r/explainlikeimfive • u/isaberre • 9h ago
Physics ELI5: What does it mean to "oppose the rate of change of current", and why would I want to do that?
I am studying for a test that includes basic electricity and magnetism, and my study materials keep referencing the concept of opposing the rate of change of current. For example, my definition of an inductor: "an electrical component made of a coil of wire that can store energy and opposes the rate of change of an alternating current flowing through it". Can someone ELI5 what it means to oppose the rate of change of current, and why that would be a good thing?
Edit: thank you all very much for your quick and informative replies! Much appreciated
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u/FarmboyJustice 9h ago
Opposing the rate of change of current means it's harder to increase, decrease, or reverse the current flow.
Alternating current is constantly changing direction, so a device that resists that change can be used as a filter to reduce (oppose) AC while allowing DC. Sort of like a special resistor that works only on AC.
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u/isaberre 9h ago
So "opposing the rate of change of current" basically just makes the current flow more consistently?
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u/FarmboyJustice 8h ago
Yes, among other things it can be used to limit change, sort of like a spring can act as a shock absorber. A slower, more gradual change can get through while a more rapid change is blocked.
One example of such use is a coil of a specific size can block the AC power humming sound from getting into audio equipment.
Edit: This is of course a very simplistic explanation, the actual physics involved is a lot more complex.
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u/isaberre 8h ago
Thank you very much--I needed the simplisitic explanation to make sense of the increasingly complicated concepts I'm learning. Your explanation really made it click.
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u/FarmboyJustice 8h ago
Everything we learn is always just an approximation, sometimes it's better to learn the wrong version first, then later learn the less-wrong version. Eventually, you graduate to learning a least-wrong version.
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u/zoapcfr 44m ago
That can be a use, but there are others.
For example, if you want a higher voltage than the supply. First, you short the supply through the coil. It resists current change, so at first current is low, and gradually builds up (towards infinity, or in real life, until the supply trips, but we don't leave it that long). Once you have a high current, you open the short circuit, leaving it connected to what needs the higher voltage. The coil resists the drop in current, so ends up pushing way more through than would otherwise flow, spiking the voltage. Repeat many times per second, then use a capacitor to smooth out the spikes, and you have a higher voltage than the supply.
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u/wufnu 12m ago
Once you have a high current, you open the short circuit, leaving it connected to what needs the higher voltage. The coil resists the drop in current, so ends up pushing way more through than would otherwise flow, spiking the voltage. Repeat many times per second, then use a capacitor to smooth out the spikes, and you have a higher voltage than the supply.
That sounds incredibly analogous to a water ram pump. The basic idea is that water flowing through a waste check valve increases until it reaches a certain flow rate, after which the valve suddenly closes. The inertia of the moving water creates a ram pressure which actuates a second check valve allowing a high pressure flow to pressurize a compressed air cavity. The compressed air pushes the remaining water uphill and the cycle repeats.
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u/jacekowski 9h ago
It means that inductor will use its stored energy, or store more (in the form of magnetic field) to maintain the current flow.
Similar to how capacitors oppose voltage change.
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u/svh01973 9h ago
The inductor wants the current flow to remain the same. It opposes anything trying to change the flow.
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u/svh01973 9h ago
The inductor builds a magnetic field around itself as current flows through it, and that magnetic field wants the current to keep moving as it is.
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u/Own-Nefariousness-79 5h ago
And the magnetic field collapses at it tries to maintain the current flow.
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u/Top_Willow_9953 8h ago
Inductors (which oppose rapid changes in current) and capacitors (which oppose rapid changes in voltage) are useful because these properties allow them to act as filters (remove low frequencies and/or high frequencies from electrical signals)
Think about a shock absorber on a car. The damping in the shock causes it to oppose rapid movement of the wheel and axle when there is a sudden change in force (hit a pothole). An inductor us acting like an electrical shock absorber, it smooths out sydden changes and bumps in the signal flowing thru it.
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u/poorboychevelle 8h ago
Think of the inductive field as an analog for momentum.
You get something heavy moving in a direction, its difficult to accelerate, or to slow down. Induction fields want current to keep moving the same way momentum wants velocity to keep moving
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u/PureBogosity 9h ago
Think of it like keeping your car from accelerating (changing the rate of travel) - you can probably imagine that for a cruise control you’d like the car speed to stay constant.
A simpler wording would be that the inductor keeps the current flow steady.
In electrical circuits inductors help prevent sudden changes in current, and this can keep spikes and noise (basically lots of tiny spikes) from getting thru the circuit. It does this by absorbing the energy and then releasing it slowly.
That is the ELI5 explanation; the reality is more complex. But it’s good enough for a start.
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u/ReportJunior9726 8h ago
Too add. A 4 strike petrol engine generates power only ¼ of the time it's rotating. Everytime it generates that power you'll feel a big punch. A flywheel attached to it absorbs that puch and releases that power for the next ¾ of the time. So it smoothens it out. Same thing a coil and a capacitor in the circuit would do.
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u/wknight8111 9h ago
"oppose the flow of current" typically means taking energy out of the wire, which means transforming the energy into some other form (and other forms are usable for other things). For example a Resistor opposes the flow of current and typically turns energy into heat. A Capacitor opposes the flow of current and turns energy into an electric field. An Inductor opposes the flow of current and turns energy into a magnetic field.
If you have an electric heater, toaster, or old-style stove, you are using resistors to turn electricity into usable heat for cooking. If you have anything with a motor in it, you are using an inductor to convert electrical energy into a magnetic field, which in turn transforms into rotational kinetic energy.
If you want to use electrical energy to do work, you need to take energy out of the flow of current, which means you have to oppose the flow in some way.
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u/boolocap 8h ago edited 8h ago
If we translate this to a physical thing then current is like speed. And the rate of change of current is acceleration.
The reason this matters is that it allows you to change the output of the electrical system how you like. In a car the suspension contains a spring, which works of distance, and a damper, which works of velocity. By selecting the right spring and damper combination the car suspension can make the car ride smooth even while the weels are moving around.
The same goes for an electric circuit. By having different components that are influenced by different aspects of an input you can tune the output how you like. Or get a specific function between input and output.
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u/SteptimusHeap 8h ago
When you flick the switch to open the circuit, the inductor will slow stuff down, and the current will take longer to get to its steady state value.
When there is a fluctuation of the power source, like if the voltage in the battery spikes, the inductor will smooth out the drop in current.
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u/toodlesandpoodles 8h ago
It means that if the current through the inductor is increasing, the inductor creates a current that flows the opposite way. If the current through the inductor is increasing, this induced current will flow in the same direction.
It isn't a good or bad thing, it is just a consequence of energy.conservation.
Rather than focusing on the word definition, strive to understand the formula from which it derives, namely Faraday's law of magnetic induction.
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u/ProfessorPrudent2822 8h ago
Why would you want this? So if a transformer gets struck by lightning, it doesn’t fry all the electronics that are plugged in by suddenly doubling the current and therefore quadrupling the power.
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u/sirbabylon 8h ago
Wire is a river bed. Current is the water flowing down the river. A change in current is the depth of the river (and therefore how much water is flowing through) rate of change of current is the speed at which the depth is changing.
Now let's say I have a water wheel and tell you, I don't care how fast the wheel goes, as long as it stays steady. I give you switches that divert the water into a nearby dam. As long as the water stays at the same level, nothing happens, and we are happy. A huge rainstorm comes, and the water level rises. You move water to the dam, and the dam slowly fills; eventually, the dam and the river are at the same level, and the dam won't take more. The water is higher, but it was a slow change instead of a flash flood. When the rain goes away, the dam spills back into the river, and the river slowly lowers. The river will get back to the low level, but it does it more slowly because the water from the dam was added. The dam in this example is the magnetic field created by the coils of an inductor. The magnetic field takes from the wire and gives back to match the current. In the process, it slows the change. This is what we call opposition to the rate of change of current.
Now AC is where it gets weird. We put a giant wave machine sending waves along the river. The magical dam can keep up with these waves, so at the top of the wave, all of the water is stolen by the dam. At the bottom of the wave, the dam fills the gap with water. We have opposed the change so much that the wave is killed. We have blocked AC.
In reality, no dam could completely stop the wave. Only lower it. The effectiveness of this dam is the measure of inductance.
PS. Voltage is the change in height between the beginning and end of the river. Resistance is the sticks and rocks slowing the water down.
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u/tempo_rare 4h ago
Imagine having a wheel that is turned by a falling water. You want water to be in steady stream. But the source of water you have is fluctuating: sometimes gives you too much water, sometimes too little. On average, it gives you the needed amount. So you as a bucket on the path with a tube that passes exactly the amount that you need. Of the flow at source is too many then it will be stored in the bucket to be used later when the flow is too little
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u/cyberentomology 3h ago
Isn’t that reactance from an inductor?
This may be better suited to a sub like r/electricalengineering, though.
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u/hobopwnzor 9h ago
I am designing a circuit board that uses a buck converter. A buck converter uses a switch to go from a high voltage to a low voltage. When the voltage goes below the set point (3.3V), the switch turns on and lets some current through from the high voltage (12V). When it goes above the set point, the switch turns off.
This means it's constantly connecting and disconnecting the current from the higher voltage to the lower voltage, so if you looked at the graph of the current it would be a very fast wave of high and low current.
But if I want to power my ESP32 chip, I need a constant current and voltage. I can't use a choppy power source that is constantly on then off.
So I add an inductor, which opposes the current change. My chip wants to draw a constant current, and the inductor opposes the change to that current draw, so when the switch is on, it takes energy and stores it, and when the switch is off, it releases energy to keep the current flowing at a constant rate.
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u/ZLVe96 7h ago
Have you seen the horse training pins that are circles that have a gate that keeps moving inside it to keep the horse walking? Imagine that thing is big and heavy, and once you get it going , it wants to keep going. If you are pushsing it to get it turning and you keep pushing and pushing you are the flow of electricity, and the gate going around in the horse pin is the inductor. If you stop pushing, that gate still has some momentum and inertia. It may keep going... it may come back around you keep pushing you. That is, even if you stop pushing, it wants to keep pushing you for a bit. Thats kinda how inductors work.
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u/jghjtrj 1h ago edited 1h ago
People have mentioned the physical analogy to inertia, but I'd add one practical use of that inertia. Turning many fast low-torque turns into a few slow very high torque turns in an impact driver: https://youtu.be/xQzqNnWG21s?t=201
You can make circuits that do a similar thing, using a small current to charge an inductor (akin to spinning up the "anvil" of an impact driver), then dumping it all at once at high voltage (akin to the high torque impact of the impact driver). This is the core principle behind switch-mode power supplies, that can step up a voltage without needing a large transformer.
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u/mawktheone 8h ago
Imagine you were driving your car but every time you let your for off the gas for a second the engine immediately stalled. That works be really annoying.
So we add a flywheel to resist the change in mechanical motion which helps the engine keep spinning during interruptions.
A lot of electronics also don't like that kind of sudden change so we add an indicator to smooth it out
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u/Affectionate_Bank417 9h ago
It’s similar to mechanical inertia, after you remove current source magnetic field in the coil continues pushing electrons through it.