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A proton(H1)and a deuteron ((H2), 鈥渉eavy鈥 hydrogen) start out far apart. An experimental apparatus shoots them toward each other (with equal and opposite momenta). If they get close enough to make actual contact with each other, they can react to form a helium-3nucleus and a gamma ray (a high-energy photon, which has kinetic energy but zero rest energy):H1+2H3He+y

This is one of the thermonuclear or fusion reactions that takes place inside a star such as our Sun.

The mass of the proton is 1.0073 u(unified atomic mass unit,1.710-27kg), the mass of the deuteron is 2.0136 u, the mass of the helium-3nucleus is 3.0155 u, and the gamma ray is massless. Although in most problems you solve in this course it is adequate to use values of constants rounded to two or three significant figures, in this problem you must keep at least six significant figures throughout your calculation. Problems involving mass changes require many significant figures because the changes in mass are small compared to the total mass. (a) The strong interaction has a very short range and is essentially a contact interaction. For this fusion reaction to take place, the proton and deuteron have to come close enough together to touch. The approximate radius of a proton or neutron is about110-15m. What is the approximate initial total kinetic energy of the proton and deuteron required for the fusion reaction to proceed, in joules and electron volts (1eV=1.610-19J)? (b) Given the initial conditions found in part (a), what is the kinetic energy of theHe3plus the energy of the gamma ray, in joules and in electron volts? (c) The net energy released is the kinetic energy of theHe3plus the energy of the gamma ray found in part (b), minus the energy input that you calculated in part (a). What is the net energy release, in joules and in electron volts? Note that you do get back the energy investment made in part (a). (d) Kinetic energy can be used to drive motors and do other useful things. If a mole of hydrogen and a mole of deuterium underwent this fusion reaction, how much kinetic energy would be generated? (For comparison, aroundare obtained from burning a mole of gasoline.) (e) Which of the following potential energy curvesin Figure 6.87 is a reasonable representation of the interaction in this fusion reaction? Why?

As we will study later, the average kinetic energy of a gas molecule is32kbT, whereis the 鈥淏oltzmann constant,鈥1.410-23J/K, andis the absolute or Kelvin temperature, measured from absolute zero (so that the freezing point of water is273K). The approximate temperature required for the fusion reaction to proceed is very high. This high temperature, required because of the electric repulsion barrier to the reaction, is the main reason why it has been so difficult to make progress toward thermonuclear power generation. Sufficiently high temperatures are found in the interior of the Sun, where fusion reactions take place.

Short Answer

Expert verified

(a) The initial total kinetic energy of the proton is 1.15410-13Jand the deuteron in joules and electron volts is 7.2105eV.

(b) The kinetic energy of the helium-nucleus is 9.231013Jand the energy of the gamma ray in joules and electro volt is5.76106eV .

(c) The net energy release in joules is 8.07610-13and electron volts is .

(d) The kinetic energy generated is 5.561011J.

(e) The 2nd potential energy curve is a reasonable representation of the interaction in this fusion reaction

Step by step solution

01

Identification of the given data

The given data is listed below as,

  • The proton鈥檚 mass is,mp=1.0073u
  • The deuteron鈥檚 mass is,md=20136u
  • The helium-nucleus鈥檚 mass is, mHe=3.0155u
  • The proton or neutron鈥檚 approximate radius is,rp=rd=110-15m

02

Significance of the law of energy conservation

The law of the energy conservation states that an isolated system鈥檚 total energy remains constant and it is conserved with time

The concept of the law of energy conservation gives the kinetic energies of the proton and the helium-3 nucleus along with the net energy release and the kinetic energy generated.

03

(a) Determination of the initial total kinetic energy of the proton and deuteron

The equation of the initial kinetic energy of the proton and the deuteron can be expressed as:

mpc2+mdc2+kp+kd=140-e2r+mdc2

Here, mpis the mass of the proton, C is the speed of light, mdis the mass of the deuteron ,kpis the kinetic energy of the proton ,kdis the kinetic energy of the deuteron,140is the Coulomb鈥檚 constant,e is the charge on the electron and r is the combined radius of the proton and the deuteron.

The above equation can be reduced as follows,

kp+kd=140-e2r

Substitute 9109Nm2/C2for14蟺蔚0,1.60210-19Cfore,and210-15mforrin the above expression.

kp+kd=9109Nm2/C21.60210-19C2210-15m=1.15410-13N-m=1.15410-13N-m1J1N-m=1.15410-13J

Covert the total initial kinetic energy of the proton and the deuteron from joules to electron volts.

kp+kd=1.15410-13J1eV1.610-19J=7.2105eV

Thus, the initial total kinetic energy of the proton and the deuteron in joules is1.15410-13J and electron volts is .7.2105eV

04

(b) Determination of the kinetic energy of the helium-3 and gamma rays

The equation of the total energy after fusion is expressed as,

T=kHe+mHec2+ky

Here,kHeis the kinetic energy of the helium,kyis the kinetic energy of the gamma rays and mHeis the mass of the helium-3 nucleus.

The above equation can be written as,

mpc2+mpc2+kp+kd=kHe+mpc2+ky

Substitute all the values in above equation.

10073uc2+2.0136uc2+1.15410-13J=kHe+3.0155uc2+kykHe+ky=0.0054uc2+1.15410-13J=0.0054u1.16610-27kg1u3108m/s+1.15410-13J=8.0610-13kg.m2/s2+1.15410-13J=8.0610-13kg.m2/s21J1kg.m2/s2+1.15410-13J=8.0610-13J+1.15410-13J=9.2310-13JConvert the total kinetic energy of the helium and the gamma ray from joules to electron volts.

kHe+ky=9.2310-13J1eV1.60210-19J=5.76106eV

Thus, the kinetic energy of the helium-3 nucleus and the energy of the gamma ray in joules is 9.2310-13Jand electro volt is 5.76106eV.

05

(c) Determination of the net energy release

The expression of the net energy release is expressed as follows,

Q=kHe+ky-kp+kd

Substitute all the values in above expression.

Q=9.2310-13J-1.15410-13J=8.07610-13J

The net energy release in electro volt is expressed as-

Q=8.07610-13J1eV1.60210-19J=5.04106eV

Thus, the net energy release in joules and electron volts are 8.07610-13Jand 5.04106eVrespectively.

06

(d) Determination of the initial kinetic energy generated

The expression for the kinetic energy generated is expressed as follows,

K.E=kHe+kyNA

Here,NA=6.0231023is the Avogadro鈥檚 number

Substitute all the values in the above expression.

K.E.=9.2310-13J6.0231023

Thus, the kinetic energy generated is 5.561011J.

07

(e) Determination of the potential energy curve

As the potential energy curve shows the potential energy of the fusion reaction in different position, hence curve 2 shows that the potential energy decreases with the increase in the radius.

Thus, the 2nd potential energy curve is a reasonable representation of the interaction in this fusion reaction.

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