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Magnets & Motors Index Magnetic and non-magnetic materials Properties of magnetic materials Magnetic Fields Electromagnetism Electric Motors Induced currents Transformers 2 Magnetic and non-magnetic materials Magnetic materials A magnetic object is a material which can be magnetized, and is attracted to magnets. All strongly magnetic materials contain iron, nickel, or cobalt. For example steel is mainly iron. A material that contains mainly iron and is a very strong magnetic material is iron. The lodestone is a naturally magnetized piece of the mineral known as magnetite. The ancient Greeks used the lodestone as a compass, by attaching it to a string, and then it would point towards the North Pole. Strong magnetic materials are also called ferromagnetic. These are divided in two categories hard and soft, this depends on how well they keep their magnetism when they are magnetized. Name an example of Hard Ferromagnetics; steel. These materials are difficult to magnetize but do not lose their magnetism readily. A practical use for these materials would be: permanent magnet. 3 Name an example of Soft Ferromagnetics; iron. These materials are easy to magnetize but their magnetism is easily lost. They are used in cores of electromagnets because their magnetic effect can be switched on/off. Non-Magnetic materials These include metals such as brass, copper, zinc, tin and aluminum as well as all other non-metals. Magnetic Metals Non-magnetic Metals Iron Brass Steel Copper mercury Zinc Nickel Tin cobalt Aluminium Where does magnetism come from? Inside the atom, the electrons orbit around the central nucleus. This orbiting and spinning of the electrons creates a magnetic effect within the material. In many types of atoms, the magnetic effect is cancelled but in some others it is not. This is where magnetism comes from. In a material which is not magnetized the atomic dipoles inside the material point in random directions. 4 What do you think happens to dipoles when the material becomes magnetized? ___________________________________________________________ ___________________________________________________________ 5 Properties of Magnetic Materials Magnetic poles When a magnet is dipped into iron filings, the small iron filings are attracted to its ends. This shows that the magnetic acts at two points of the magnet. These points are called ‘the poles’. Every magnet has two poles; the north and south pole. Assume that you have two similar magnets, what will happen when you bring; Two South poles next to each other _________________ A south and a north pole next to each other _________________ Two north poles next to each other _________________ This is because like poles repel, and unlike poles attract each other. Therefore, the north pole is attracted to the south pole, and vice versa. Magnetic poles always exist in pairs. It is not possible to produce just an isolated north pole or isolated south pole. If a bar magnet is broken into two, each piece will have a north and south pole. 6 If you had to tie a bar magnet to a piece of string, and you suspended it in mid-air, it swings around and stops with its north pole facing the Earth’s North, and the south of the magnet facing the South of the Earth. This means that the Earth exerts a weak magnetic force on the poles of the magnet. Induced magnetism: Which materials are attracted to magnets? Why is this so? When these materials are brought close to magnets, magnetism is induced in them as shown in the diagrams. This means that iron acts as a temporary magnet, while steel acts as a permanent magnet. Making a magnet: There are various ways how to make a magnet, as mentioned a piece of steel can by magnetized if it is simply put close to a magnet however, this makes steel a weak magnet. The following are two ways in which steel can be made a permanent magnet. 7 1. Stroking: This method consists of stroking with a magnet a piece of steel from end to end in the same direction for several times. 2. Electrically: take the magnetic material and place it inside a coil of wire which has numerous turns of copper wire which is connected to a low voltage, high current supply. The alignment of the dipoles depends on the direction of the current supplied. By which rule is the direction of the current given?___________________. How can a stronger magnet be produced electrically? ___________________________________________________________ ___________________________________________________________ 8 Demagnetization of magnets There are 3 main ways to demagnetize a magnet which are: 1. Hammering: this is done by hammering the magnet which would cause the dipoles to lose their alignment. 2. Heating: heating the magnet would cause the atoms to move rapidly which in turn would cause the dipoles to lose their alignment. 3. Electrically: this is the most effective method of demagnetization. The magnet is placed inside a solenoid connected to an ac supply this alternating current changes current direction rapidly. The magnet is then slowly withdrawn in the east west direction with the alternating current still flowing through the solenoid. 9 Magnetic Fields List 3 cases were a magnetic field is present: ____________________, ____________________, or _______________________. In these cases a magnetic field can be observed around the magnets and this exerts forces on magnetic materials in it. Magnetic field of a bar magnet: The magnetic field lines run from the north pole to the south pole. The field direction is represented by an arrowhead, which indicates the direction of the magnetic field.. Draw the field lines on this magnet: 10 Magnetic fields can also be investigated using a small compass. When a compass is placed inside the magnetic field it points in same direction as the arrowhead of the magnetic field How can you trace the magnetic field lines of a magnet using a compass? 11 There is a way in which we can see a representation of the magnetic field lines of a bar magnet, this is by using iron filings. Iron filings are sprinkled on a paper which is placed over a bar magnet, the filings will now become tiny magnets each having their own north and south and these are pulled into position from the poles of the magnet. Can you now draw the magnetic fields of two magnets when they are placed next to each other in the following ways? 12 When two magnets are placed next to each other their magnetic fields combine to produce a single field as you have displayed above. There is what is called a neutral point this is where the field of one magnet exactly cancels the field of the other, this occurs only when there are two like poles facing each other. Can you mark with an X on one of the above diagrams where there is a neutral point? Magnetic field of a straight wire carrying a current When a current is passed through a wire a magnetic field is produced. The magnetic field is in the form of circles around the wire and the field is the strongest _______________ to the wire. What happens to the field lines if you increase the current? _______________________________. 13 There is a way how we can find the direction of the magnetic field around the wire and this is called the right hand grip rule. For this rule imagine grabbing the wire with you _________hand, so that your thumb points in the direction of conventional current. So it follows that your fingers now point in the same direction as the field lines around the wire. Finding the shape of the magnetic field around a wire: A wire is passed through a piece of cardboard with a hole in the middle, iron filings are then sprinkled over the cardboard. A large current is passed through the wire WITH CAUTION, while tapping the card so that the iron filings show the shape of the magnetic field around the wire. How can you determine the direction of the magnetic field lines? ___________________________________________________________ ___________________________________________________________ 14 How does the current change the strength of the magnetic field in a straight wire? Magnetic field of a solenoid: When a current is passed through a solenoid it also produces a magnetic field in the same shape as that produced by a bar magnet. The field inside the solenoid is very strong and uniform. This magnetic field also has magnetic poles at the ends of the coil. If the current in increased, the strength of the field ________________. What happens if you increase the number of turns of the coil? ___________________________________________________________ 15 Why does this happen? ___________________________________________________________ ___________________________________________________________ Here we can once again use the right hand grip rule to find the poles produced inside the solenoid. Can you explain how you can use this rule to find the north/south poles of the coil? ___________________________________________________________ ___________________________________________________________ 16 Finding the shape of the magnetic field around a solenoid: A solenoid is passed through a piece of cardboard with holes in it. Iron filings are then sprinkled over the cardboard. A large current is passed through the wire WITH CAUTION, while tapping the card so that the iron filings show the shape of the magnetic field around the wire. How can you determine the direction of the magnetic field lines? ___________________________________________________________ ___________________________________________________________ 17 The strength of the magnetic field can be increased by; increasing the current in the circuit, using more turns on the wire of the coil. Also using a soft iron core aids to produce a stronger field. The iron core helps he solenoid by acting as a temporary magnet. How does the current affect the strength of the magnetic field in a solenoid? Draw the field lines of the solenoids below: Current = 10A Current = 30A 18 Electromagnetism What do you think is the difference between a permanent magnet and an electromagnet? ___________________________________________________________ ___________________________________________________________ An electromagnet is made up from a coil which is wound round an iron core (much like that of a solenoid) but the difference is that it can be switched on and off using a simple switch. When a current flows through the coil, this produces a magnetic field. A DIY solenoid using an iron nail 19 Consider this circuit below; What happens to the core when the circuit is switched on? __________________________________ __________________________________ __________________________________ Why is it important that the core is made from iron and not steel? ___________________________________________________________ ___________________________________________________________ The strength of the electromagnet can be increased by; Increasing the current in the circuit, Using more turns on the wire of the coil, And by using a bigger iron core. An electromagnet picking up iron 20 HW! Uses of electromagnets: Three uses of electromagnets in everyday life are: 1. Circuit breaker 2. Electric bell 3. Loudspeaker 4. Electromagnetic door lock 5. Electromagnetic for lifting Describe in detail one of the above: It is suggested that you use diagrams in the space below: ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 21 Electric Motors Magnetic Force on a Current: Let us assume that a copper wire is placed perpendicular to a magnetic field. The wire only Is copper a magnetic material feels a force from the magnet when the current is or not? passing through it. This happens because the current produces its own magnetic field which acts on the poles of the magnets. The force is increased if: The current is increased A stronger magnet is used The length of the wire in the field is increased Discuss: Do you think that the wire will move when there is a current passing through it? This force experienced by the wire is called The Catapult Effect. The ‘’catapult effect’’ 22 Fleming’s Left Hand Rule: The direction of motion of the copper wire can be found by using Fleming’s Left hand rule. Using the diagram on the right, and representing the direction of the field with the first finger, and the direction of the current with the second finger, through Fleming’s left hand rule, we can see that the wire moves upwards (the direction of the thumb). What needs to be altered for the wire to move downwards? N.B. The wire is not attracted to either pole. In applying this rule it is important to remember that 1. The field direction of the magnet is from the North Pole to the South Pole. 2. The current direction is from the positive to the negative; following conventional current direction. 3. This rule only applies if the current and the field are at right angles. N.B. If the current and the field are in the same direction (parallel to each other, then there is no force). 23 Turning Effect on a coil: If a coil is carrying a current in a magnetic field, the forces on it produce a turning effect. Many electric motors use this principle As you can see from the diagram, on opposite sides of the coil, current flows in opposite directions. So if we apply Fleming’s left hand rule on the coil next to the North side of the magnet, this wire is pushed up. Likewise on the other side, it is pushed down. This creates a turning effect. What happens to this turning effect if the coil has more turns? ___________________________________________________________ ___________________________________________________________ 24 Test your knowledge: There is a force on the wire in the diagram on the right; a. Give two ways in which the force could be increased. b. Use Fleming’s left hand rule to work out the direction of the force. c. Give two ways in which the direction of the force can be reversed. A simple DC Motor The simple DC motor runs on direct current. What do you understand by direct current? _________________________ _________________________ _________________________ The coil is made up of insulated copper wire, and is allowed enough room to rotate between the poles of the magnet. Attached to the coil there is a split ring, also known as the commutator. There are two brushes that act as contacts which rub against the commutator and keep the coil connected to the battery. Suggest a suitable material for these brushes: __________________ 25 A simple DC motor The maximum turning effect on the coil is when it is horizontal, this is because the forces here are the furthest apart. If there was no change to the forces, the commutator would come to rest in the vertical position, however when the coil just passes the vertical position, the commutator changes the direction of the current through it, and so do the forces. This continues to push the coil further round until it is once again vertical. And so on. Thus, the coil is rotating clockwise, half a turn at a time. What would happen if the battery or the poles of the magnet, were reversed? ___________________________________________________________ ___________________________________________________________ 26 List 4 ways in which the turning effect can be increased: a.__________________________________________________________ b._________________________________________________________ c._________________________________________________________ d._________________________________________________________ 27 Induced Currents Induced EMF A current produces a magnetic field, is it possible for a magnetic field to produce a current? If a bar magnet is pushed into a coil when there is no battery attached to the coil, an emf is induced in the current. This effect is called electromagnetic induction. This is because the field lines of the magnet are being cut by the wire and therefore it makes a complete circuit. This induced emf can be increased: Moving the magnet faster Increasing the number of turns Using a stronger magnet The above statement is summed up by Faraday’s Law of Electromagnetic Induction, which states that; “The emf induced in a conductor is directly proportional to the rate at which the magnetic field lines are being cut by the conductor“ 28 An application of Faraday’s law is the electric guitar; the pick up under the string are tiny coils with magnets inside them the steel string become magnetized. When they vibrate, current is induced in the coils boosted by an amplifier to produce sound. Lenz’s law If a magnet is moved in or out of a coil the direction of the current can be predicted using Lenz’s law which states that an induced current always flows in a direction such that it opposes the changes which produces it. From the diagram above, the induced current turns the coil into a weak electromagnet. The North pole of this electromagnet opposes the North pole of the actual magnet. This creates repulsion between the two. 29 As the magnet is being pulled out of the coil, the induced current changes direction, and so the poles of the coil are reversed, this time there is attraction between the coil and the magnet as it is being pulled away. What can we deduct from both cases? (Refer to Lenz’s law) ___________________________________________________________ ___________________________________________________________ Lenz’s law and the law of conservation of energy Lenz’s law is a direct example of the law of conservation of energy. Energy is used when a current flows round a circuit, so energy must be used to induce this current in the first place. As it can be seen above, energy was being used to move the magnet against the opposing force. Eddy Currents What if the conductor you're moving through the magnetic field isn't a wire that allows the electricity to flow neatly away? You still get electric currents, but instead of flowing off somewhere, they swirl about inside the material. These are what we call eddy currents. They're electric currents generated inside a conductor by a magnetic field that can't flow away so they swirl around instead, dissipating their energy as heat. 30 One of the interesting things about eddy currents is that they're not completely random: they flow in a particular way to try to stop whatever it is that causes them. This is an example of another bit of electromagnetism called Lenz's law Here's an example. Suppose you drop a coin-shaped magnet down the inside of a plastic pipe. It might take a half second to get to the bottom. Now repeat the same experiment with a copper pipe and you'll find your magnet takes much longer (maybe three or four seconds) to make exactly the same journey. Eddy currents are the reason. When the magnet falls through the pipe, you have a magnetic field moving through a stationary conductor (which is exactly the same as a conductor moving through a stationary magnetic field). That creates electric currents in the conductor—eddy currents, in fact, which cause the magnet to slow down. 31 Transformers From what we have discussed previously we know that a magnet pushed into a coil induces an emf. We that if the _____________ is complete, a current is also induced. Mutual Induction If an electromagnet is placed next to coil, as shown above, and the electromagnet is switched on an emf is induced in the coil next to it for a fraction of a second. What happens to the galvanometer? ___________________________________________________________ When the electromagnet is switched off, an emf is produced in the opposite direction for a fraction of a second. This is like pulling the magnet away from the coil. 32 List two methods of how the induced emf in the second coil can be increased: ______________________________________________________ ______________________________________________________ When coils are magnetically linked, so that a change in current in one causes an induced emf in the other this is called mutual induction. A heart pacemaker uses mutual induction. Pulses of current through a coil in the pacemaker unit include pulses in a coil fitted in the patients’ chest. These trigger heartbeats. A Simple Transformer Insert diagram of a transformer. AC Voltages can be increased or decreased by using a simple transformer. A simple transformer works by mutual induction. What is the difference between AC and DC? ___________________________________________________________ When alternating current flows in the primary coil this sets an alternating magnetic field which induces an alternating current in the secondary coil. This is because the magnetic field lines from the primary coil are ‘cutting’ the secondary. Provided that no energy is lost the following equation applies for transformers 33 𝑜𝑢𝑡𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑛𝑝𝑢𝑡 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 = 𝑡𝑢𝑟𝑛𝑠 𝑜𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑜𝑖𝑙 𝑡𝑢𝑟𝑛𝑠 𝑜𝑛 𝑖𝑛𝑝𝑢𝑡 𝑐𝑜𝑖𝑙 𝑉2 In symbols: 𝑉1 = 𝑛2 𝑛1 Draw the field inside the transformer using the diagram below: Questions: 1. In the experiment on the right: what happens when a. The switch is closed (turned on) b. The switch is left closed (in the on position) c. The switch is opened again 2. A transformer has a turns ration of 1:4. Its input coil is connected to a 12 volt AC supply. Assuming there are no field line or energy losses. a. What is the output voltage? b. What turns ratio would be required of an output voltage of 36V? 34 Step Up and Step Down Transformers How is a transformer able to step up or step down the voltage supplied? ___________________________________________________________ ___________________________________________________________ What is the difference between a step up and a step sown transformer? ___________________________________________________________ ___________________________________________________________ Power through a transformer If no energy is wasted in a transformer, the input power will delivered equally to the output power. So: 𝒊𝒏𝒑𝒖𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 × 𝒊𝒏𝒑𝒖𝒕 𝒄𝒖𝒓𝒓𝒆𝒏𝒕 = 𝒐𝒖𝒕𝒑𝒖𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 × 𝒐𝒖𝒕𝒑𝒖𝒕 𝒄𝒖𝒓𝒓𝒆𝒏𝒕 In symbols: 𝑽𝟏 𝑰𝟏 = 𝑽𝟐 𝑰𝟐 Since Voltage x Current is the same on either side of the transformer increases the voltage and respectively reduces the current in the same proportion or vice versa. 35 National Grid Power for the AC mains generated in power stations and transmitted through long distance cables, and then distributed to consumers. The power supplied by the power station is first passed through a step up transformer to raise the voltage and reduce current to minimize loss of electricity. This power from is then distributed by a series of substations. These contain step down transformers which reduce the voltage to the level needed by the consumers. In Europe and Malta, this is 230V. 36