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How to calculate magnetic field strength?

How to calculate Electric field strength at a point between two charges?

  • If I have two charges of -4nC and -6nC, and they are 40cm apart, what is the electric field strength at the midpoint between these two charges? How do I know if it's positive or ...show more

  • Answer:

    To find the electric field at any point you use the formula that's derived from the Coulomb Force, you should have this in your book. E=kQ/(r^2), where k is 8.99*10^9 Nm^2/C^2, Q is the magnitude of the charge, and r is the distance from the charge. When dealing with two charges (or any number) all you have to do is add the electric field of the two charges to get one net electric field. Positive charges have an electric field that points outward in all directions from the charge. Negative charges have electric fields that point inward towards the charge from all directions. In this case imagine to the left is the -4nC charge, then 20cm to the right is the point where you are measuring the field, and 20 more cm to the right is the -6nC charge. Because both charges are negative, we should expect one electric field line to point left towards the -4nC charge (negative charges point inward towards the charge) , and the other to point right towards the -6nC charge (becuase the other charge is also negative, and negative charges have Electric fields hat point inward towards the charge), but this second line will be larger becuase the charge is larger. Thus when we calculate the total electric field, the total electric field should face the right towards the larger -6nC charge. If both charges were positive we would expect the electric field lines to point away from each charge, and the net eclectic field would point to the left away from the larger +6nC charge. If one charge was say +1C and the other was -1C, we should expect one field line pointing away from the +1C charge and one pointing towards the -1C charge. In this case, where we are between the two, they will both be facing the same direction, and the total field would be twice as large as each field line individually, facing the negative -1C charge.

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first consider the midpoint to be a positive test charge. (NOTE: always consider the midpoint "test charge" to be positive) to find the field strength midway between the charges you must assume there is a positive test charge present. im assuming that by -4nC you mean -4 "nanocoulombs" (1 x 10^-9). not "microcoulombs" (1 x 10^-6) convert to coulombs: -4nC = -4 x 10^-9 C -6nC = -6 x 10^-9 C you should know: E = (kQ)/r^2 k = 9 x 10^9 {its just general a constant} Q = magnitude of charge (units: coulombs C) r = radius or distance from charge (units: m) Also, you should be able to add simple 2D vectors. Calculations I'm assuming that if you drew a diagram the -4nC would be on the left and the -6nC on the right, the midpoint charge located on the direct line between them -4nC -------- x -------- -6nC magnitude of E between -4nC and point charge E = (kQ)/r^2 E = ((9 x 10^9)(4 x 10^-9)) / ((0.2)^2) E = 900 N/C to the right magnitude of E between -6nC and point charge E = (kQ)/r^2 E = ((9 x 10^9)(6 x 10^-9)) / ((0.2)^2) E = 1350 N/C to the left Let "left" be the positive direction E(total) = 1350 - 900 {consider positive direction to be to the left, therefore making 1350N/C to the left positive and 900N/C to the right negative (i.e. -900 N/C to the left)} = 450 N/C to the left EFS : electric field strength If the charges were opposite in sign (i.e. one negative and one positive) yes, you simply "add" the vectors in whatever direction you state to be positive {its best to go in the direction of the greatest EFS magnitude}. In your presented case this was the EFS between the point charge and the -6nC charge.

Ben T

Electric field potential is based on what a +1 coulomb charge would experience at that point. Since it's a vector field, you sum up vector charges. So, if you have two positive charges, at the midpoint both vectors will be facing away from each other and they will act in opposition. If you have one positive and one negative, the vectors will act in the same direction - towards the negative charge. Use Coulomb's law to figure out what the field strength is.

astazangasta

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