Exam 2 practice problems solutions

Exam 2 practice problems solutions - Exam #2 practice...

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Exam #2 practice problems solutions Physics 227 1. So far in the course we have not been able to design an experiment that could decide if the moving charged particles that make up currents in wires were positive or negative. We’ve been using the convention that they are positive and we’ve defined electric current to be in the direction of motion of these positive charges. However, we also know that it could be that it is actually negatively charged particles traveling in the opposite direction . Suddenly, you have an idea! A straight wire carrying a current is placed between the poles of a strong horseshoe magnet as shown. You realize that if you take a voltmeter and touch its two terminals to opposite sides of the wire (see the side view) you can actually answer the question of what the sign of the moving charges are. Explain in detail what is happening in this situation and why it can be used to finally answer the question. Hint: If the moving charges are positive, what will be happening to them? If they are negative, what will be happening to them? Since electric current is the collective motion of large numbers of electric charges then these charges should feel a magnetic force because of the presence of the magnet. (I’ve only drawn the magnetic field between the poles of the magnet.) If current truly is the motion of positive charges then their velocity is out of the page and according to the RHR they feel a magnetic force downwards. Therefore, the wire will have an excess of positive charges at the bottom and a lack of positive charges at the top. This creates an electric field inside the wire in the direction shown. If current is actually the movement of negative charges then their velocity is into the page and according to the RHR they will also feel a magnetic force downwards. This creates an electric field in the opposite direction! This means the top and bottom of the wire are at different electric potentials. If you touch the positive terminal of the voltmeter to the top of the wire and the negative terminal to the bottom of the wire you can decide whether it’s positive or negative charges moving in the wire. If the voltmeter reads a positive value then it’s negative charges moving. If the voltmeter reads a negative value it’s positive charges moving. 2. Consider the circuit below. You can consider the wires and battery as ideal, having negligible resistance. Predict what the currents through each of the resistors will be. S N I S N Side view S N B Positive charges moving E S N B Negative charges moving E
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3 R 2 R 1 R 1 2 1 2 1 2 3 3.00 7.00 6.00 4.00 2.00 V V R R R 
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3. Two long straight wires are carrying currents as shown in the diagram. The wires are in contact but are insulated from each other. The wires are free to move with respect to each other (slide, twist, rotate, curl up, do nothing, etc.). Initially there are no currents flowing in the wires. Predict what will happen when the currents are turned on and justify your prediction in detail.
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Exam 2 practice problems solutions - Exam #2 practice...

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