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I know how to calculate the field at any point from each sphere, but not sure how to work backwards and find the charge.

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I know how to calculate the field at any point from each sphere, but not sure how to work backwards and find the charge.

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Hootenanny

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Hootenanny

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Also, what is the definition of the electric field?

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Electric field is an area where a charge experiences a force (F=Eq). The field strength from a point is kq/r^2 so the field strength between the points would be the vector sum of the field from each sphere. If they are equal and opposite charges this is just 2kq/r^2. I don't think the two spheres will create a uniform field though, which the question sort of implies.

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Hootenanny

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All is correct. However, you can still calculate the potential of some point in an elctric field can you not?

Electric field is an area where a charge experiences a force (F=Eq). The field strength from a point is kq/r^2 so the field strength between the points would be the vector sum of the field from each sphere. If they are equal and opposite charges this is just 2kq/r^2. I don't think the two spheres will create a uniform field though, which the question sort of implies.

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Hootenanny

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Correct!The potential difference V is the product of the field strength and the distance (I could be wrong), V=Ed. The question says the air ionises when the field strength is 30,000V/cm which means E=(30000/0.015)=2*10^6 N/C. So the air ionises causing a spark when there is a field of that magnitude between the two spheres.

Why must the field be uniform? What would you see if the field was uniform?But if the field is not uniform, that doesnt make sense.

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Hootenanny

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CorrectIf the field isnt uniform then some parts of the air are ionised and other arent.

You can still say this, except that it is not a requirement that theI was assuming that it would be uniform so I could say, when all the area between the spheres had a field of x, the air ionised and a spark happened.

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Does the whole 'path' from one sphere to the other need to have this field? I just made a graph of position between the spheres vs field strength and is looks the shape of the letter u. The minimum field is in the centre of the two spheres so if a path of 'ionised air' is to be made, this would be the area last to be ionised and hence what I use to calculate the answer.

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