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# Electric field strength watch

If you have a +ve charge and a -ve charge, and want to find the resultant electric field strength at a point between, the field strength would be non-zero as the forces act in the same way.

My book says to find the resultant field strength add the two individual fields strengths - but one would be negative and one positive so how does that make sense as it means at some point it could be zero. Same applies for the forces...

Should I just be ignoring the signs and considering directions? Or ignoring the signs and always subtracting?

Thanks...
2. (Original post by fayled)

If you have a +ve charge and a -ve charge, and want to find the resultant electric field strength at a point between, the field strength would be non-zero as the forces act in the same way.

My book says to find the resultant field strength add the two individual fields strengths - but one would be negative and one positive so how does that make sense as it means at some point it could be zero. Same applies for the forces...

Should I just be ignoring the signs and considering directions? Or ignoring the signs and always subtracting?

Thanks...
you need to consider directions though

which way would a positive test charge accelerate if it was placed in a negative field? towards the field source

which way would it accelerate if placed in a positive field? away from it

so there can't be a point where the resultant field strength is zero - probably best to just consider the directions when calculating the resultant field
3. (Original post by boner in jeans)
you need to consider directions though

which way would a positive test charge accelerate if it was placed in a negative field? towards the field source

which way would it accelerate if placed in a positive field? away from it

so there can't be a point where the resultant field strength is zero - probably best to just consider the directions when calculating the resultant field
Yeah I knew that was the case. However If there was a way to do it simply by considering the signs rather than considering the magnitudes and directions separately it would be simpler.

But I will just use the directions approach anyway.
4. (Original post by fayled)
Yeah I knew that was the case. However If there was a way to do it simply by considering the signs rather than considering the magnitudes and directions separately it would be simpler.

But I will just use the directions approach anyway.
Maybe always subtract while considering the signs?
5. (Original post by boner in jeans)
Maybe always subtract while considering the signs?
Aha yeah as I suggested in the OP. Thanks anyway.
6. You would add the magnitude of the individual strengths... (i.e., ignore the signs, and just add the numbers).

But a general rule that would work for everything, regardless of the charges, would be to subtract field strengths.
7. (Original post by Qwertish)
You would add the magnitude of the individual strengths... (i.e., ignore the signs, and just add the numbers).

But a general rule that would work for everything, regardless of the charges, would be to subtract field strengths.
Hmm yeah, the first is the most logical approach to use for me, but if I could make sense of the latter in terms of why it works I could use it.

I can see that it always works in terms of the numbers but why does subtracting them work in a sense of what is actually going on?
8. (Original post by fayled)
Hmm yeah, the first is the most logical approach to use for me, but if I could make sense of the latter in terms of why it works I could use it.

I can see that it always works in terms of the numbers but why does subtracting them work in a sense of what is actually going on?
same as when you consider forces, it's the direction that's important

imagine an object in the centre and force A is on the left of the object, applying a force to the right. and force B is on the right, applying a force to the left. as the forces are opposite, the resultant you can find by subtracting them, it's the direction that's important
same thing if the force on the right applied applied a force to the right, then you would actually add the forces

try and imagine the electric fields in a similar situation

now try to imagine if there is a small positive test charge and the positive and negative electric fields are both on the left of the test charge. you would actually have to add them now rather than subtract to find the overall position

it's just most important to consider direction here
9. (Original post by fayled)

If you have a +ve charge and a -ve charge, and want to find the resultant electric field strength at a point between, the field strength would be non-zero as the forces act in the same way.

My book says to find the resultant field strength add the two individual fields strengths - but one would be negative and one positive so how does that make sense as it means at some point it could be zero. Same applies for the forces...

Should I just be ignoring the signs and considering directions? Or ignoring the signs and always subtracting?

Thanks...
Field strength is a vector and must always be added as a vector. This means you include the direction and use vector addition.
In the simple case (most often asked in questions) where the two vectors are in the same straight line, you can add or subtract them depending on their direction. First work out the direction. It's the direction a positive charge would move.
Forget magnitudes. Use directions. They are vectors.

When dealing with electric potential, on the other hand, this is a scalar and just adds or subtracts as a number. Magnitude only.
10. (Original post by boner in jeans)
same as when you consider forces, it's the direction that's important

imagine an object in the centre and force A is on the left of the object, applying a force to the right. and force B is on the right, applying a force to the left. as the forces are opposite, the resultant you can find by subtracting them, it's the direction that's important
same thing if the force on the right applied applied a force to the right, then you would actually add the forces

try and imagine the electric fields in a similar situation

now try to imagine if there is a small positive test charge and the positive and negative electric fields are both on the left of the test charge. you would actually have to add them now rather than subtract to find the overall position

it's just most important to consider direction here
Yeah because the - and + really are indicative of whether the field is attractive or repulsive from the charge considered towards a positive test charge, as opposed to telling us which direction the fields are acting (I mean it does tell you which way it acts, but say if you consider two charges, then different signs can mean the same direction, so in the bigger picture it doesn't), so two fields can act in the same direction but have opposite signs because its telling us if the field attracts or repels towards that charge, not its direction.

Correct?
11. (Original post by fayled)
Yeah because the - and + really are indicative of whether the field is attractive or repulsive from the charge considered towards a positive test charge, as opposed to telling us which direction the fields are acting (I mean it does tell you which way it acts, but say if you consider two charges, then different signs can mean the same direction, so in the bigger picture it doesn't), so two fields can act in the same direction but have opposite signs because its telling us if the field attracts or repels towards that charge, not its direction.

Correct?
The direction is defined as the way in which it acts on a positive test charge. Therefore, direction of the field strength is positive if it is formed by a positive charge (and repels a +ve test charge), and negative if it is formed by a negative charge (and attracts and -ve test charge).

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