Positive test charge and electric field.

[GRod18]

can someone explain this statement.

The length of this vector (electric field vector) will tell us the magnitude, or strength, of the field at that point, and the direction of the vector will tell us the direction of the resulting electric force that a positive test charge would feel if it were placed at that point.

The positive test charge thing I just don't grasp fully, can someone list an easy example?

Thanks

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[Rabolisk]

Electric field is dependent upon a source charge, but is independent of any "test" charge. Mathematically E = kq/r^2. By convention, electric fields radiate outward from a positive source charge. Electrostatic force is electric field times a test charge. Mathematically F = kqq0/r^2 = Eq0, where q0 is a test charge. If q0 was positive, then the direction of the field and the force are equal. If q0 was negative, then they are the opposite. If an electric field was directed to the right, the electric force on a positive test charge will also be directed to the right. Hope that helped.

 

[nUgnoY]

Just as an analogy to go along with the example, think of the situation as dandelion seed in air

A dandelion seed is blown to the direction of the wind. It's so small and light that its presence doesn't change the wind. You can draw an arrow towards the direction that the seed is flying which will be the direction of the wind.

A test charge would be moved by the electric field. Its charge is so small that its presence doesn't change the electric field. You can draw an arrow towards the direction that the test charge would move which will be the direction of the electric field.

It's a little more complex but I think that's an easy way to grasp the basic. The main point is you want to know the direction and strength of a field WITHOUT changing the field.

 

[Rabolisk]

Well, a test charge's presence may or may not change the electric field, but that is irrelevant, because the test charge's own electric field cannot affect it. The force on the test charge is affected by the magnitude of the test charge, though. Also it is important to distinguish between a positive and a negative test charge, because one moves with the electric field vector, and one moves against it.

Dandelion seed, really?

 

[GRod18]

Somehow I managed a A out of Physics 2 without learning the nitty gritty of this stuff, the special relativity chapter test really helped me.. Too bad thats not on the MCAT lol..

 

[nUgnoY]

Well, a test charge's presence may or may not change the electric field, but that is irrelevant, because the test charge's own electric field cannot affect it. The force on the test charge is affected by the magnitude of the test charge, though. Also it is important to distinguish between a positive and a negative test charge, because one moves with the electric field vector, and one moves against it.

Dandelion seed, really?

It's very important for a test charge to not influence the electric field. If it generated its own field, the electric field would be changed after addition of the test charge. If you measured it, it would not be the same as the original electric field you wanted to measure. Measuring something would not very effective if the method of measuring created inconsistencies or changes in the variable.

You're right, it's important to distinguish between charges, but the OP just asked for an easy example, and I felt that an easy example was not provided in the thread. But it seems that my simple model offended you. Please PM me if you want to!