Electric Fields

Flashcards about concepts of Electric Fields

Polarization

The charges in the ball can be separated by induction (polarization), leading to a net attraction even if the ball is neutral.

Charge Polarization in Conductors

While the conductors are connected, positive charge will flow from one conductor to another due to polarization. Once disconnected, the charges will remain on the separate conductors even when the rod is removed.

Coulomb's Law Symmetry

The force between two charges is symmetric with respect to the two charges involved. The force of one on the other is the same as the reverse (Newton's 3rd Law).

Net Electric Force of Zero

The point where the net force on Q0 is zero can be found regardless of the sign of Q0, as long as it is on the line between +Q and +4Q.

Electric Force and Distance

Since +4Q is 4 times bigger than +Q, Q0 needs to be farther from +4Q. Q0 must be twice as far from +4Q, since the distance is squared in Coulomb’s law.

Electric Force Cancellation

A charge (positive or negative) can be placed to the left of the +Q charge, such that the repulsive force from the +Q charge cancels the attractive force from –4Q.

Acceleration of Proton vs. Electron

The particle with the smaller mass will have the larger acceleration. This is the electron because F=ma.

Meeting Point of Proton and Electron

The electron and proton feel the same force due to Newton's 3rd law with F = ma. Since the proton’s mass is much greater, the proton’s acceleration will be much smaller! Thus, they will meet closer to the proton’s original position.

Net Force Direction

The charge +2Q repels +Q toward the right. The charge +4Q repels +Q upward, but with a stronger force. Therefore, the net force is up and to the right, but mostly up.

Electric Field Equation

E = kQ/r^2. Doubling the charge puts a factor of 2 in the numerator, but doubling the distance puts a factor of 4 in the denominator because it is distance squared!!

Electric Field Vector

For the upper charge, the E field vector at the center of the square points toward that charge. For the lower charge, the same thing is true. Then the vector sum of these two E field vectors points to the left.

Net Electric Field at Center of Square

The four E field vectors all point outward from the center of the square toward their respective charges. Because they are all equal, the net E field is zero at the center!!

Uniform Electric Field Calculation

E = F / q = 12 N / 4 C = 3 N/C. Once the 4 C charge is replaced with a 6 C charge, this new charge will feel a force of: F = q E = (6 C)(3 N/C) = 18 N

Electric Field Lines Density

The field lines are denser around the red charge, so the red one has the greater magnitude.