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Electricity and Magnetism Science Posse University of Wyoming What is ELECTROMAGNETISM? The fundamental relationship between electric fields and magnetic fields Electromagnetism A moving electric field creates a magnetic field that rotates around it The Right Hand Rule helps describe this The Right Hand Rule First define positive electric current as flowing from the positive (+) end of a battery, through an electric circuit, and back into the negative (-) end. Next define a magnetic field as always pointing away from a North pole and towards a South pole. The Right Hand Rule Extend your thumb. This is the direction of the current. If your thumb is the direction of the electric current, your fingers are curling along with a rotating magnetic field! The Right Hand Rule A transformer is a device for increasing or decreasing voltage. Structure of Transformer Circuit Symbol for Transformer How Transformer works Laminated soft iron core Input voltage Output voltage (a.c.) (a.c.) Primary coil Secondary coil All transformers have three parts: 1. Primary coil – the incoming voltage Vp (voltage across primary coil) is connected across this coil. 2. Secondary coil – this provides the output voltage Vs (voltage across the secondary coil) to the external circuit. 3. Laminated iron core – this links the two coils magnetically. Notice that there is no electrical connection between the two coils, which are constructed using insulated wire. Two Types of Transformer A step-up transformer increases the voltage - there are more turns on the secondary than on the primary. A step-down transformer decreases the voltage - there are fewer turns on the secondary than on the primary. To step up the voltage by a factor of 10, there must be 10 times as many turns on the secondary coil as on the primary. The turns ratio tells us the factor by which the voltage Formula for Transformer voltage across the primary coil number of turns on primary voltage across the secondary coil number of turns on secondary Vp Vs Np Ns Where Vp = primary voltage Vs = secondary voltage Np= Number of turns in primary coil Ns = Number of turns in a secondary coil. The diagram shows a transformer. Calculate the voltage across the secondary coil of this transformer. Step-up transformer! Solution VP N P VS N S Substituting 12 180 VS 540 Crossmultiplying 180.VS 12 x 540 12 x 540 VS 180 VS 36 V Worked example No. 2 A transformer which has 1380 turns in its primary coil is to be used to convert the mains voltage of 230 V to operate a 6 V bulb. How many turns should the secondary coil of this transformer have? VP = 230 V VS = 6 V NP = 1380 NS = ? Obviously, a Step-down transformer!! Solution VP N P VS NS Substituting 230 1380 6 NS Crossmultiplying 2300.N S 6 x 13800 6 x 1380 NS 230 N S 36 turns The Electromagnet By the Right Hand Rule, a coil of wire with current flowing in it will create a magnetic field The strength of the magnetic field depends on The amount of current in a wire – More current means stronger magnetic field The number of turns in the coil – More turns means stronger magnetic field The material in the coil – Magnetic materials like iron and steel make the magnetic field stronger The Electromagnet A simple electromagnet can be made by coiling some wire around a steel nail, and connecting a battery to it. As current rotates around the nail, a magnetic field is created with the North pole at the bottom and the South pole at the top The Electromagnet Electromagnets are found in many everyday objects such as Motors and generators Doorbells Speakers and headphones Hard drives VHS and Audio cassette tapes Telephones The Electric Motor An electric motor is a device that converts electromagnetic energy into mechanical energy It works by using special electromagnets in the case of the motor that create North and South poles that are constantly spinning in a circle. These spinning poles pull along magnets that are on the rotor (the part that spins), causing it to rotate The Electric Generator An electric generator works in the opposite of a motor; it converts mechanical energy into electromagnetic energy An outside force moves magnets in a circle past several coils of wire. By the Right Hand Rule, electric currents are developed in the coils of wire.