# 31.7 Tunneling  (Page 3/6)

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## Phet explorations: quantum tunneling and wave packets

Watch quantum "particles" tunnel through barriers. Explore the properties of the wave functions that describe these particles.

## Section summary

• Tunneling is a quantum mechanical process of potential energy barrier penetration. The concept was first applied to explain $\alpha$ decay, but tunneling is found to occur in other quantum mechanical systems.

## Conceptual questions

A physics student caught breaking conservation laws is imprisoned. She leans against the cell wall hoping to tunnel out quantum mechanically. Explain why her chances are negligible. (This is so in any classical situation.)

When a nucleus $\alpha$ decays, does the $\alpha$ particle move continuously from inside the nucleus to outside? That is, does it travel each point along an imaginary line from inside to out? Explain.

## Problems-exercises

Derive an approximate relationship between the energy of $\alpha$ decay and half-life using the following data. It may be useful to graph the log of ${t}_{1/2}$ against ${E}_{\alpha }$ to find some straight-line relationship.

Energy and half-life for $\alpha$ Decay
Nuclide ${E}_{\text{α}}\phantom{\rule{0.25em}{0ex}}\text{(MeV)}$ ${t}_{\text{1/2}}$
${}^{\text{216}}\text{Ra}$ $\text{9.5}$ $\text{0.18 μs}$
${}^{\text{194}}\text{Po}$ $\text{7.0}$ $\text{0.7 s}$
${}^{\text{240}}\text{Cm}$ $\text{6.4}$ $\text{27 d}$
${}^{\text{226}}\text{Ra}$ $\text{4.91}$ $\text{1600 y}$
${}^{\text{232}}\text{Th}$ $\text{4.1}$ $1.4×{\text{10}}^{\text{10}}\phantom{\rule{0.25em}{0ex}}\text{y}$

Integrated Concepts

A 2.00-T magnetic field is applied perpendicular to the path of charged particles in a bubble chamber. What is the radius of curvature of the path of a 10 MeV proton in this field? Neglect any slowing along its path.

22.8 cm

(a) Write the decay equation for the $\alpha$ decay of ${}^{235}\text{U}$ . (b) What energy is released in this decay? The mass of the daughter nuclide is 231.036298 u. (c) Assuming the residual nucleus is formed in its ground state, how much energy goes to the $\alpha$ particle?

(a) ${}_{92}^{235}{\text{U}}_{143}\to {}_{\text{90}}^{\text{231}}{\text{Th}}_{\text{141}}+{}_{2}^{4}{\text{He}}_{\text{2}}$

(b) 4.679 MeV

(c) 4.599 MeV

Unreasonable Results

The relatively scarce naturally occurring calcium isotope ${}^{48}\text{Ca}$ has a half-life of about $2×{10}^{16}\phantom{\rule{0.25em}{0ex}}\text{y}$ . (a) A small sample of this isotope is labeled as having an activity of 1.0 Ci. What is the mass of the ${}^{48}\text{Ca}$ in the sample? (b) What is unreasonable about this result? (c) What assumption is responsible?

Unreasonable Results

A physicist scatters $\gamma$ rays from a substance and sees evidence of a nucleus $7.5×{10}^{–13}\phantom{\rule{0.25em}{0ex}}\text{m}$ in radius. (a) Find the atomic mass of such a nucleus. (b) What is unreasonable about this result? (c) What is unreasonable about the assumption?

a) $2.4×{\text{10}}^{8}$ u

(b) The greatest known atomic masses are about 260. This result found in (a) is extremely large.

(c) The assumed radius is much too large to be reasonable.

Unreasonable Results

A frazzled theoretical physicist reckons that all conservation laws are obeyed in the decay of a proton into a neutron, positron, and neutrino (as in ${\beta }^{+}$ decay of a nucleus) and sends a paper to a journal to announce the reaction as a possible end of the universe due to the spontaneous decay of protons. (a) What energy is released in this decay? (b) What is unreasonable about this result? (c) What assumption is responsible?

(a) $–1.805 MeV$

(b) Negative energy implies energy input is necessary and the reaction cannot be spontaneous.

(c) Although all conversation laws are obeyed, energy must be supplied, so the assumption of spontaneous decay is incorrect.

Consider the decay of radioactive substances in the Earth’s interior. The energy emitted is converted to thermal energy that reaches the earth’s surface and is radiated away into cold dark space. Construct a problem in which you estimate the activity in a cubic meter of earth rock? And then calculate the power generated. Calculate how much power must cross each square meter of the Earth’s surface if the power is dissipated at the same rate as it is generated. Among the things to consider are the activity per cubic meter, the energy per decay, and the size of the Earth.

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is the study of matter in relation to energy
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power P = Work done per second W/ t. It means the more power, the stronger machine
Sphere
e.g. heart Uses 2 W per beat.
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1.75cm
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my name is Abu m.konnek I am a student of a electrical engineer and I want you to help me
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the magnification k = f/(f-d) with focus f = R/2 =16 cm; d =12 cm k = 16/4 =4
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Godfred
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the same behavior thru the prism out or in water bud abbot
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What is the far point of a person whose eyes have a relaxed power of 50.5 D?
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try to read several books on phy don't just rely one. some authors explain better than other.
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And don't forget to check out YouTube videos on the subject. Videos offer a different visual way to learn easier.
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hope that helps
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