ExamLearn.ie. Magnetism

Transcription

1 ExamLearn.ie Magnetism

2 Magnetism If you hold a pin close to a magnet, you will feel a pull. This pulling force is called magnetism. A magnet is a piece of metal that can attract other substances to it. Only three of the elements on the Periodic Table can be magnetized. These are iron, nickel and cobalt. Attraction and repulsion Magnets are strongest at either end. These ends are called poles. They are referred to as the north pole and the south pole. Experiment (not mandatory) *To demonstrate the attraction and repulsion of magnets Apparatus required: two bar magnets, paper stirrup, string, non-metal retort stand 1. Hang one bar magnet in paper stirrup from retort stand. 2. Bring south pole of second magnet close to north pole of hanging magnet. Note whether the magnets attract or repel each other. 3. Now bring north pole of second magnet near north pole of hanging magnet. Record result on table. Then try other combinations. Result Hanging magnet Second magnet Result North pole South pole Attract North pole North pole Repel South pole South pole Repel

3 South pole North pole Attract Hanging magnet Second magnet Result *Like poles repel, unlike poles attract. Magnetic fields A magnetic field is the space around a magnet over which it has a magnetic effect. Mandatory Experiment

4 *To plot magnetic field of bar magnet using iron filings Apparatus required: bar magnet, iron filings, card 1. Put bar magnet on bench. Put card on top of it. Draw the outline of position of magnet on card. 2. Sprinkle iron filings evenly all over card. 3. Tap paper gently with fingers. Result The iron filings are influenced by strong magnetic forces from poles of bar magnet beneath. They are forced to point themselves along direction of magnetic field. Thus, they show the magnetic field around the bar magnet. Magnetic field lines always go from north pole to south pole.

5 The magnetic field of the earth The earth has a magnetic field. This magnetic field forces all free magnets to line up in a northsouth direction. This is why we can use a free magnet to locate north and south. We call this magnet a compass. A compass is a magnet, which is free to rotate and indicate direction. Experiment (not mandatory) *To demonstrate that earth has magnetic field and to locate north/south direction Apparatus required: bar magnets, paper stirrup, string, wooden retort stand 1. Hang bar magnet in paper stirrup from retort stand. The magnet should be horizontal and free to swing. 2. When magnet comes to rest, note position in which it is pointing. 3. Repeat experiment for other bar magnets. Result and conclusion Each bar magnet comes to rest with one end pointing in a northerly direction. This end is north pole of magnet. The opposite end, or the south-seeking' pole, is referred to as the south pole. The magnets are responding to the magnetic field of the earth. The Magnetic effect of a current Experiment (not mandatory) *To show the magnetic effect of a current

6 Apparatus required: battery, switch, plotting compass, wire 1. Allow compass needle to come to rest in its usual north-south direction. 2. Hold straight wire parallel to compass needle over compass. Connect wire in circuit to a battery and switch. 3. Close the switch for a few seconds. This allows current to flow in the wire. Result The compass needle moves at right angles to wire. Conclusion This movement is caused by the magnetic effect of current in the wire. Because it is very near, the magnetic effect of current is greater than the magnetic effect of earth. The electromagnet An electromagnet consists of an iron rod surrounded by an insulated coil of wire. An electromagnet also demonstrates the magnetic effect of an electric current. The simple electromagnet consists of an insulated wire wrapped closely around an iron nail. A power pack is used to drive a current through this wire. When the power supply is switched on, the iron nail attracts the paper clips. When the current is switched off, the nail loses its magnetism. The paper clips fall off. While a current flows through the wire, the iron nail is a strong magnet.