Magnetism And Electromagnetism
Übersicht
Somewhere on a hillside in thick fog, with no landmark in sight and no signal on her phone, a walker takes a small plastic case out of a pocket and rests it flat on her glove. A slim needle inside swings, hesitates and settles. She turns the case right round and the needle holds its direction as though it had been pinned to the horizon. Nothing touched it, nothing is attached to it, and it will do exactly the same thing on any hillside on the planet.
That needle is a tiny magnet, and it is being steered by an object twelve thousand kilometres across: the ground under her boots. By the end of this lesson you will be able to say what fills the space between a magnet and the thing it pulls, describe the invisible patterns around one magnet and around two well enough to sketch them from memory, predict from a picture whether a pair of magnets will snap together or shove apart, and explain the one fact that lets a magnet lift a steel paper clip that was never a magnet at all.
Ziele
- Magnetic forces are strongest at the poles of a magnet. When two magnets are brought close together they exert a force on each other. Two like poles repel each other and two unlike poles attract. Attraction and repulsion between two magnetic poles are examples of non-contact forces. Students should be able to predict the interaction between magnets given their physical arrangement.
- The space around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt, nickel) is called a magnetic field. The strength and direction of a magnetic field change from one point to another. Students should be able to recognise magnetic field patterns using one or two bar magnets. In a uniform magnetic field the lines of the magnetic field are parallel.
- An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly. Students should be able to explain how a magnet attracts a magnetic object by inducing a magnetic field around it.
- The earth has a magnetic field that is most concentrated at the magnetic north and south poles. Students should be able to explain how a plotting compass can be used to detect the earth’s magnetic field and to assist in navigation. 34
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- Which list contains only magnetic materials? A. Iron, steel, cobalt, nickel B. Iron, copper, steel, nickel C. Aluminium, cobalt, steel, iron D. Nickel, brass, cobalt, iron Answer: A
- A grey bar may or may not be a magnet. Which observation proves that it is a magnet? A. It is attracted to the north pole of a known magnet B. It is attracted to the south pole of a known magnet C. It is pushed away from the north pole of a known magnet D. It picks up a steel paper clip Answer: C
- How are the magnetic field lines arranged in a uniform magnetic field? A. Parallel to each other and evenly spaced B. Concentric circles about a single point C. Crowded together at one end and spread out at the other D. Crossing each other at regular intervals Answer: A
- An unmagnetised iron nail is held close to the north pole of a bar magnet. Which statement is correct? A. The nail is repelled, because a north pole is induced in its near end B. The nail is attracted, because a south pole is induced in its near end C. The nail is unaffected, because it is not a magnet D. The nail is attracted, because a north pole is induced in its near end Answer: B
- Two bar magnets are placed in line with their north poles facing each other across a small gap. What is found at the midpoint of the gap? A. The strongest field anywhere in the arrangement B. A neutral point, where the two fields cancel C. A point where two field lines cross D. An induced south pole Answer: B
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