;
- explain how a plotting compass gives evidence that the Earth's core is magnetic;
- describe how a current creates a magnetic field, and how a solenoid and an iron core make an electromagnet.
A few skills are for Higher tier only: the motor effect and Fleming's left-hand rule, the force on a current-carrying conductor, electric motors, the generator effect, transformers and the national grid, moving-coil microphones and loudspeakers.
Where it is used
Magnets are hidden inside many machines. A motor turns because a current in a magnetic field feels a force. A generator does the opposite, and makes a current from movement. Transformers move electrical energy around the country at high voltage. A loudspeaker, a microphone and a doorbell all use the link between magnetism and electric current. The scrapyard crane is one more example of the same ideas.
Your turn to connect it
Where in your own week does a magnet hold, move or switch something? Write two examples. One should be a magnet you can see or feel, and one should be hidden inside a device.