Slide 1 / 162 Slide 2 / 162 Magnetism www.njctl.org Slide 3 / 162 Slide 4 / 162 Table of Contents How to Use this File Click on the topic to go to that section Each topic is composed of brief direct instruction · The Nature of Magnetism There are formative assessment questions after every topic · · Magnetic Fields denoted by black text and a number in the upper left. · · Origin and direction of Magnetic Fields Students work in groups to solve these problems but use student · · Magnetic Field force on a moving Electric Charge responders to enter their own answers. · Magnetic Field force on a current carrying wire Designed for SMART Response PE student response systems. · · Magnetic Field due to a long, straight current carrying wire Use only as many questions as necessary for a sufficient · · Magnetic Field force between two current carrying wires number of students to learn a topic. · Mass Spectrometer Full information on how to teach with NJCTL courses can be · · Summary found at njctl.org/courses/teaching methods Slide 5 / 162 Slide 6 / 162 History The Nature of Magnets were first discovered over 2000 years ago by the Chinese and the Greeks and were used for various non scientific Magnetism purposes. The name was coined by the Greeks, as certain magnetic rocks (magnetite) were found in the province of Magnesia. Unlike electrical effects due to the rubbing of various substances, like amber, to separate the electrical charges so there would be attractive and repulsive forces, these magnets came out of the ground already attracting and repelling certain materials. Return to Table of Contents
Slide 7 / 162 Slide 8 / 162 Magnet Properties History Magnets have two It wasn't until after the 1000 A.D. that Chinese, European and ends (poles) called Persian mariners separately used magnets for navigation. north and south. When a magnetic material, shaped in the form of a needle and Like poles repel; floated on the surface of water, it always pointed in the same unlike poles attract. direction - towards the north. This attraction or Always being able to tell which direction was north was a critical repulsion is the factor in ushering in the age of exploration. magnetic force. It wasn't until 1600 when this phenomenon was explained by William Gilbert. But first, the nature of magnetism will be discussed. These are examples of bar magnets. Slide 9 / 162 Slide 10 / 162 Magnetic Poles Magnetic Poles and Electric charges When a magnet is cut in half, each piece still has a north and a south pole. No matter how many times the magnet is cut, the pieces still The behavior of magnetic poles (north and south) are similar to have a north and south pole. electric charges (positive and negative) where opposite poles/ charges attract and like poles/charges repel. This works all the way down to the atomic level! There are two significant differences between these effects. One, certain materials are naturally magnetic, where electrical properties result from physical rubbing. And secondly - there are independent positive and negative charges, but magnetic materials always contain a north and a south pole. Slide 11 / 162 Slide 12 / 162 1 What are the two kinds of magnetic poles? 2 Which of the following combination of magnetic poles will exert an attractive force on each other? A North and Negative. A North and North. B South and Positive. B North and South. C Postive and Negative. C South and South. D North and South.
Slide 13 / 162 Slide 14 / 162 4 Explain the similarities and the differences between 3 It is possible to find a magnet that only has a north pole? electric charges and magnetic poles. Students type their answers here Yes No Slide 15 / 162 Slide 16 / 162 Magnetic Fields Electric field lines were used to show how electric charges would exert forces on other charges. A similar concept will be used in Magnetism. Magnetic Fields What's nice about Magnetic field lines is that they are more easily Return to Table "seen." The above is a picture of iron filings sprinkled on a paper of Contents on top of a bar magnet. Slide 17 / 162 Slide 18 / 162 Magnetic Fields Magnetic Fields The iron filings act like little bar magnets, and align with the Arbitrarily, magnetic field lines are defined as leaving the north magnetic field of the large magnet. pole of the magnet and reentering at the south pole as seen below. The lines specify the direction that the north pole of a magnet will The blue line point to. shows a magnetic The more lines per unit field line going area, the stronger the field. through the magnet, and The red line completing the shows a The lines that seem not to be loop outside of the complete in loops are - we just ran out picture. magnetic field N S of room on the slide. All line loop. magnetic field lines form complete loops. The field exits one end of the magnet and returns to the other The field lines also go right through the magnet, but are end. Note also, that the field lines extend through the magnet, left out of the picture so you can see the pole labels. making a complete loop (unlike Electric Field Lines).
Slide 19 / 162 Slide 20 / 162 Magnetic Fields Magnetic Fields Like Electric Fields, different configurations of magnets will Like Electric Fields, different configurations of magnets will produce interesting Magnetic Fields. produce interesting Magnetic Fields. Here are two magnets with their opposite poles next to each other Here are two magnets with their north poles next to each other - - these magnets are attracting each other. these magnets are repelling each other. Slide 21 / 162 Slide 22 / 162 5 Magnetic field lines are drawn to represent the direction 6 Magnetic field lines: of the magnetic field at points in space. What convention is used for the directions? Select two answers . A start on a north pole and extend to infinity. B start on a north pole and extend to infinity. A Magnetic field lines originate on a south pole. C start on a south pole, loop around and return to a B Magnetic field lines originate on a north pole. north pole. C Magnetic field lines terminate on a south pole. D start on a north pole, loop around and return to a D Magnetic field lines terminate on a north pole. south pole. Slide 23 / 162 Slide 24 / 162 The Earth's Magnetic Field The Earth's Magnetic Field The Magnetic Field extends from the core to the outer limits of The Earth’s magnetic field is similar to that of a bar magnet. the atmosphere (magnetosphere). It is caused by the circulation of molten iron This picture (not to scale) shows the interaction of the solar alloys in the earth's outer wind (ions and electrons) with the earth's magnetic field that core (more on this later). produces the magnetosphere. The Earth’s “North Pole” is really a south magnetic pole as the north ends of magnets are attracted to it. The magnetic poles are not located along the earth's axis of rotation.
Slide 25 / 162 Slide 26 / 162 The Earth's Magnetic Field The Earth's Magnetic Field The Magnetic Field protects life on earth from being subjected This kinetic energy to light energy transformation produces the Aurora to ionizing radiation that would cause great harm. The ions Borealis and Aurora Australis. coming from the sun are deflected by the earth's magnetic field at the poles (where it's the strongest), and as they spiral down, they give off much of their energy as light. Slide 27 / 162 Slide 28 / 162 7 The earth's magnetic north pole is located at: 8 Why are the Aurora Borealis and the Aurora Australis produced over the south and north magnetic poles? A the North geographic pole. A Because of the earth's angle of inclination in its B the South geographic pole. orbit about the sun. C in Alaska. B The earth is slightly flattened at both poles. D in Australia. C The earth's magnetic field is weakest at the poles. D The earth's magnetic field is strongest at the poles. Slide 29 / 162 Slide 30 / 162 Magnetic Field Units Magnetic Field Units The symbol for the Magnetic Field is B. The field is a vector and has both magnitude and direction. The unit of B is the Tesla, T, where Because the Tesla is such a large magnitude, another unit is frequently used, the Gauss, G, where To gain perspective, the magnetic field of the Earth at its surface is around 0.5 x 10 -4 T or simply 0.5 G. Nikola Tesla Carl Friedrich Gauss 1856-1943, Inventor, Engineer, 1777-1855 - Mathematician and Physicist. Physicist.
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