1Chapter 25Electromagnetic Induction22Electromagnetic Induction• Changing magnetic fields cause a potential difference (voltage) through loops of wire• This voltage produces current in the loops– The greater the change in magnetic field, the greater the voltage– The more loops of wire, the greater the voltage• Therefore magnetism can induce electricity just like electricity can induce magnetism!33Induction44Faraday’s Law• “The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops”• The amount of current is then determined by this voltage and the circuit’s resistance by Ohm’s Law timefield magnetic N ~ Vloops∆∆55Problem Solving: Faraday’s Law• If you double the rate of change in the magnetic field, how do you change the induced voltage?• If you cannot change the rate of change of the magnetic field, how can you increase the induced voltage?–If you double the change in magnetic field, you also double the induced voltage.–You can increase the number of turns and/or increase the induced voltage66Generators and Alternating Current• When you plunge a magnet into coils of wire, you induce a current going in a certain direction• When you take the magnet back out, a current is induced going in the opposite direction• This changing current is the alternating current• A method similar to this is used in power plants to generate our electricity77Power Production• Electricity can also be generated by turning a coil of wire inside the magnetic field– The magnetic field then changes inside the loop as it rotates•The turbogenerator uses steam to turn a fan. This fan is connected to a loop of wire. As the loop turns the voltage changes as the magnetic field inside the loop changes thus producing our alternating current88Induction of Alternating Current99Transformers• The magnetic field from one coil of wire can induce a voltage in another without the coils touching• This device is called a transformer– The coil attached to a voltage source is called the primarywhile the coil where the current is induced is called the secondary• A different number of coils between the primary and secondary can produced a higher or lower voltage in the secondary– Power is always conserved1010Transformers• Relationship between the voltages and number of turns in the primary and secondary• Conservation of energy (power):turnssecondary ofnumber voltagesecondary rnsprimary tu ofnumber ltageprimary vo=secondaryprimaryVI)((VI) secondary in thepower primary in thepower ==1111Transformer1212Problem Solving: Transformers• A primary coil has 50 turns and a voltage of 120 V. The secondary coil has 100 turns. What is the voltage in the secondary?• If the current is the primary is 1 A, what is the current is thesecondary?V 240 turns50 turns)V)(100 (120 voltagesecondary turns100voltagesecondary turns50V 120===A 5.0V 240A) V)(1 120(I)V)(I (240 A) V)(1 120(secondarysecondary===1313Self-Induction• The coils of wire not only feel the effects of external magnetic fields, but the also feel the magnetic fields produced by their own current• When the current suddenly stops in an electromagnet, there is a large change in magnetic field• This produces a back emf that can cause a spark over the switch– This is why you should always turn off an appliance before unplugging itEmf stands for electromotive force. This is really a voltage and why batteries are sometimes referred to as an emf source.1414Field Induction I• You do not need a wire to induce a magnetic field and make charges move– “An electric field is created in any region of space in which a magnetic field is changing with time. The magnitude of the induced electric field is proportional to the rate at which the magnetic field changes. The direction of the induced electric field is at right angles to the changing magnetic field”1515Field Induction II• The magnetic equivalent then is:– “A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric