- When an electromagnet is connected to a circuit, the electric current does not reach the required strength immediately, but gradually. Why?
- When the battery terminals of the flashlight are briefly connected and turned off, we notice a spark. It is much stronger if an electromagnet is connected to the battery. Why?
- You can feel almost nothing by touching the battery pole of your flashlight with your fingers. When an electric call is connected to the battery, electric punctures are felt with the fingers. Why?
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$\begingroup$ keyword: "inductance". You have just learned about inductors and now your teacher is quizzing you. $\endgroup$– Stack Exchange Supports IsraelCommented Sep 21, 2020 at 16:31
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1 Answer
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When an electromagnet is connected to a circuit, the electric current does not reach the required strength immediately, but gradually. Why?
Because the electromagnet is basically an inductor with a magnetic core. For an inductor
$$v(t)=L\frac{di(t)}{dt}$$
or
$$i(t)=\frac{1}{L}\int v(t)dt$$
Which tells you it takes time to build the current in an inductor. Or to put it another way, the current in an inductor lags the voltage.
When the battery terminals of the flashlight are briefly connected and turned off, we notice a spark. It is much stronger if an electromagnet is connected to the battery. Why?
When you connect and disconnect the flashlight from the battery terminals the arc is due to the fact that the contacting surfaces are not perfectly flat. A microscopic view would show surface irregularities. The current density at the points of contact is greater than the average current density over the entire contacting surfaces. It is at these points of contact where the arcing is occurring when making and breaking the circuit. This is why mechanical switches always produce some arcing when making and breaking a circuit. The switch contacts are never perfectly flat.
Now if in addition you add the inductor the arcing is greater because, once again, an inductor resists a change in current. The first equation tells you if you attempt to quickly disrupt the current in and inductor (a high value of $di/dt$), it induces a large voltage. The greater intensity arc is due to the large induced voltage, in addition to the arc caused by the irregular contact surfaces.
You can feel almost nothing by touching the battery pole of your flashlight with your fingers. When an electric call is connected to the battery, electric punctures are felt with the fingers. Why?
By electric cell I assume you mean the electromagnet. Again, it's because of the large voltage produced by the coil when attempting to break the circuit.
Hope this helps.
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$\begingroup$ Thank you so much! I didn't have a lot of time to focus on physics the past week, so your answer helps a lot. $\endgroup$ Commented Sep 21, 2020 at 16:59
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$\begingroup$ "electric call" sounds like the OP might be talking about an electric bell such as you might find used in the home of a wealthy family to call a servant. A bell of this type once was common in the junk box of any young person who had an interest in "electricity." (Don't ask me how I know!) When you applied power, the armature would rattle back and forth, interrupting the circuit (and causing high-voltage pulses across the points) 10 to 20 times per second. $\endgroup$ Commented Sep 21, 2020 at 17:05
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$\begingroup$ @SolomonSlow Yeah I thought the OP said "cell", but now I see the OP said "call". In any case, it still sounds like the same inductance phenomenon as the electromagnetic. $\endgroup$– Bob DCommented Sep 21, 2020 at 17:09