615-0300 (20-030) and 615-0305 (20-035) Electromagnet Written by Dr

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615-0300 (20-030) and 615-0305 (20-035) Electromagnet Written by Dr. P.G. Mattocks Introduction: Activities few inches above the surface. This compact electromagnet is able If conditions are right, sur- to lift 100 times its weight - up to 200 faces smooth, tray loaded and pounds (20-030) or 500 pounds (20- Demonstrate Holding Force 035), with only one or two 1 1/2 volt D A quick way to demonstrate holding hanging correctly, you can cell batteries as a power source! This is force is to connect batteries, place core carry over 200 pounds. due to precision machining of the flat and yoke together, and try to pull apart Reverse connections and confirm surfaces (core and yoke). Compare it to with your two hands. If your surfaces the same load can be carried. cranes that can lift derelict automobiles are flush and connections good, you Variations: Stand on a bathroom scale around a junk yard. These cranes use will not be able to separate the parts. underneath the magnet and pull electromagnets similar in principle to Now disconnect battery. Parts remove down on the yoke. (Take care the this one, although with higher power easily. yoke does not hit your head. Avoid requirements. This style of crane For a quantifiable demonstration, the in ropes!) The reading on scale is practical since it can be switched you will need: will decrease by amount of the force on and off remotely and loaded and • Weights between magnet and yoke. If you unloaded by one person. • Tray for holding weights weigh 240 pounds and the yoke Warranty and parts: • Safety strap separates at a reading of 40 pounds, We replace all defective or missing parts free • Connection to beam1. the maximum holding force is 200 of charge. Additional replacement parts may be pounds. If you weigh less than 200 ordered toll-free. We accept MasterCard, Visa, Hang magnet as shown in Figure 1.2. pounds, this little magnet should be checks and School P.O.s. All products warranted Add known weights to tray until able to bear your full weight. to be free from defect for 90 days. Does not ap- yoke falls off. ply to accident, misuse or normal wear and tear. Intended for children 13 years of age and up. The safety strap will catch the This item is not a toy. It may contain small parts yoke but not the tray, provided Dependence on Current that can be choking hazards. Adult supervision 1. Attach as in first demonstration is required. the tray was initially only a 2. Change the current as shown: For best results: • Do not scratch mating surfaces. batteries or heavy duty Keep mating surfaces clean, smooth beam power supply variable resistor and undamaged. If surfaces do not mate properly, poles appear which connection reduce strength. Smear grease or safety strap to beam oil between use or while storing. Remove before using. electromagnet • Do not operate near a watch, color coil TV or other electrical appliance. Core Figure 2: Connecting Battery & Resistor • Do not switch on current until magnet surfaces are aligned to prevent Yoke damage from sudden impact. 3. Place known weight on tray and vary • To calculate the force required current until it falls. to remove the yoke, compare the Graph load against current and note that weight pan energy contained in the magnetic load does not increase linearly with field with separation of the yoke. It current. This is because the iron is should agree with the 200 pounds well magnetized and approaching found experimentally. table top saturation. (See Theory in following sections.) Operation Requires: Warning: The coil will overheat in a one or two D cell batteries Figure 1: How to Hang Electromagnet few minutes with currents of 2 or more amps.

Dependence on Gap terminal will therefore be negative during cause them to be magnetic. 1. Repeat procedure above. induction and vice versa. These thermal properties are employed 2. Place one or more layers of alu- Because the induction is brief, the in magnetic heat engines. minum foil (about .005 inch, depending value obtained cannot be predicted be- Exploring magnetic field upon type of foil) between electromagnet cause it is too dependent on voltmeter around electromagnet and yoke. response-time. An ammeter can also be 1. Place sheet of cardboard (or Because aluminum is nonmagnetic used to detect the brief flow of current of other nonmagnetic material) over and no free poles appear at iron surface, a few milliamperes that results. upturned face of magnet. the effect of coil current on iron is re- 2. Sprinkle iron filings over the duced. Even one thousandth of an inch of Dependence on magnet face. foil will reduce the holding force to 100 Temperature(Curie Use a small hand-held compass to lbs. This confirms the need for smooth Temperature) identify poles. The North pole of the surfaces in an electromagnet. 1. Hold steel wire coat hanger close compass (that which seeks the Earth’s to electromagnet. Observe attraction. Replace iron yoke with other North Pole) will point to the South pole 2. Heat same short section of wire of the magnet (which seeks the South substances with propane torch. pole of the Earth). 1. In place of yoke try other materials Warning: Do not heat electromagnet! - copper, aluminum, brass etc. Confirm Observe that red-hot steel is no longer that these materials are nonmagnetic.2. Theory attracted but that on cooling magnetism Part One: Background Use core of magnet to lift other iron is restored. At high temperatures, objects in addition to yoke. Use iron Sailors have known of naturally oc- thermal agitation destroys alignment objects with flat surfaces free of rust and curring permanent magnets for thousands within each domain. paint (i.e: base of iron cooking pot; base of years. In the 1800’s, connections When a few spins are shaken out of carpenter’s wood plane). Try to attain between magnetism and electricity were of alignment, their influence is lost holding forces of 40 lbs. studied by Ampere, Maxwell, Biot-Sa- over neighboring spins which in turn vark, Curie etc. Together their equations Remanence are more easily shaken. The magnetic “solve” (accurately predict outcome of) character does not trail off over an ill- 1. Put yoke in place and connect coil given physical situations. But these were defined temperature range but drops to cell for a few seconds so domains are of a phenomenological nature. While at an increasing rate until it vanishes aligned as before. equations quantified the observed math- at a particular material-dependent 2. Disconnect coil. You will need to ematical relationships between electricity temperature (the or apply a few pounds of force to pull yoke Curie Temperature, and magnetism in the form of “laws”, ) For iron, Tc is 770 ° C; in contrast, off. Not all domain walls moved back Tc. they did not give insight into why there red heat becomes just barely visible in when cell was disconnected. Due to the should be two seemingly different but a dark room at about 700° C. There closed magnetic circuit, there are no free related effects as magnetism and elec- is no connection between these two poles even in iron to drive domain walls tricity. Both were seen to depend upon effects but red heat does make a useful back past the few pinning sites. electric charge. thermometer. 3. Pull yoke off to break magnetic Most people believed these inter- There are two other common circuit. Reapply yoke. You’ll see that lit- twined but apparently different effects magnetic materials. The found in tle force is required to pull it off again. flint had common explanations. Einstein’s cigarette lighters, an alloy of iron and Theory of Relativity (1905) provided the Demonstrate Induction curium, is magnetic with Tc of about missing connection. It stated that length, 100° C. Suspend a cigarette lighter on The remanence noted above time and mass transform according to cotton thread and observe as it pulls to the relative velocity between observer may be used to demonstrate the electromagnet. Dip in boiling water; and observed. Even though a stationary induction. the attraction disappears until the flint electric charge produces certain effects 1. When disconnecting coil from cell, cools. on its surroundings (described as an connect coil to DC voltmeter. Some brass screws are nickel-plated. electric field) the electric charge, when 2. Pull off yoke. Voltmeter will The nickel is ferromagnetic with Tc of moving, produces a different effect due briefly register aninduced voltage. 360° C. Despite thin plating, if you to those transformations. This effect is This is because magnetism in the iron is suspend the screw by a brass wire, what is described as the original electric collapsing as domain walls are driven back you’ll see attraction to the electromagnet. field due to static charge plus a magnetic by free poles. This, according to Lenz’s Gentle heating - take care not to melt field caused by the movement of the Law, induces a voltage in the coil in order wire! - will cause the nickel to become charge. In this case, the magnetic field is a to sustain the magnetism. It tries to replace nonmagnetic below red heat but above relativistic correction to the electric field. current that magnetized the iron in the first boiling water temperature. Canada uses In other situations you can regard place. The clip connected to positive cell enough nickel in their “silver” coins to

the electric as a relativistic correction not filled, cancellation is not inevitable, spins of millions of its atoms are aligned to the magnetic field. Generally, neither and each atom, even in a solid, may have in parallel fashion. The piece of Fe, electric nor magnetic fields are special. the properties of a tiny bar magnet. At for instance, even though one continu- Tensor equations in Einstein’s Theory this level thermal agitation causes the tiny ous solid, contains subdivisions called relativistically transform electric and magnets to point in different directions domains. Within one domain, all spins magnetic fields into each other accord- with an overall cancelling effect. The which are essentially parallel differ from ing to velocity. You can regard them as only survivors are those tiny magnets neighboring domains which may point in different manifestations of the same thing which have the right crystal structure different directions, as shown. connected by relativity. In a sense this is and separation between atoms. In such Figure 4 is greatly simplified by the same as the phenomenological laws. cases the Quantum Mechanical Effect enlarging domains and reducing their Relativity was postulated because the (exchange interaction is strong enough number. Actually domains are a frac- velocity of light was the same for all ob- to overcome the thermal agitation. QM tion of a millimeter in size with many servers. But relativity is also connected to asserts that there can be no chance of thousands represented. and between electricity and magnetism. finding 2 electrons spinning in the same The situation can be compared to Einstein’s aim was to connect as many direction at the same position. throwing many magnets (corresponding observations with as small a set of pos- All electrons have a negative charge to domains) into a box (corresponding to tulates as possible. Further theories link which causes mutual repulsion. Antipar- the solid piece). They clump together electromagnetism to nuclear and gravity. allel electrons that QM allows to come trying to stay opposite each other, thus A further advance in the study of close together have a higher energy than cancelling each other out. magnetism unfolded during the early the parallel ones that QM has kept apart. To overcome cancellation, take a short twentieth century. Quantum Mechanics The parallel spin arrangement favors piece of wire (ex. copper) carrying an (QM) “explains” the different types of lower energy. The strength of this effect electric current i. The current is a flow of magnetism found in elements and com- depends on the energy difference which electrons along the wire. The direction of pounds and the fact that other materials in turn depends on how electrons move current is, by convention, that of positive were essentially nonmagnetic. “Explain” around atomic nuclei and crystals. charges opposite to the electron flow. The in this sense means to provide the connec- In pure Fe, Co, Ni and Gd, and in com- current produces a magnetic effect shown tions with other known phenomena such pounds containing one or more of these, by imagining lines (magnetic field) as in as periodicity of the elements, atomic the effect can be strong, leading to paral- Figure 5, below. spectra, electrical conductivity, and, later lel alignment of spins and orbital motion. The line in Figure 5 represents the superconductivity and semiconductivity. These are known as ferromagnets. Pd path the North pole of a magnet follows Part Two: Magnetic Materials and Pt are almost ferromagnet. Other if allowed to move slowly. (If allowed effects come into play in Cr, Mn and the to move freely, it would move too quickly Moving electric charge produces rare earths (other than Gd), La through and overshoot.) magnetic effects. Atoms of all elements Lu, giving rise to complex and beautiful each have at least on electron which spins spiral or alternating alignment of spins. on its own axis and orbits the nucleus. In all cases, at elevated temperatures, Why, then, is magnetism limited in the thermal agitation will eventually destroy real world? In most materials, one or the alignment and cause cancellation. By the 1980’s the vast majority of low more of the possible levels cancel each N S N S other out. Cancellation at all levels is temperature aligned configurations were avoided in only a few materials. understood and predicted (retroactively) from the starting point of knowing how N S The lowest level of cancellation oc- S N curs in the inner orbitals of atoms. These many electrons were on each atom. domain orbitals, when full, have even numbers of (Note that the very first level of domain wall cancellation - that of full orbitals with electrons with equal numbers spinning in Figure 4 - 4 magnetic domains in one opposite directions. The magnetic effects equal numbers of opposite spins - was solid piece of iron of countermovements cancel. (When really an example of the antiparallel orbitals which are not full become full, pairs of spins having the lowest energy as a consequence of electron motion atoms combine to form solids. Cancel- B S lation results.) around the nucleus, the division B into orbitals and the structure of the i i The only atoms that survive this level N of cancellation are the transition and acti- Periodic Table itself.) vide metal atoms. Here the d and f orbit- One more level of cancellation exists. A piece of Fe, Co, Ni or Gd may not Figure 5 - Magnetic fieldB produced by als are not fully involved in the formation current i in straight wire of solids. When the d and f orbitals are necessarily be a magnet, even though the

The lines are clockwise diodes centered on der these conditions, switching off the current Energy Considerations the wire. Those lines close together near the still leaves a bulk magnet. These, called hard It is a common misconception that mag- wire indicate a strong force; those far apart, ferromagnets, are the materials used to make nets provide power at no expense. a weak force. The South pole of a magnet permanent magnets. Of course, the pinning According to Quantum Mechanics, the would move the opposite way with the same of the domain walls make it correspondingly electrons prefer to be aligned within a do- force. However, North and South poles can- difficult to magnetize in the first place. Very main. A domain is a lower energy state. For not be isolated. Put a small magnet as shown high currents are used in short pulses to pre- example, if you heat iron to bright red, you can in Figure 5; it will line up but not move. The vent overheating. measure the extra energy needed to destroy effect of a current carrying coil of wire (an this magnetic state. Conversely, energy is electromagnet) can move. The electromagnet Part Three: Magnetic Circuits given off on cooling. is therefore the sum of all contributions from and the Electromagnet Aligning all domains in the iron core of the all little pieces of wire. 20-030 and 20-035 Electromagnet requires The domain wall movement shown in energy. Such energy is supplied by the battery. Figure 7 will not progress very far. As the pre- Once aligned, no further power is required: ferred domains grow, they build up North and a superconducting coil could carry the same South poles at the ends of the iron. Although current to keep the domains aligned at no that appears desirable, it impedes the magnet’s energy cost. With the copper coil, however, strength. Those “free” poles produce their its finite resistance requires the power of the own magnetic field through the iron in the battery to sustain the current. The electrons in opposite direction. The coils’ effectiveness the iron spin forever. They cannot stop and is therefore impaired. The field is not strong so require no more energy. enough to completely magnetize the iron. To You must therefore supply a significant combat this negative feedback, the 20-030 amount of work to pull the yoke from the & 20-035 electromagnet is designed with a Figure 6 - Magnetic Field produced by magnet. You get most of it back again when continuous circuit of iron. Ideally there are no Solenoid the yoke is placed back on the electromagnet. ends or free poles, as in Figure 8 below. All energy can be accounted for in every Put a piece of non-magnetic material, such conceivable situation. as copper, inside the coil depicted in Figure The electromagnet is therefore a practical 6, above. Little will happen because the field demonstration and use of the magnetic forces is too weak compared with thermal agitation that have been described theoretically. to change the spin orientations. Add iron, on the other hand, and you benefit from the alignment that has already taken place. The field How to Teach with Electromagnet from the coil reorients the already aligned Concepts: Electromagnet; solenoid; spins. When the spins in the domain walls ro- electromagnetic attraction and repulsion; tate towards the field, the domain walls move magnetic and nonmagnetic material; through the iron piece. Some domains grow magnetic domain & energy level; mating surfaces while others shrink - as in Figure 7. yoke magnetic circuits. Curriculum Fit: PS/ Electricity & i into page i out of page Magnetism. Unit: Moving Charge & Figure 8 - Electromagnet Magnets. Grades 9-10.

Figure 8 represents the case in which the iron is almost completely magnetized but with no poles. The spinning electrons, and not the poles, after all, do the attracting. Since each side of the mating surface has the same orien- tation, these electrons attract. If you increase the coil current you accomplish little because Figure 7 the ion is already almost fully aligned (or saturated). Examples of complete magnetic The last level of cancellation is incom- circuits may be found in transformers and in plete. The iron has thereby become a magnet. the inductive pickup devices that automobile The effect is temporary; switch off the current mechanics clip around sparkplug leads to and domain walls move back. The magnetism trigger stroboscopic lights. is lost. The iron in a magnet of this type is a soft ferromagnet. With appropriate impurities (C or Cr in P/N 24-20030 steels) or with appropriate texturing of materi- ©Science First/Morris & Lee All als into powder (as in ceramics or samarium Rights Reserved. Science First© is cobalt) the domain wall can be pinned so that a registered trademark of Morris & it does not move back past the impurities. Un- Lee.

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