AP Physics 2 : AP Physics 2
Study concepts, example questions & explanations for AP Physics 2
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Example Questions
Example Question #1 : Charge And Electric Force
Two point charges, Q1 and Q2, are placed on the x-axis. Q1 is placed at x = 0m and Q2 at x = 1m. If a positive charge, Q3, is placed on the x-axis at x = 0.25m, it experiences a net electric force of 0. Which of the following could be determined based on this observation?
Q1 and Q2 have the same sign, but Q1 has a greater magnitude than Q2
Q1 and Q2 are both positive and have the same magnitude
Q1 and Q2 have opposite signs and the same magnitude
Q1 and Q2 have the same sign, but Q1 has a smaller magnitude than Q2
Q1 and Q2 are both negative and have the same magnitude
Q1 and Q2 have the same sign, but Q1 has a smaller magnitude than Q2
In order for there to be 0 net force on Q3, the forces from Q1 and Q2 must be in opposite directions. So Q1 and Q2 must either be both attracting Q3 or both repelling Q3 (that is, Q1 and Q2 are either both negative or both positive).
Also, since these forces must have equal magnitude in order to cancel, such that 
Example Question #4 : Electrostatics And Electrical Fields
Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
Electronegativity is based on the principle that the closer an electron is to the atomic nucleus, where postive charge is concentrated, the more attractive force the positive charge can exert on the electron.
As a result, as atomic radius decreases by one half, what happens to the force pulling the electron toward that atom?
It is reduced by half
It quadruples
It goes up by a factor of eight
It stays constant
It doubles
It quadruples
This is a function of the equation 
In other words, as the distance between two charges doubles, the force between them goes increases, as the denominator decreases from 
Here, we are decreasing the distance by 1/2, decreasing the denominator by 1/4, so our force goes up by 4 times.
Example Question #11 : Electrostatics And Electrical Fields
If the electrical potential energy between two equal charges quadruples, describe the change in the distance between the particles.
The distance was doubled
The distance was halved
The distance was quartered
The distance was quadrupled
The distance was not changed
The distance was quartered
Electrical potential energy is given by the equation 
Electrical potential energy is inversely proportional to the distance between the two charges. If the energy is quadrupled, then 
For the energy to be quadrupled, the radius must be quartered.
Example Question #1 : Electric Potential Energy
For the following:
If a charge of magnitude +4e is being held in place 3 nm from a charge of -5e which is also being held in place. What is the potential energy of the system?
Given the equation and plugging in the values of e and k, we get that
It is important to keep in mind that the charge 
Example Question #1 : Electric Fields
Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1 is 0.5 μF and the capacitances of C2 and C3 are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.
Using the plate diagram above, what direction do the electric field lines point?
Out of the page
To the right
To the left
Into the page
To the right
As with a point charge, remember that electric field lines point from areas of high potential to areas of low potential. Areas of high potential have positive (+) charge and areas of low potential have negative (-) charge, thus the electric field lines point to the right.
Example Question #2 : Electric Fields
Which of the following equations best relates capacitance, charge, and distance between parallel plates as they pertain to the electric field, 
This question forces us to combine two equations.
Using substitution, we can solve for the electric field with the variables given in the question stem.
Example Question #13 : Electrostatics And Electrical Fields
Which of the following is not true regarding electric potential?
When a positive charge moves from a region of low potential to high potential, the electric field does positive work on the charge
Electric potential can be expressed with units of Volts or Joules per Coulomb
The positive terminal of a battery has higher electric potential than the negative terminal
A negative charge moving from low potential to high potential will accelerate
All of these are true statements
When a positive charge moves from a region of low potential to high potential, the electric field does positive work on the charge
A positive test charge will naturally move from high potential to low potential. If it is moved in the opposite direction, then the electric field will do work against its motion (negative work). This be seen from the equation for electric field work:
Example Question #1 : Magnetic Fields
Which of the following best describes magnetic field lines?
More than one of the other options is correct
They show the direction in which a positive charge will experience a force
They show both the relative strength and direction of the magnetic field
They can only cross when multiple magnets are present
They always begin at a north pole and terminate at a south pole
They show both the relative strength and direction of the magnetic field
First, magnetic field lines, like electric field lines, can never cross. Also, unlike electric field lines, magnetic field lines are continuous—they do not have starting or ending points. Next, the force experienced by a charge in a magnetic field depends on the charge's velocity direction, not just the magnetic field. So the only remaining choice is that magnetic field lines show both the relative strength and direction of the field.
Example Question #53 : Circuits
Batteries and AC current are often used to charge a capacitor. A common example of capacitor use is in computer hard drives, where capacitors are charged in a specific pattern to code information. A simplified circuit with capacitors can be seen below. The capacitance of C1 is 0.5 μF and the capacitances of C2 and C3 are 1 μF each. A 10 V battery with an internal resistance of 1 Ω supplies the circuit.
While the capacitor is charging, does the capacitor generate a magnetic field?
No
Yes
Cannot be determined
Yes
This question asks us to consider electromagnetism and what happens when a capacitor is charging. When a capacitor is charging, charge is accumulating on the surface over a period of time. Given that the electric field due to a capacitor is given by the formula E = σ/ε0, where σ is the charge per unit area and ε0 is the constant of permeability of free space, we can see that E is directly proportional to σ; therefore, the more charge that builds up, the higher the resulting E field.
Because σ is changing during charging, and thus E is changing, we also know that a magnetic field, B, must be created.
Remember the principle—a changing electric field induces a changing magnetic field, and a changing magnetic field induces a changing electric field.
Example Question #21 : Ap Physics 2
An electron is traveling at constant velocity directed to the the left through a magnetic field directed into the page as shown below.
Which way would the force vector on the electron due to the magnetic field be directed?
Out of the page
Towards the top of the page
Towards the bottom of the page
Towards the right of the page
Towards the top of the page
When finding the direction of the force on a POSITIVELY charged particle due to a magnetic field, we can use the right hand rule. Holding your thumb perpendicular to the rest of your fingers (as if motioning to stop), orient your fingers parallel to the lines of the magnetic field, and your thumb with the velocity vector of the charge. For a positive charge, the force would be oriented directly out of the palm; HOWEVER, the charge of an electron is negative and therefore the force will be applied in the opposite direction. This would mean that the force would be coming directly out of the backside of your hand (in this case, towards the top of the page) using the right hand rule.
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