Home Page Textbooks Physics: Principles and Problems Page 685: Section Review

Physics: Principles and Problems

9th Edition

Elliott, Haase, Harper, Herzog, Margaret Zorn, Nelson, Schuler, Zitzewitz

ISBN: 9780078458132

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Page 685: Section Review

Exercise 18

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Once we plunge the magnet into the coil the magnetic field will be induced in order to reduce the change of flux induced by the moving of the magnet. The field lines will be in opposite direction from the ones of the magnet. Therefore the coil will swing away from the magnet.

Result

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The coil will swing away from the magnet.

Exercise 19

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The reason for this is that the voltage needed for running a vacuum cleaner is much higher than the voltage needed for having the lamp on. Since the spark occurrence depends on whether or not applied electric field exceeds the dielectric breakdown strength. That is more likely to happen with higher voltages.

Result

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Because the voltage needed for running the vacuum cleaner is much higher.

Exercise 20

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The reason for this is the fact that AC intensity oscillates with time, thus creating a time dependent magnetic field which can induce the current in the secondary coil. On the other hand DC has the constant intensity and thus it cannot create a time dependent magnetic field i.e. it can not induce the current in the secondary coil.

Result

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AC induces the current in secondary circuit and DC does not.

Exercise 21

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The reason for this we find the transformer equation which states that

$$
frac{I_p}{I_s}=frac{N_s}{N_p}
$$

So we see that more turns do we have, the lower is the current and vice versa, less turns higher the current. Now, the resistance comes into play. If we assume that the material the wires are made of is the same, than thicker wires means less resistance. So, in the regions of the high currents we want to reduce the resistance as much as possible in order to prevent heating and energy loss in general.

Result

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Because the thicker wires have lower resistance and since few turns coils will have high currents flowing through them, the low resistance becomes a priority.

Exercise 22

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Since the transformer is defined via the ratio of the turns in its primary and secondary circuit, the ratio of the current will be constant. I.e. change in one current will lead to the change in another.
If the secondary coil is short-circuited, at first, the current will briefly rise followed by the increase of the EMF. However, briefly after that, the total induced EMF will be lower since the current in the secondary circuit will stat to go down, so the primary current will be lower, too.

Result

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The primary current will rise for a very short period of time and then it will be lower than usual.

Exercise 23

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Permanent magnets are definitely a poor choice for a transformer core since their magnetic properties are very much constant and transformer relies on changing the polarity of the core material and thus inducing the current in the secondary coil.

Result

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Permanent magnets would not make a good transformer core.

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