Could someone please help me understand the charging capacitor and discharging capacitor? and switching on and off an inductor? and how to answer it NCEA style?
Hi @muhc, welcome to studyit.
I was going to prepare a video to respond to your question, then I remembered this gem of a video produced by a friend and previous colleague Kent Hogan.
In terms of answering an NCEA style question on it, you could check out this exam review by Mr Whibley. I doubt you’d have to deal with switching on and off an inductor in the same question, but hopefully after reviewing those two videos, you’d be prepared for it.
If you would like to write an example answer, post it here and I’ll give you feedback on it.
Best wishes!
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Thank you so much! I’ll have a look at these videos.
Hey so I wrote an example answer for 2020 Q2 b and c could you please mark it and give me some feedback please:
Explain why the current takes time to reach a maximum once the switch is closed
When the switch is closed there is a great change in current as it increases which means there is a change in flux inducing a large back emf that opposes the current change and slows down the rate of current change.
Spark Plug needs a very high voltage 20 000 V to produce a spark.
Explain why a park is produced when the switch is opened, but not when it is closed.
Once the switch is opened there is a huge change in current as it drops to 0 rapidly. to oppose this change a large emf is produced increasing voltage and producing a large spark. The energy to produce this spark comes from the change in flux created when there was a change in current.
Hi @muhc,
Sorry I didn’t see this before your exam, I hope it went well today.
Those answers look good, you seem to understand the concept.
Explain why the current takes time to reach a maximum once the switch is closed
When the switch is closed there is a great change in current as it increases which means there is a change in flux inducing a large back emf that opposes the current change and reduces the rate of current change.
You could add (but don’t need to): After 5 time constants, the current would be so close to it’s maximum value we would consider it to be at maximum
Spark Plug needs a very high voltage 20 000 V to produce a spark.
Explain why a park is produced when the switch is opened, but not when it is closed.Once the switch is opened there is a huge change in current as it drops to 0 rapidly. This would cause a rapid change in flux. To oppose this change a large back emf is produced This high voltage across a small gap produces a spark. The energy to produce this spark comes from the change in flux created when there was a change in current. Since τ = L/R the high resistance of the air gap causes the time constant to be very short, so the change happens very rapidly. Kirchoff’s Voltage Law only applies to closed circuits, and since this circuit is now open, the maximum voltage can exceed the original supply voltage.