-I understand when there is an on-going electric field, if it is produced by a positive charge, the direction of electric field is away from it etc. And, if we get a +ve test charge, it repels it, and the force is also the same direction as the field. However, I can't understand why and how we know the main electric field is + or – charged? What causes it? And, if electric field is produced by electric charge, how come the test charges don't change or affect the main field? (Sorry, if I make no sense!) For instance, what causes and determines the direction of electric field in a DC power line?
Electronic – What determines the direction and cause of electric field exerted on test charges
chargefield
Related Solutions
The electrostatic field is zero AFTER equilibrium is established (after the internal E-field cancels the external E-field.)
Remember the equation that describes the introduction of charges at some interior point of a given material:
What it means is that any charge introduced at the interior of the conductor will decay immediately and appear on the the surface.
Stated in other way, if some charges are introduced in the interior of a conductor, the charges will move to the surface and redistribute themselves quickly in such a manner that the field inside the conductor vanishes.
I think you must use the superposition principle of the field in this case, and consider the problem as a distribution of charge along a line from a to -a, because the contribution to the field of the charge along the z axe is 0, the field in this case is always parallel to the surface xy. The problem is to build the right integral considering a dE and a dL (along the y axe).
Note: there is a little mistake, I put r=x^2+y^2 instead r^2=x^2+y^2, I have forgot to put the ^2 on the r, but I have taken into account in the rest of the calculation.
Related Topic
- Electronic – What exactly are surface charges and what do they do
- What happens when I put a charge inside a conductor that is already in static equilibrium
- Electrical – How do laser printers produce the positive and negative charges on the paper
- Electronic – Why do charges not lose potential as they travel through the circuit before reaching a resistor
- Electronic – What causes the electric field to change over series resistors
- Electronic – Capacitor in presence of an external electric field
Best Answer
One way is to place free test charge in the field and see which way it moves.
One instrument that essentially does this is the electrometer.
In physics textbooks, we an study a case where a single charge sits in an infinite universe with no other charge, producing an electric field that radiates away from it in all directions with perfect symmetry.
In the real world, we almost always deal with situations where charge is balanced. There is a negative charge somewhere to terminate whatever field emanates from a positive charge and vice versa.
So if we see an electric field pointing from A to B, there is almost always a positive charge somewhere in the neighborhood of A, and a negative charge near B.
Whoever designed the power system designed the system to produce either a positive or negative voltage on the line (relative to earth).
If they produced the DC voltage by rectifying an AC source (like a generator), then they can choose the polarity of the DC voltage by how they connect the rectifier to ground.
If they produced the DC voltage from a battery, then the chemical reactions going on in the battery determine which terminal is positive and which is negative. We can connect our power line to either of these and connect the other to earth (and possibly also to a return conductor) to get whatever polarity we want on the DC power line.