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Section 22.2 in Matter and Interactions (4th edition) | Section 22.2 in Matter and Interactions (4th edition) | ||
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===== Changing Flux from an Alternating Current ===== | ===== Changing Flux from an Alternating Current ===== | ||
As we said before, one of the most important sources of a changing magnetic field is an alternating current. This is what actually comes out of the wall outlets; as opposed to the current from a battery which is a constant current (or a direct current). We are only briefly going to talk about alternating current as it refers to induction and changing magnetic flux, but there are many more applications of alternating current, especially with regard to circuits, resistors, and capacitors. For the purposes of these notes, we will talk about how we represent an alternating current, how that alternating current can produce an induced current/ | As we said before, one of the most important sources of a changing magnetic field is an alternating current. This is what actually comes out of the wall outlets; as opposed to the current from a battery which is a constant current (or a direct current). We are only briefly going to talk about alternating current as it refers to induction and changing magnetic flux, but there are many more applications of alternating current, especially with regard to circuits, resistors, and capacitors. For the purposes of these notes, we will talk about how we represent an alternating current, how that alternating current can produce an induced current/ | ||
==== Alternating Current ==== | ==== Alternating Current ==== | ||
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When a current is constantly switching between positive and negative values, we call this an **alternating current**. Typically, this means that we have a current that is represented by a sine graph (shown in the figure to the right), though there can be other types of alternating current that look like step functions (also called [[https:// | When a current is constantly switching between positive and negative values, we call this an **alternating current**. Typically, this means that we have a current that is represented by a sine graph (shown in the figure to the right), though there can be other types of alternating current that look like step functions (also called [[https:// | ||
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If you have an oscillating current, this would also mean that you would have an oscillating magnetic field everywhere around the wire (since currents create magnetic fields). If there is an oscillating magnetic field, this means that there will also be an induced potential/ | If you have an oscillating current, this would also mean that you would have an oscillating magnetic field everywhere around the wire (since currents create magnetic fields). If there is an oscillating magnetic field, this means that there will also be an induced potential/ | ||
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As you figured out in the project last week, a rotating loop in a magnetic field will create a oscillating current and oscillating voltage. This idea is exactly what happens in a power generator, just on a much larger scale. In a power generator there are thousands of loops, rotating very quickly in a large magnetic field to produce a very large current with a frequency of 60 Hz (in the U.S. at least). However, because power (or the energy transferred to heat) is related to $P=I^2R$, sending a very large current over large distances (like from the generator to your house) would result in a large loss of power over that wire. This is a problem because any power lost on the wires can't be used in your house and if that power is large enough it could heat up the wires to point where they are damaged. | As you figured out in the project last week, a rotating loop in a magnetic field will create a oscillating current and oscillating voltage. This idea is exactly what happens in a power generator, just on a much larger scale. In a power generator there are thousands of loops, rotating very quickly in a large magnetic field to produce a very large current with a frequency of 60 Hz (in the U.S. at least). However, because power (or the energy transferred to heat) is related to $P=I^2R$, sending a very large current over large distances (like from the generator to your house) would result in a large loss of power over that wire. This is a problem because any power lost on the wires can't be used in your house and if that power is large enough it could heat up the wires to point where they are damaged. | ||
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To get around this problem, a step up transformer is used to change a low voltage, high current circuit (like what comes out the generator) into a high voltage, low current circuit for transport from the generator to the neighborhoods or wherever it is needed. A step down transformer is then used close to the neighborhoods to return the high voltage, low current back to a low voltage, high current circuit that is then used in your house. You may have seen these around your neighborhood - they look like small boxes attached to the power lines overhead, generally on the lines going from a larger street into a residential area (shown in the figure to the left). | To get around this problem, a step up transformer is used to change a low voltage, high current circuit (like what comes out the generator) into a high voltage, low current circuit for transport from the generator to the neighborhoods or wherever it is needed. A step down transformer is then used close to the neighborhoods to return the high voltage, low current back to a low voltage, high current circuit that is then used in your house. You may have seen these around your neighborhood - they look like small boxes attached to the power lines overhead, generally on the lines going from a larger street into a residential area (shown in the figure to the left). | ||
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In these notes, we will go through how a step up transformer works and how it uses induction to change the voltage from a low voltage to a high voltage. We will use a basic transformer, | In these notes, we will go through how a step up transformer works and how it uses induction to change the voltage from a low voltage to a high voltage. We will use a basic transformer, | ||
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Because there is an oscillating potential/ | Because there is an oscillating potential/ | ||
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If we put the secondary solenoid on the end of the iron ring, this changing magnetic field will be the same as that from the primary solenoid: $B_P=B_S$. This changing magnetic field (from the primary solenoid) will induce a voltage ($V_S$) in the secondary solenoid. We can use Faraday' | If we put the secondary solenoid on the end of the iron ring, this changing magnetic field will be the same as that from the primary solenoid: $B_P=B_S$. This changing magnetic field (from the primary solenoid) will induce a voltage ($V_S$) in the secondary solenoid. We can use Faraday' | ||
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==== Examples ==== | ==== Examples ==== | ||
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