A uniform electric field of magnitude points in the positive x-direction.
(a) Find the change in electric potential between the origin and the point (0, 2.1m).
E is the electric field
V is the voltage
d is the distance
is the angle between the electric field vector and the position vector
The answer is zero because the electric field is along the x-axis, while our test point is on the y-axis, giving an angle of and the is zero.
(b) Find the change in electric potential between the origin and the point (2.1m, 0).
(c) Find the change in electric potential between the origin and the point (2.1m, 2.1m).
A particle with a mass of 0.44g and a charge of is released from rest at point A in the figure .
(a) In which direction will this charge move?
- Positive y-direction
- Negative y-direction
- Negative x-direction (correct)
- Positive x-direction
(b) What speed will it have after moving through a distance of 19.0cm? The electric field has a magnitude of 4137NC
In the last part where we found the velocity, we used the old kinematic formula ( ). Speed is a scalar which is why our number is not negative even though it is accelerating in the negative x direction.
Is the electric potential at point 1 in the figure greater than, less than, or equal to the electric potential at point 3?
- Greater than
- Less than
- Equal to (incorrect)
(b) Choose the best explanation from among the following:
- The value of the electric potential is large where the electric field lines are close together, and small where they are widely spaced. Therefore, the electric potential is the same at points 1 and 3. (incorrect)
- The electric field lines point to the right, indicating that the electric potential is greater at point 3 than at point 1.
- The electric potential decreases as we move in the direction of the electric field, as shown in the figure. Therefore, the electric potential is greater at point 1 than at point 3.
Cell Membranes and Dielectrics
Many cells in the body have a cell membrane whose inner and outer surfaces carry opposite charges, just like the plates of a parallel-plate capacitor. Suppose a typical cell membrane has a thickness of , and its inner and outer surfaces carry charge densities of and , respectively. In addition, assume that the material in the cell membrane has a dielectric constant of 3.1.
(a) Find the direction of the electric field within the cell membrane.
- Into the cell (correct)
- Out of the cell
(b) Find the magnitude of the electric field within the cell membrane.
(c) Calculate the potential difference between the inner and outer walls of the membrane.
(NOTE: Express your answer in millivolts, mV.)
(d) Indicate which wall of the membrane has the higher potential.
- Inner wall
- Outer wall (correct)