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Magnetism What are magnets? • A magnetic field is created around any moving charged object. What is charged that is moving within an atom? • Electrons (e-) • The atoms within most materials have paired up electrons spinning in opposite directions so the magnetic field that is created by one is cancelled out by the other. • Some materials like iron, nickel, and colbalt have a single electron or pair of electrons that spin in the same direction creating a magnetic field or a small atomic magnet. The atomic sized magnets line up to create domains within the material. The individual domains line up to form a magnet. A magnet has two ends called poles (dipoles), where the magnetic force is the strongest. • No matter how many times a magnet is broken, each piece always has a north pole and a south pole. • There is no such thing as a monopole. A magnetic field exists around a magnet or any moving charged object. Magnetic fields are like electric fields or gravitational fields in that they allow magnets to interact without touching. Imaginary lines that map out the magnetic field around a magnet are known as magnetic field lines or magnetic flux lines. The direction of the magnetic field is determined by using a compass. The needle of the compass points in the direction of the magnetic field. Rules for drawing magnetic field lines: 1. Magnetic field lines always go from the north pole to the south pole (outside a magnet). Bar Magnet Horseshoe Magnet Rules for drawing magnetic field lines: 2. Magnetic field lines are closed loops and never cross or intersect. Rules for drawing magnetic field lines: 3. Where the magnetic field lines are closer the magnetic field is stronger. Draw magnetic flux lines around the magnets below: S S N N S N N S Magnetism • Magnetism is the force of attraction or repulsion between magnetic poles. Magnetic Field around the Earth What we do know… The north magnetic pole and the geographic North Pole do not coincide. The magnetic pole is about 1500 km (930 mi) south of the geographic North Pole and it wanders. A compass actually indicates the direction of magnetic north, not true north. Therefore a navigator must need to know the magnetic declination for a specific area. This is the angular difference between magnetic and true north. The details and mechanisms of how and why the Earth has a magnetic field are unclear. They do believe that it may have something to do with motions in the liquid outer core. …Electromagnet An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases. An electromagnet is most commonly made by coiling wire around a piece of iron. This electromagnet is called a solenoid. The shape of the magnetic field is the same as a bar magnet. As electrons move through the coil of wire, the magnetic field of one electron adds to the field of any others moving in the same direction. Factors affecting the magnetic field of a solenoid: 1. The magnetic field around a solenoid is directly related to the current through the coil. Factors affecting the magnetic field of a solenoid: 2. The magnetic field around a solenoid is directly related to the number of turns or coils around the solenoid. Factors affecting the magnetic field of a solenoid: 3. The magnetic field around a solenoid is directly related to the material around which the wire is coiled. The soft iron is more permeable to the magnetic field than the air is. Electromagnetic Induction This is the process of generating a potential difference (voltage) in a conductor (wire) due to the motion of the conductor in a magnetic field. Generating a voltage in a wire would mean generating areas of uneven charge within a wire. How can this be done? What can be forced to move within a wire that will create a potential difference? Moving a wire through a magnetic field will cause a magnetic force which will move electrons towards one end of the wire creating a potential difference. Let’s take a closer look… Motion of wire B - - - wire If this wire is moved up or down (perpendicular to the field), a voltage will be generated and if the wire is part of a complete circuit then current is induced. If this wire is moved left or right (parallel to the field), no current will be generated because no force is generated to move electrons and create a voltage. Other ways to induce voltage… This is how Niagara Falls was used to produce electricity… Water flow is used to turn a turbine through a magnetic field inducing a potential difference and producing electrical current. This is how an electric motor works… An electric motor utilizes the property of electromagnetic induction to convert electricity into mechanical energy to make things move. The conductor itself, a coiled wire, will move to oppose the magnetic field. Just when it gets into position the current is reversed, and the coil spins round and round and round full of mechanical energy. A Generator… A generator is simply the same process in reverse, converting mechanical energy into electricity. Almost all of the electrical energy we use in our daily lives is supplied by electric generators.