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QCE Physics Questions Thread

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jasmine24:
hi, i was wondering how Lenz's law is consistent with the principle of conversation of energy
Thank you

Bri MT:

--- Quote from: jasmine24 on November 09, 2020, 07:51:07 am ---hi, i was wondering how Lenz's law is consistent with the principle of conversation of energy
Thank you

--- End quote ---

Hi,

Hopefully this reaches you before paper 2!

I encourage you to draw out how you could get a situation with infinite electrical energy if Lenz's law wasn't true by having positive feedback loop of: more current -> more change in flux -> more current -> more change in flux etc.

Another way to think about it is to take the situation of moving a coil towards a bar magnet. As the coil moves the change in flux will induce current in the coil and thus the coil will generate a magnetic field. This magnetic field will have a north and south end and consequently there will be attraction or repulsion between the coil and bar magnet. This attractive or repulsive force will act over the displacement of the coil (W=Fx) and this work balances the change in electrical energy.

I've included a rough diagram below where you can see that as the distance between the bar magnet and loop decreases the generated magnetic field is such as to generate repelling force.

jasmine24:
For my SI, i'm doing inclined planes and one of the suggested improvements was using a longer incline (I'm measuring time taken for object to travel down the inclined plane) because it would give more consistent results but I'm not sure how it would and was wondering if anybody knew why  :)

fun_jirachi:
The longer the plane is, the longer it takes for the object to roll down the plane. This increases the consistency of results because random error will comparatively be a smaller percentage of the time taken than if you had a smaller incline.

Consider fixing random error at ±0.1s. A longer plane may take 3s + random error, while a smaller plane may take 2s + random error. This is an unrealistic example but hopefully, this demonstrates that longer inclines will minimise the impact of random error on your experiment (thus providing more consistent results). It's the same reason why we choose compounds of high molar mass for standard solutions in chemistry.

Hope this helps :)

jasmine24:
what is the relationship between the length of an inclined plane and acceleration? All ik is that as displacement increases, acceleration decreases but I'm not sure why
TIA

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