AP Physics 2 : Atomic and Nuclear Physics

Study concepts, example questions & explanations for AP Physics 2

varsity tutors app store varsity tutors android store

Example Questions

Example Question #11 : Quantum And Nuclear Physics

What is the speed of an electron in the first Bohr orbit in meters per second?

Possible Answers:

Correct answer:

Explanation:

To find the speed of the electron, use the following formula:

Substitute all the knowns and solve for velocity.

Example Question #1 : Electron Energy Levels

Suppose that an electron within a hydrogen atom moves from the fourth energy level to the second energy level. What is the wavelength of the photon emitted during this process?

Possible Answers:

Correct answer:

Explanation:

To answer this question, we'll need to utilize the equation that specifies the energy level of electrons within a hydrogen atom.

Where  is equal to the electron energy level within the hydrogen atom. Also notice that this equation has a negative sign. This is because in its ground state, an electron is closest to the positively charged nucleus and thus has the lowest energy. As the energy level increases, the electron moves further and further away from the nucleus, thus gaining increasing energy. At an infinitely far away energy level, the electron will have a maximum energy value of zero. To find the difference between the second and fourth energy levels, we'll simply use the above equation for different values of .

The negative sign for the change in energy just means that energy is being released in this process. We can drop the negative because we know that energy is being released.

Now that we've found how much energy is contained in the released photon, we'll need to calculate its wavelength.

Example Question #2 : Electron Energy Levels

An electron collides with an atom, exciting an electron in the atom from it's ground state . The initial velocity of the incoming electron is  and after the collision it has a velocity of . What is the energy of the excited electron in the atom after the collision in electron-volts?

Possible Answers:

Correct answer:

Explanation:

The incoming electron will lose kinetic energy during the collision, transfering this energy to the potential energy of the bound electron in the atom. Conservation of energy can be used to solve this problem. The general statement that energy is conserved is

where  is the kinetic energy and  is the potential energy. The incoming electron has kinetic energy and no potential energy. We are defining the initial state of the bound electron to be at  so the total initial potential energy of the system is zero.

The incoming electron will still have kinetic energy after the collision but the bound electron will not since it is not a free electron. This means that

where

plugging this in - 

 is the mass of the electron. Plugging everything in and converting to  gives

Example Question #1 : Atomic And Nuclear Physics

Calculate the energy released as a photon when an electron falls from the  energy level to the  energy level.

Possible Answers:

Correct answer:

Explanation:

During a energy level change in a hydrogen atom, the amount of energy either lost of gained is given by the following equation with respect to the initial and final energy levels shown below. 

Recall that whenever electrons drop from higher energy levels to lower ones, energy can be released in the form of a photon. To obtain the amount of energy released, we mst take the difference in energy of the electrons at the particular energy levels:

It is important to note that the negative energy difference corresponds to how much energy the photon is "taking away" as it leaves. Therefore, the photon leaves the atom with  of energy.

Example Question #1 : Electron Energy Levels

An electron in a hydrogen atom falls from the level to the level. What is the energy of the photon emitted?

Possible Answers:

None of these

Correct answer:

Explanation:

Using

Plugging in values:

This will be the change in energy of the electron, which is the negative of the energy of the photon released.

Thus, the energy of the photon is

Example Question #2 : Atomic And Nuclear Physics

How much energy would it take to raise an electron from the to the energy level of a hydrogen atom?

Possible Answers:

None of these

Correct answer:

Explanation:

Using the formula for the energy of an electron in a hydrogen atom's nth energy level:

Plug in  and  then find the difference:

Convert electronvolts to Joules:

Example Question #1 : Atomic And Nuclear Physics

One mole of hydrogen atoms have electrons drop from the  energy level to the  energy level. Determine the energy released.

Possible Answers:

None of these

Correct answer:

Explanation:

Using the following equation for the energy of an electron in Joules:

And

Combining equations and plugging in values:

would be released

Example Question #1 : Electron Energy Levels

What is the difference in energy for a hydrogen atom with its electron in the ground state and a hydrogen atom with its electron in the  state?

Possible Answers:

Correct answer:

Explanation:

For this question, we need to compare the difference in energy levels of hydrogen atoms with electrons in different orbitals.

First, we will need to use the equation that describes the energy of an electron in a hydrogen atom.

In the above expression,  represents the orbital in which the electron resides.

First, let's see what the electron energy level is in the ground state, which corresponds to .

Next, let's do the same thing for the  orbital.

Next, we can find the difference in the energy values.

Example Question #1 : Atomic And Nuclear Physics

By what process is  converted into ?

Possible Answers:

 decay

 decay

 decay

 capture

Correct answer:

 decay

Explanation:

To answer this question, we'll need to consult the periodic table. From the table, we know that magnesium's atomic number (the number of protons it contains in its nucleus) is 12, and sodium's is 11. We also need to realize that the mass number for each (the number of protons plus neutrons contained in the nucleus) is the same. Since the mass numbers are the same but the atomic numbers differ by one, then we can infer that a neutron is undergoing a decay into a proton and a so called positron, . The overall reaction is as follows:

Furthermore, it cannot be alpha decay, because in this process an alpha nucleus is released and the reactant's mass number and atomic number would both change. It also cannot be gamma decay, because in this process there is no change in atomic or mass numbers. Finally, it cannot be electron capture because in this process, an electron combines with a proton to generate a neutron. Thus, the mass number would not change, but the atomic number would increase by one. But in the question stem, we know the atomic number is decreasing by one rather than increasing.

Example Question #21 : Quantum And Nuclear Physics

Which of the following particles has a charge that is fractions of an electron charge?

Possible Answers:

Hadron

Tau

Quark

Graviton

Tachyon

Correct answer:

Quark

Explanation:

The correct answer is quarks. Quarks usually have charges of  or . They are usually bound with other quark particles and could be mixed to form hadrons. Tau is part of the leptons family and has a charge of . Graviton does not have a charge, and is a hypothetical particle. The tachyon is a hypothetical particle assumed to be faster than light. Hadrons are strong composite particles that are composed of quarks and will result to a net integer charge.

Learning Tools by Varsity Tutors