AQA physics paper question: Where is the potential zero near these charges?

[bonbon22]

Homework Statement

The diagram shows two charges, +4 µC and -16 µC, 120 mm apart. What is the distance from the +4 µC
charge to the point between the two charges, where the resultant electric potential is zero?
A 24 mm
B 40 mm
C 80 mm
D 96 mm

Relevant Equations

f=kq1q2/r^2

This question is a bit werid because would there be a resulatant electric potential equal to zero as we have a positive and negative charge in which case there's no neutral point. As E1-E2 , they have to be acting in oppostie directions , in this case would they be acting in the same direction.

 

[PeroK]

This question is a bit werid because would there be a resulatant electric potential equal to zero as we have a positive and negative charge in which case there's no neutral point. As E1-E2 , they have to be acting in oppostie directions , in this case would they be acting in the same direction.

Why do you think zero potential implies zero electric field (at a given point)?

 

[bonbon22]

Why do you think zero potential implies zero electric field (at a given point)?

i think their is no neutral point but apparently their is, but these two charges are opposite yet the questions suggested theirs a neutral point so i don't quite understand. i understand how to get the answer of 24 mm.

 

[PeroK]

i think their is no neutral point but apparently their is, but these two charges are opposite yet the questions suggested theirs a neutral point so i don't quite understand. i understand how to get the answer of 24 mm.

What do you mean by a "neutral" point? Where the field is zero? Or, where the potential is 0?

Note that you can always add a constant to the potential, so that the value of the potential has no physical significance in itself.

 

[bonbon22]

What do you mean by a "neutral" point? Where the field is zero? Or, where the potential is 0?

Note that you can always add a constant to the potential, so that the value of the potential has no physical significance in itself.

i mean that the when you have two positive or negative charges inbetween there's a point where F1= F2
the neutral point, in this case as the forces are acting in together its f1+f2, but would their potentials not also act in the same direction? hence no neutral point. to get the answer 24 mm i minused the potentials.

 

[PeroK]

i mean that the when you have two positive or negative charges inbetween there's a point where F1= F2
the neutral point, in this case as the forces are acting in together its f1+f2, but would their potentials not also act in the same direction? hence no neutral point. to get the answer 24 mm i minused the potentials.

A potential has no direction. It's not a vector. The potential and the field are very different things.

 

[bonbon22]

A potential has no direction. It's not a vector. The potential and the field are very different things.

So why does minusing the potentials give me the answer ? I assumed as f = k q1q2/r^2 and potential has the equation v=kq1/r that's they are similar , different from graviational where its mass which can only have a magnitute but charge can be positive and negative right? so the potential can be positive and negative am i missing something?

 

[PeroK]

So why does minusing the potentials give me the answer ? I assumed as f = k q1q2/r^2 and potential has the equation v=kq1/r that's they are similar , different from graviational where its mass which can only have a magnitute but charge can be positive and negative right? so the potential can be positive and negative am i missing something?

A sign is not a direction. You have found a point where the potential is 0. That says nothing about the field at that point.

Try calculating the electric field at that point.