course Phy 122
course Phy 122 \ǗV}W߅Wassignment #008
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02:15:37 Query introductory problem set 54 #'s 8-13 Explain how to determine the magnetic flux of a uniform magnetic field through a plane loop of wire, and explain how the direction of the field and the direction of a line perpendicular to the plane of the region affect the result.
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RESPONSE --> The flux of any vector field through an area A is equal to the product of the field strength and the area, as long as the vector field is directed perpendicular to the area. If the field is directed at an angle (theta) from the perpendicular, then the flux will be reduced by the factor of cosine of theta. flux = B * A * cos('theta)
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02:17:43 To do this we need to simply find the area of the plane loop of wire. If we are given the radius we can find the area using Pi * r ^2 Then we multiply the area of the loop (In square meters ) by the strength of the field (in tesla). This will give us the strength of the flux if the plane of the loop is perpendicular to the field. If the perpendicular to the loop is at some nonzero angle with the field, then we multiply the previous result by the cosine of the angle.
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RESPONSE --> You find the area by using the radius and the formula 'pi * r^2. I left that part out.
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02:23:22 Explain how to determine the average rate of change of magnetic flux due to a uniform magnetic field through a plane loop of wire, as the loop is rotated in a given time interval from an orientation perpendicular to the magnetic field to an orientation parallel to the magnetic field.
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RESPONSE --> First you have to find the initial flux which is equal to B * A. Then after the field is suddenly rotated until it is parallel to the plane the flux will become zero. So then you find the change in flux which is -B * A. Then you have to find the average rate of flux change by -(B*A)/'dt. This is the average voltage that results from the flux change.
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02:24:32 ** EXPLANATION BY STUDENT: The first thing that we need to do is again use Pi * r ^ 2 to find the area of the loop. Then we multiply the area of the loop (m^2) by the strength of the field (testla) to find the flux when the loop is perpendicular to the field. Then we do the same thing for when the loop is parallel to the field, and since the cos of zero degrees is zero, the flux when the loop is parallel to the field is zero. This makes sense because at this orientation the loop will pick up none of the magnetic field. So now we have Flux 1 and Flux 2 being when the loop is perpendicular and parallel, respectively. So if we subtract Flux 2 from flux 1 and divide this value by the given time in seconds, we will have the average rate of change of magnetic flux. If we use MKS units this value will be in Tesla m^2 / sec = volts. **
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RESPONSE --> So if we use MKS units the value will be in Tesla m^2/sec, which equals volts.
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02:31:04 Explain how alternating current is produced by rotating a coil of wire with respect to a uniform magnetic field.
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RESPONSE --> A magnet creates magnetic lines of force on either side of it that move in opposite directions. As the metal coil passes through the magnetic field in a generator, the electrical power that is produced constantly changes. At first, the generated electric current moves in one direction. Then, when the coil reaches a position where it is parallel to the magnetic lines of force, no current at all is produced. As the coil continues to rotate, it cuts through magnetic lines of force in the opposite direction, and the electrical current generated travels in the opposite direction. A spinning coil in a fixed magnetic field will produce an alternating current, one that travels first in one direction and then in the opposite. The rate at which the current switches back and forth is known as its frequency. Reference: Scienceclarified.com
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02:34:20 ** STUDENT RESPONSE WITH INSTRUCTOR COMMENT: Y ou rotate a coil of wire end over end inside a uniform magnetic field. When the coil is parallel to the magnetic field, then there is no magnetic flux, and the current will be zero. But then when the coil is perpendicular to the field or at 90 degrees to the field then the flux will be strongest and the current will be moving in one direction. Then when the coil is parallel again at 180 degrees then the flux and the current will be zero. Then when the coil is perpendicular again at 270 degrees, then the flux will be at its strongest again but it will be in the opposite direction as when the coil was at 90 degrees. So therefore at 90 degrees the current will be moving in one direction and at 270 degrees the current will be moving with the same magnitude but in the opposite direction. COMMENT: Good. The changing magnetic flux produces voltage, which in turn produces current. **
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RESPONSE --> So coils that are parallel to the magnetic field have zero current and no magnetic flux. Coils that are perpendicular to the field or at 90 and 270 degrees will have the strongest flux and current will be moving in one direction...but at 90 and 270, the current will be moving in opposite directions but have the same magnitude. Which is why it is calle alternating.
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02:35:19 Query Principles and General College Physics 18.04. 120V toaster with 4.2 amp current. What is the resistance?
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RESPONSE --> I used the formula Resistance equals Voltage divided by Amps. R = V/A = 120 V / 4.2 A = 28.6 ohms
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02:35:31 current = voltage / resistance (Ohm's Law). The common sense of this is that for a given voltage, less resistance implies greater current while for given resistance, greater voltage implies greater current. More specifically, current is directly proportional to voltage and inversely proportional to resistance. In symbols this relationship is expressed as I = V / R.{}{}In this case we know the current and the voltage and wish to find the resistance. Simple algebra gives us R = V / I. Substituting our known current and voltage we obtain {}{}R = 120 volts / 4.2 amps = 29 ohms, approximately.
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RESPONSE --> I agree.
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02:37:42 Query Principles and General College Physics 18.28. Max instantaneous voltage to a 2.7 kOhm resistor rated at 1/4 watt.
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RESPONSE --> I used the formula Voltage = 'sqrt (Power * Resistance), which came from the original formula P = V^2 / R V = 'sqrt [.25W * (2.7 * 10 ^ 3 Ohms)] = 26 Volts
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02:37:52 Voltage is energy per unit of charge, measured in Joules / Coulomb. {}Current is charge / unit of time, measured in amps or Coulombs / second.{}Power is energy / unit of time measured in Joules / second.{}{}The three are related in a way that is obvious from the meanings of the terms. If we multiply Joules / Coulomb by Coulombs / second we get Joules / second, so voltage * current = power. In symbols this is power = V * I.{}{}Ohm's Law tells us that current = voltage / resistance.In symbols this is I = V / R. So our power relationship power = V * I can be written {}{}power = V * V / R = V^2 / R. {}{}Using this relationship we find that {}{}V = sqrt(power * R), so in this case the maximum voltage (which will produce the 1/4 watt maximum power) will be{}{}V = sqrt(1/4 watt * 2.7 * 10^3 ohms) = 26 volts.
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RESPONSE --> I agree
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02:38:03 Query general college physics problem 18.39; compare power loss if 520 kW delivered at 50kV as opposed to 12 kV thru 3 ohm resistance.** The current will not be the same at both voltages. It is important to understand that power (J / s) is the product of current (C / s) and voltage (J / C). So the current at 50 kV kW will be less than 1/4 the current at 12 kV. To deliver 520 kW = 520,000 J / s at 50 kV = 50,000 J / C requires current I = 520,000 J/s / (50,000 J/C) = 10.4 amps. This demonstrates the meaning of the formula P = I V. To deliver 520 kW = 520,000 J / s at 12 kV = 12,000 J / C requires current I = 520,000 J/s / (12,000 J/C) = 43.3 amps. The voltage drops through the 3 Ohm resistance will be calculated as the product of the current and the resistance, V = I * R: The 10.4 amp current will result in a voltage drop of 10.4 amp * 3 ohms = 31.2 volts. The 43.3 amp current will result in a voltage drop of 40.3 amp * 3 ohms = 130 volts. The power loss through the transmission wire is the product of the voltage ( J / C ) and the current (J / S) so we obtain power losses as follows: At 520 kV the power loss is 31.2 J / C * 10.4 C / s = 325 watts, approx. At 12 kV the power loss is 130 J / C * 43.3 C / s = 6500 watts, approx. Note that the power loss in the transmission wire is not equal to the power delivered by the circuit, which is lost through a number of parallel connections to individual homes, businesses, etc.. The entire analysis can be done by simple formulas but without completely understanding the meaning of voltage, current, resistance, power and their relationships it is very easy to get the wrong quantities in the wrong places, and especially to confuse the power delivered with the power loss. The analysis boils down to this: I = P / V, where P is the power delivered. Ploss = I^2 R, where R is the resistance of the circuit and Ploss is the power loss of the circuit. So Ploss = I^2 * R = (P/V)^2 * R = P^2 * R / V^2. This shows that power loss across a fixed resistance is inversely proportional to square of the voltage. So that the final voltage, which is less than 1/4 the original voltage, implies more than 16 times the power loss. A quicker solution through proportionalities: For any given resistance power loss is proportional to the square of the current. For given power delivery current is inversely proportional to voltage. So power loss is proportional to the inverse square of the voltage. In this case the voltage ratio is 50 kV / (12 kV) = 4.17 approx., so the ratio of power losses is about 1 / 4.17^2 = 1 / 16.5 = .06. Note that this is the same approximate ratio you would get if you divided your 324.5 watts by 5624.7 watts. **
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RESPONSE --> Principles of Physics not assigned.
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02:38:14 Query univ 25.62 (26.50 10th edition) rectangular block d x 2d x 3d, potential difference V. To which faces should the voltage be applied to attain maximum current density and what is the density?
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RESPONSE --> Principles of Physics not assigned.
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02:38:16 ** First note that the current I is different for diferent faces. The resistance of the block is proportional to the distance between faces and inversely proportional to the area, so current is proportional to the area and inversely proportional to the distance between faces. Current density is proportional to current and inversely proportional to the area of the face, so current density is proportional to area and inversely proportional to the distance between faces and to area, leaving current inversely proportional to distance between faces. For the faces measuring d x 2d we have resistance R = rho * L / A = rho * (3d) / (2 d^2) = 3 / 2 rho / d so current is I = V / R = V / (3/2 rho / d) = 2d V / (3 rho). Current density is I / A = (2 d V / (3 rho) ) / (2 d^2) = V / (3 rho d) = 1/3 V / (rho d). For the faces measuring d x 3d we have resistance R = rho * L / A = rho * (2d) / (3 d^2) = 2 / 3 rho / d so current is I = V / R = V / (2/3 rho / d) = 3 d V / (2 rho). Current density is I / A = (3 d V / (2 rho) ) / (3 d^2) = V / (2 rho d) = 1/2 V / (rho d). For the faces measuring 3d x 2d we have resistance R = rho * L / A = rho * (d) / (6 d^2) = 1 / 6 rho / d so current is I = V / R = V / (1/6 rho / d) = 6 d V / (rho). Current density is I / A = (6 d V / (rho) ) / (6 d^2) = V / (rho d) = V / (rho d). Max current density therefore occurs when the voltage is applied to the largest face. **
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02:38:19
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