Electromagnetics
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Physics › Electromagnetics
What size resistor should be connected across the terminals of a 
Explanation
Ohm's law is 
In this case, 
Solving for the resistance, 
Substitute known values and solve for the unknown resistance:
Three equal charges are at three of the corners of a square of side d. A fourth charge of equal magnitude is at the center of the square as shown in Figure above. Which of the arrows shown represents the net force acting on the charge at the center of the square?
B
A
C
D
Explanation
Because of the principles of superposition, each electric force that acts from the charges at the corners on to the charge at the center can be broken into components. Since all the charges are positive, all the forces will be repulsive. The forces acting from the top left and bottom right corners will cancel, leaving only the repulsive force coming from the bottom left corner.
A charged particle traveling along the +x axis enters an electric field directed vertically upward along the +y-axis. If the charged particle experiences a force downward because of this field, what is the sign of the charge on this particle?
It is negative
It is positive
It is neutral
None of these
Explanation
Positive charges in an electric field will experience an electric force that is in the same direction as the electric field. If the charge is negative, the force will be in the opposite direction of the electric field. Since the charged particle experiences a force which is opposite to the electric field, the sign of the charge must be negative.
An x-ray machine emits electromagnetic photons carrying a frequency of 
Explanation
The energy of a photon of a given frequency is determined by the equation
So plug in Plank's constant and the frequency of the x-ray photons to get an energy of very near
A charged particle traveling along the +x axis enters an electric field directed vertically upward along the +y-axis. If the charged particle experiences a force downward because of this field, what is the sign of the charge on this particle?
It is negative
It is positive
It is neutral
None of these
Explanation
Positive charges in an electric field will experience an electric force that is in the same direction as the electric field. If the charge is negative, the force will be in the opposite direction of the electric field. Since the charged particle experiences a force which is opposite to the electric field, the sign of the charge must be negative.
Suppose that a magnetic field is oriented such that it is pointing directly to the left, as in the picture shown below. If a positively charged particle were to begin traveling through this magnetic field to the right, in which direction would the particle's trajectory begin to curve?
The particle would continue to move to the right unaffected
The particle would move out of the page
The particle would move down the page
The particle would move into the page
Explanation
In order to answer this question, it's important to understand the factors that determine the magnetic force experienced by a charge. We can begin by writing out the equation for magnetic force.
As shown in the above equation, the magnetic force is directly proportional to the particle's charge, its velocity, and the strength of the magnetic field itself. But, for the purposes of the this question, the most important factor is the angle of the particle's velocity with respect to the magnetic field.
Notice that if theta is equal to zero, then the sine of theta will be equal to zero as well. This, in turn, will cause the magnetic force to also be zero. This is also true if we were to define theta as 
Since the particle is moving in a direction that is parallel to the magnetic field lines but in the opposite direction, we have a situation in which theta is equal to 
A single string of wire has a resistance of 
Explanation
So this is all about the magnetic field strength around a current carrying wire.
The equation for this is:
But you must use Ohm's Law 
Since the wire has 
Being sure to change 
A conductor is placed in an electric field under electrostatic conditions. Which of the following statements is correct for this situation?
All of these
The electric field is zero inside the conductor
All valence electrons go to the surface of the conductor
The electric field on the surface of the conductor is perpendicular to the surface
Explanation
A conductor is defined as a object free to move charges. In particular, valence electrons, which are the outer most electron in each atom and the most free to move, travel inside the conductor until the net electric field inside the conductor is zero. These electrons will move until this condition has been met. Because of the presents of charged particles at the surface and the condition that they are no longer moving, any electric field at the surface must be perpendicular to that surface.
An x-ray machine emits electromagnetic photons carrying a frequency of
Explanation
The energy of a photon of a given frequency is determined by the equation
So plug in Plank's constant and the frequency of the x-ray photons to get an energy of very near
Three equal charges are at three of the corners of a square of side d. A fourth charge of equal magnitude is at the center of the square as shown in Figure above. Which of the arrows shown represents the net force acting on the charge at the center of the square?
B
A
C
D
Explanation
Because of the principles of superposition, each electric force that acts from the charges at the corners on to the charge at the center can be broken into components. Since all the charges are positive, all the forces will be repulsive. The forces acting from the top left and bottom right corners will cancel, leaving only the repulsive force coming from the bottom left corner.