Electric field

Why the answer is B?

Posted from TSR Mobile

Electric field strength is uniform.
Field strength is equated to (negative) potential gradient.
So the potential between the plates varies uniformly (straight line) with a gradient equal to the (negative) field strength.

The graph shows a negative gradient line.

In other words
the potential varies uniformly from one plate to the other
when the field is uniform.

Why negative??
And does uniform mean same electric field?

Posted from TSR Mobile

The question says with reference to the +ve plate.

So at that +ve plate the field strength is maximum as shown on the y-axis.

The question also says the field varies uniformly between the plates. i.e. a straight line.

So the field strength at distance d from the +ve plate must be zero at the opposite plate. i.e. V=0 at distance d on the x-axis.

Join the two points and the gradient is -ve. i.e. reducing wrt to the +ve reference plate.

Does uniform electric field mean Constant And does not change?

Posted from TSR Mobile

Yes.
The value of E is equal everywhere in the field.
The force on a charge anywhere in the field is the same value and direction.

The force on the charge, let's say it's positive, is made up of repulsion from the positive plate and attraction to the negative plate. So when you get nearer the positive plate, yes, the repulsion gets larger, but the attraction to the negative plate gets smaller. The result, in a uniform field, is that the total resultant force is constant.

The question says with reference to the +ve plate.

So at that +ve plate the field strength is maximum as shown on the y-axis.

The question also says the field varies uniformly between the plates. i.e. a straight line.

So the field strength at distance d from the +ve plate must be zero at the opposite plate. i.e. V=0 at distance d on the x-axis.

Join the two points and the gradient is -ve. i.e. reducing wrt to the +ve reference plate.

I jsut wanted to ask, if E=V/d will give the pd anywhere in a uniform electric field?

Thanks!

No it gives E, the field strength, anywhere in the field.

The force on the charge, let's say it's positive, is made up of repulsion from the positive plate and attraction to the negative plate. So when you get nearer the positive plate, yes, the repulsion gets larger, but the attraction to the negative plate gets smaller. The result, in a uniform field, is that the total resultant force is constant.

Is there a formula that gives pd anywhere is a uniform electric field?

You don't need a formula, surely.

If the field is uniform the pd varies uniformly from the one plate to the other.

If it's 100V on one plate and 0 V on the other, for example, and there was 100mm distance between the plates, the the pd would vary by 1 volt for every mm. So 20mm from the zero plate the potential would be 20V.
50mm (half way) it would be 50V. And so on. It's just simple proportion.