Unit 3.6 - Nuclear energy Flashcards
Binding energy
The energy required to completely separate the nucleus to its constituent nucleons
What can we know about the nucleus of an element from looking at it on the periodic table?
Number of protons and neutrons (top number)
Number of protons (bottom number)
How do we work out the number of neutrons in an element?
Top number - bottom number
How many protons and neutrons are there in a helium nucleus?
2 protons, 2 neutrons
1u
1 atomic mass unit
1 atomic mass unit (1u) definition
A mass equal to 1/12th of the mass of a carbon atom
What is it important NOT to do in this unit and why?
Round up or down since the numbers are so small
Mass defect symbol
Δm
Δm
Mass defect
Mass defect
The difference between the mass of the constituents and the mass of the nucleus - the energy put into the system to break it apart
What’s happened to the mass defect?
Has been turned into energy, usually kinetic
Equation for calculating the binding energy of a nucleus
E = mc^2
What does E = mc^2 allow us to calculate?
The binding energy of the nucleus
m in E = mc^2
Mass defect (u) x conversion to kg
c in E = mc^2
Speed of light
Converting from J to eV
Divide by the charge of an electron
1eV
The energy transferred when an electron travels across a p.d of 1V
Since energy transferred = charge x pd
MeV meaning
Mega electron volts
What’s the shortcut equation for working out binding energy in MeV?
Δm x 931 = E(MeV)
How come we can use a shortcut equation for working out binding energy?
Since there’s so many constants
What does the shortcut equation for binding energy give th answer in?
MeV
Equation for the binding energy per nucleon
Total binding energy/amount of nucleons
Nucleons
Things in the nucleus (protons and neutrons)
Protons and neutrons name
Nucleons
Why is mass that’s converted into energy huge?
By examining E=mc^2, we see that it’s huge since it’s multiplied by 3x10^8 squared
Does a nucleon have less mass in a nucleus or at rest outside the nucleus?
Less mass in the nucleus
What’s the lowest - the total mass of a nucleus or the total mass of the nucleons that make it?
The total mass of the nucleus is less
Describe the mass defect of larger atoms
Larger
What does it mean if an atom has a larger mass defect?
It requires more energy to pull them apart
How is the energy taken to bind together an atom and why?
The energy is taken directly from the mass of individual nucleons, since if all the nucleons were brought together from infinity, then the repulsive forces (between the protons) would be huge
What holds together nucleons once they’re in a very small distance of arch other?
The strong force
What are the forces acting inside the nucleus of atoms?
Strong force
Electromagnetic force
Strong force
Pulls protons and neutrons together
Electromagnetic force
Pulls protons and neutrons apart
Which force is stronger - the strong force or the electromagnetic force?
The strong force
Under which condition is the strong force stronger than the electromagnetic force?
At short ranges
Range of the strong force
Roughly 21/2 nucleons in diameter
Which force dominates at larger distances inside the nucleus of atoms?
The electromagnetic force
What happens to the nucleus of an atom as we initially add nucleons and work up the periodic table?
Initially, each nucleus is more tightly bound than the on before = increasing binding energy
Up to which element does the binding energy of the nucleus of atoms increase?
Iron
What does it mean if a nucleus is more tightly bound?
Higher binding energy
Which element has the highest binding energy of all of the elements?
Iron
Which element has the most stable nucleus and why?
Iron
Has the highest binding energy
Requires the greatest energy to form
What does a Higher binging energy mean for a nucleus?
More stable nucleus
Past which element does the electromagnetic force begin to dominate over the strong force in nuclei and why?
Past iron
The nucleus now has a radius more than 2 1/2 nucleons wide
Why does the electromagnetic force begin to dominate in nuclei past iron?
Since the nucleus now has a radius more than 2 1/2 nucleons wide
What happens when nucleons are added beyond iron? Why?
The electromagnetic force is winning and so each added nucleon is less tightly bound
At which element does the electromagnetic force beat the strong force?
Lead
Which elements are radioactive? why?
Anything past lead since the electromagnetic force and they’re now nuclei are unstable and come apart by themselves
What does it mean that the elements past lead are unstable and come apart by themselves?
Are radioactive
How can unstable nuclei become more stable?
Can split into other more stable nuclei —> fission
Fission
When unstable nuclei splint into other more stable nuclei
Which side of a binding energy graph shows the nuclei available for fusion?
LHS of iron
Which side of a binding energy graph shows the nuclei available for fission?
RHS of iron
What do both nuclear fission and fusion do?
Release energy
How can we understand the differences between nuclear fission and fusion?
We need to look at the binding energy
What happens during nuclear fission?
In order to “force” a nucleus to break up, a neutron is introduced
How is a nucleus broken up during nuclear fission?
By introducing a neutron
Are many neutrons required for nuclear fission?
No
Why is a neutron used during nuclear fission?
Because of its electrical properties, since a proton would be repelled from any nucleus
What would happen if a proton was used in nuclear fission instead of a neutron?
The proton would be repelled from the nucleus
How can we easily explain the idea of nuclear fission?
The liquid drop model of the nucleus:
Large drop of water on a surface
Very small amount of water added to the drop = becomes unstable and breaks up
Explain nuclear fission in uranium-235
When a neutron is added to uranium-235, it forms the very unstable uranium-236
This splits apart to produce 2 lighter atoms
Releases more neutrons than were put in
Moves up the binding energy curve —> binding energy per nucleon increases from about 7.5MeV to. Little under 8.5MeV
Since there are over 200 nucleons involved in the reaction, this represents the release of approximately 200MeV of energy per uranium-235 atom fissioned
What happens when a nucleus is added to uranium-235?
It forms the very unstable uranium-236
What does unstable uranium-236 do?
Splits apart to produce 2 lighter atoms
Are more neutrons put in or taken out during nuclear fission?
More neutrons released than those put in
How does nuclear fission cause a chain reaction?
More neutrons are released than those put in
What happens to the binding energy per nucleon during nuclear fission?
Increases
What is a good absorber do neutrons to slow down the reactions of nuclear fission?
Boron control rod
Why is a boron control rod used during nuclear fission?
It’s a good absorber of neutrons to slow down the reaction
What happens when a control rod fails during nuclear fission?
A nuclear accident occurs
How do we calculate the energy released per nucleus during nuclear fission or fusion?
∆m = sum of mass on LHS - sum of mass on RHS (or the other way around, it’s just the difference between them)
Energy released (MeV) = ∆m x 931
Why is uranium-236 not included in the calculation for the energy released during nuclear fission?
Very unstable and very brief so no need to include it in the calculation
Why does nuclear fusion occur?
Because the sum of the mass on the left hand side is greater than that on the right hand side
Do the products of nuclear fission have an increased or decreased binding energy per nucleon? Why?
Products have an increased binding energy per nucleon
Remember, binding energy is the energy given out when a nucleus is formed
Nuclear Fusion
The joining of two light nuclei to form a heavier nucleus
How does nuclear fusion produce energy?
Just like in nuclear fission, the production of energy results from the increase in binding energy of the products of the reaction over the reactants (the RHS having less mass than the LHS)
Deuterium
Heavy hydrogen
Heavy hydrogen
Deuterium
Deuterium symbol
2
H
1
When can we round up in this unit?
At the very end of working out the energy
What’s highest, the energy released during the fusion of deuterium or the energy released during the fission of uranium-235?
The energy released during the fission of uranium-235
What is highest - the energy released per nucleon during fission or fusion?
Fission
What’s another advantage to using fission over fusion other than that it releases more energy per nucleon?
The products are more stable and aren’t radioactive
What do we move up during fission?
The binding energy curve
Is a lot of energy released during nuclear fusion? Explain
Although it’s less than that released per nucleon during nuclear fission, the mass is several times less than the energy released, so a lot of energy can be obtained from very little fuel
How has nuclear fusion been achieved so far?
In an uncontrolled way in thermonuclear explosions (the hydrogen bomb)
Why is it difficult to harness nuclear fusion for more peaceful purposes?
The difficulty comes in providing the energy to enable two positively charged nuclei to overcome huge repulsion caused by the electric force between protons in the nuclei so that they can come close enough for the strong nuclear force to cause them to fuse
Why do we need energy for nuclear fusion to occur?
We need energy to enable two positively charged nuclei to overcome the huge repulsion caused by the electric force (between protons in the nuclei) so that they can come close enough for the strong nuclear force to cause them to fuse
How are nuclei given kinetic energy for nuclear fusion?
Through the use of extremely high temperatures, between 10^8k and 10^10k
What’s plotted on a binding energy per nucleon curve?
Negative binding energy per nucleon
By plotting negative binding entry per nucleon on the binding energy curves, what does it mean?
The more either process moves the resulting nucleus up the curve, the more energy is released
What fact shows why fusion releases much more energy per unit mass?
The fact that it is energy released per nucleon
Assumption made in nuclear fusion/fission calculations
That the energy just about causes the reaction with no leftover energy (conservation of energy)
Isotopes
Nuclei with the same atomic number (proton number) but different nucleon number (due to different neutron numbers)
What happens on a binding energy per nucleon curve at a helium nucleus?
There’s a spike
Why is there a spike at the helium nucleus on a binging energy per nucleon curve?
Very high binding energy
What does a high binding energy mean for a nucleus?
Stable nucleus
How do we know if an element has a stable nucleus?
High binding energy
Why is alpha decay likely?
As the helium nucleus is a very stable nucleus (high binding energy) and alpha decay emits helium nuclei
Explain why nuclear fission is possible in the case of some elements and nuclear fusion in others
Nucleons to the left of iron-56 with a lower nucleon number will move up the binding energy per nucleus curve if they fuse together. Those to the right of the iron-56 with a higher nucleon number will move up the curve by fissioning.
The higher up the curve, the more stable the nucleus.
What happens to the nucleus the higher up the binding energy per nucleon curve we are?
The more stable
How do we know if a nucleus is more stable?
Higher binding energy per nucleon
What does a positive energy output imply in a nuclear equation?
Getting more stable
What is higher - the binding energy of the products or reactants when energy is released during the reaction? Why?
The total binding energy of the products is greater as if energy is released, they’re getting more stable so the binding energy is higher
Which conservation laws do nuclear reactions have to follow?
Conservation of momentum
Conservation of energy
Conservation of momentum
Momentum before = momentum after
What is the binding energy of n and p? Why?
Zero
They are the constituents
How come energy is released during fusion and fission?
When a reduction in mass occurs, energy is released (mass converted into energy)
What do we do in a question where we’re asked to “determine a value” and we’re given a graph?
Calculate the gradient
What do we do in a question where we’re asked to “justify” a claim?
Do calculations
When would we have to use the E=mc^2 method to calculate the binding energy of a nucleus or the energy released in a reaction?
If given masses in kg instead of u
How do both fusion and fission release energy?
Mass is converted into energy
u to kg
MULTIPLY by conversion factor in data book
MeV to J
x e
x 10^6
How do we know when energy has been released?
BE per nucleon/stability increases
What happens when BE per nucleon or stability Increases?
Energy is released
What happens per revolution in a synchrotron?
Two energy kicks
What’s m in F = mv x no of electrons per second when working out the force an electron beam exerts on a target?
The mass of one electron
What do we need to mention about fusion and fission every time?
Energy released = binding energy per nucleon increases = stability increases, lower mass
