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09:55:32 If we know the number of conduction electrons in a wire, the length of the wire and the average drift velocity of the electrons how to we determine the current in the wire?
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RESPONSE --> Using the velocity we can find that the electrons will drift 'dL per second. We get a ratio of the length 'dL/L and use this to multiply by the number of electrons in the entire length to find the number of electrons for that small length of time. Once we get this we can multiply by the charge of an electron to find the current flowing.
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09:57:40 GOOD STUDENT SOLUTION:
Given: # of electrons, L, vAve of the drift: From the velocity, we find that the electrons will drift a certain `dL per second. We find the ratio of the length `dL/L, and multiply this ratio by the number of electrons in the entire length `dL, to find the number of electrons for that small increment of length and time (1 sec). We can then mulitply that number of electrons by the charge of an electron to find the current flowing past a point at any given second. **......!!!!!!!!...................................
RESPONSE --> ok i understand
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09:59:14 For a given potential difference across two otherwise identical wires, why is the current through the longer wire less than that through the shorter wire?
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RESPONSE --> The potential gradient will be higher for the shorter length. So in the shorter wire the electrons are accelerated by a greater average force and build more velcoity.
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10:01:20 ** The potential gradient, which is the potential difference per unit of length, will be higher for the shorter length.
The potential gradient is the electric field, which is what exerts the accelerating force on the electrons. So in the shorter wire the electrons are accelerated by a greater average net force and hence build more velocity between collisions. With greater average drift velocity, more electrons therefore pass a given point in a given time interval. **......!!!!!!!!...................................
RESPONSE --> I understand that the potential gradient is the electric field which exerts the accelerating force on the electrons. Also, the greater average drift velocity will allow more electrons to pass between a given point in a given time.
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10:05:41 For a given potential difference across two otherwise identical wires, why is the current through the thicker wire greater than that through the thinner wire?
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RESPONSE --> More electrons are availabe in the thicker wire. So the potential gradient is the same because the lenght is the same so more electrons respond the the same field.
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10:08:49 ** The key is that more electrons are available per unit length in the thicker wire. The potential gradient (i.e., the electric field) is the same because the length is the same, so more electrons respond to the same field.
GOOD STUDENT SOLUTION: If we know the diameters of both the wires (d1 and d2), we know that the cross-sectional area of the second diameter is (d2/d1)^2 times the cross-sectional area of the first wire. This means that the second wire will have (d2/d1)^2 times as many charge carriers per unit length. This also means that the current in the second wire will be (d2/d1)^2 times that of the first. Therfore the thicker wire will have a greater current.......!!!!!!!!...................................
RESPONSE --> ok i understand and see why the thicker wire will have a greater current from the solution. From this I see that the current in the second wire will be (d2/d1)^2 times that of the first.
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10:12:29 If we know the length of a uniform wire and the potential difference between its ends, how do we calculate the average net force exerted on a conduction electron within the wire?
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RESPONSE --> From the charge and potential difference we can find the work done over the entire length of the wire. Then we multiply the voltage by the charge to get the work done. Since we know that F= W/s we can divide the work found by the distance which will give us the force.
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10:15:34 STUDENT RESPONSE WITH INSTRUCTOR COMMENT: First determine the work done and then divide it by the distance to get the average net force
INSTRUCTOR COMMENT: The work done on an electron is the product of its charge and the potential difference. Having this information we can then do as you indicate. GOOD STUDENT SOLUTION: From the charge and voltage (potential difference) we can find how much work is done over the entire length of the wire. We multiply the voltage by the charge to get work done over the full length. We know that F = W/s, so we can divide the work we found by the distance and this will give us the amount of force.......!!!!!!!!...................................
RESPONSE --> ok i understand
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10:18:27 If we know the voltage and the resistance in a circuit, how do we find the current, and how do we use this result to then reason out the power required to maintain the current?
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RESPONSE --> From the equation I=V/R you can find the current.
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10:19:42 ** Reasoning in terms of units:
Power is work per unit of time. P = `dW / `dt, in J / s. Current is voltage / resistance. I = V / R, in C / s. Voltage is work per unit of charge: V = `dW / Q, in J / C. If we multiply voltage in J / C by current in C / s we get power in J / s. CORRECT STUDENT SOLUTION: you find the current by solving the equation I= V/R for I which divided the volts by the resistance of a circuit and to maintain it you multiply the volts by the current determined before.......!!!!!!!!...................................
RESPONSE --> ok i understand the reasoning in units
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10:21:30 Describe the effect of a magnetic field on a current. Note the relative directions of the magnetic field, the current and the force exerted on the current. Note whether a sustained current experiences a sustained force.
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RESPONSE --> A magnetic field exerts a force equal to I * L * magentic field on the current. This force is perpendicular to the magnetic field and to the current.
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10:22:40 ** A uniform magnetic field B oriented perpendicular to a current I in a straight current carrier of length L exerts a force equal to I * L * B on the current. This force is perpendicular to the magnetic field and to the current by the right-hand rule where I is crossed with B.
If B and the current make angle theta then the force is I * L * B * sin(theta). Again the right-hand rule applies. **......!!!!!!!!...................................
RESPONSE --> ok i understand if the B(magnetic field) and the current make angle theta then the force is I*L*B*sin(theta). Also, I know that a uniform magnetic field B orients a perpendicular to a current I in a straight current carrier of length L. "