# Pair Production: Which Conservation Law?

In summary, a single photon cannot produce an odd number of electrons and positrons because it would violate charge conservation. While energy and momentum conservation can still be achieved, the resulting charge would not have a net charge of zero. This is why another external particle is needed for pair production.

## Homework Statement

One of the reasons a single photon could not produce an odd number of electrons and positrons is

a) energy would not be conserved
b) unattainable photon energies would be needed
c) matter would be created
d) charge would not be conserved

## The Attempt at a Solution

This is for AP physics by the way. I would think at first that it would be charge, because logically an odd number would create a charge that doesn't have a net charge of zero. Obviously it isn't B (it can be any energy) or C (matter and energy together are what is conserved). But he has said over and over in class that conservation of energy and momentum is what is important for pair production and this is why there must be a pair. Is some other particle produced and that is why charge is conserved?

But he has said over and over in class that conservation of energy and momentum is what is important for pair production and this is why there must be a pair.
This is incorrect. Conservation of energy and momentum is why you cannot pair produce in vacuum (you need another external particle to satisfy both). You are correct that producing an odd number would violate charge conservation because there cannot be the same number of positrons and electrons.

## Homework Statement

One of the reasons a single photon could not produce an odd number of electrons and positrons is

a) energy would not be conserved
b) unattainable photon energies would be needed
c) matter would be created
d) charge would not be conserved

## The Attempt at a Solution

This is for AP physics by the way. I would think at first that it would be charge, because logically an odd number would create a charge that doesn't have a net charge of zero. Obviously it isn't B (it can be any energy) or C (matter and energy together are what is conserved). But he has said over and over in class that conservation of energy and momentum is what is important for pair production and this is why there must be a pair. Is some other particle produced and that is why charge is conserved?

The "energy" part is conserved via the mass of the created particles (i.e. mc2) and the kinetic energies of the particles. So it isn't just the mass content that needs to be accounted for. If the particles all take up the appropriate total kinetic energy, then there should be energy conservation (and momentum conservation as well since those particles are moving).

This means that in the given situation for odd number of electrons+positrons, it will not be impossible for energy and momentum to be conserved. However, it will be impossible for total charge to be conserved.

Zz.

## 1. What is pair production?

Pair production is a phenomenon in particle physics where a high-energy photon interacts with a nucleus or an electron, resulting in the creation of an electron-positron pair. This process is governed by the fundamental laws of quantum mechanics.

## 2. How does pair production violate conservation of energy?

In the process of pair production, a high-energy photon is converted into a pair of particles with rest mass, which implies that energy is not conserved. However, this is allowed by the law of conservation of energy because the energy is converted into the mass of the particles created.

## 3. Which conservation law is conserved in pair production?

The conservation law that is conserved in pair production is the law of conservation of charge. This means that the total charge of the particles before and after the process remains the same. In other words, the number of positive charges (protons) is equal to the number of negative charges (electrons and positrons).

## 4. What are the conditions required for pair production to occur?

Pair production requires a high-energy photon, typically in the form of gamma rays, and a target particle, such as an atomic nucleus or an electron. The target particle must have enough energy to produce the mass of the new particles. Additionally, the process must take place in the presence of an electric field so that the charges of the new particles can be separated.

## 5. What are the applications of pair production in modern technology?

Pair production has several applications in modern technology, including medical imaging and cancer treatment. It is also used in particle accelerators to produce high-energy particles for research purposes. Additionally, pair production is a crucial process in understanding the structure and behavior of matter at the subatomic level.

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