Magnetism And Electromagnetism
Visão Geral
In a scrapyard, a crane swings a flat steel disc over a heap of wrecked cars, lowers it, and lifts a tonne of metal into the air with nothing gripping it. No hook, no chain, no clamp. Then the operator flicks a switch and the whole load drops into the wagon below. Whatever was holding that car up was switched on, and then it was switched off, and in between it reached across a gap of empty air and pulled.
That disc is a coil of wire wound on a lump of iron, and by the end of this lesson you will be able to say exactly why it works, what makes it stronger, and why it lets go the instant the current stops. Along the way you will learn to read the invisible patterns around magnets well enough to sketch them from memory, to work out which end of a coil has become a north pole using nothing but your own right hand, and to explain the one fact that lets a magnet pick up a paper clip that was never a magnet at all.
Objetivos
- 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.
- A magnetic field is produced when an electric current flows through a wire. The magnetic field lines are concentric circles in a plane, perpendicular to the wire: the field is stronger closer to the wire; increasing the current makes the magnetic field stronger; reversing the current reverses the direction of the magnetic field lines.
- Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.
- The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the magnetic field strength. An electromagnet consists of a solenoid with an iron core. Students should be familiar with some typical uses of electromagnets. Required practical: Investigate the factors that determine the strength of an electromagnet.
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Nota de Aula
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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
- What happens to the magnetic field lines around a straight wire when the direction of the current is reversed? A. The lines become straight and parallel to the wire B. The lines keep the same shape and spacing but point the opposite way round C. The lines move twice as far apart D. The field disappears Answer: B
- What is the magnetic field like inside a solenoid carrying a steady current? A. Zero everywhere B. Strong and uniform C. Weak and uniform D. Strong only at the two ends Answer: B
- 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
- Which change would NOT increase the strength of an electromagnet? A. Increasing the current in the coil B. Increasing the number of turns on the coil C. Replacing the iron core with a plastic core D. Winding the turns of the coil more closely together Answer: C
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