# Introduction to Electromagnetism

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• Published : February 27, 2013

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Unit 1
What is inside an atom?
* A positively charged nucleus composed of protons and neutrons * Electrons that surround the nucleus
What are isotopes?
Isotopes are atoms with the same number of protons and different numbers of electrons How do we represent different atoms?

What is specific charge?
The specific charge of a particle is defined as its charge divided by its mass What keeps protons and neutrons in a nucleus together?
A stable isotope has nuclei that do not disintegrate, so there muse be a force holding them together. We call this force the strong nuclear force because it overcomes the electrostatic force of repulsion between the protons in the nucleus and (except in unstable nuclei) keeps the protons and neutrons together. Why are some nuclei stable and others unstable?

Whether nuclei are stable or unstable depend on their neutron:proton ratio. What happens when an unstable nucleus emits an alpha or beta minus particle? Alpha radiation consists of alpha particles which each

comprise of 2 protons and 2 neutrons.

Beta radiation consists of fast moving electrons. A beta minus particle is emitted because a neutron is changing into a proton.

What is a photon?
A photon is a ‘packet’ of quantum of electromagnetic waves. An electromagnetic wave consists of an electric wave and a magnetic wave which travel together and vibrate: *at right angles to each other and to the direction in which they are travelling. *in phase with each other. As you can see the two waves reach peak together so they are in step. When waves do this we say they are ‘in phase’.

What is the energy of a photon?
Photon energy E=hf
How many photons does a light source emit every second?
A laser beam consists of photons of the same frequency. The power of a laser beam is the energy per second transferred by the photons. For a beam consisting of photons of frequency f, the power of the beam=nhf where n is the number of photons in the beam passing a fixed point each second. What is antimatter?

Antimatter is antiparticles that each have the same rest mass and, if charged, have equal and opposite charge to the corresponding particle. What happens when a particle and an antiparticle meet?
When they meet, antimatter and matter particles destroy each other and radiation is released; this is called annihilation. 2 photons are produced in this process. The minimum energy of each photon produced is hfmin =E0. The opposite of annihilation is pair production. This is when a photon creates a particle and its corresponding antiparticle, and vanishes in the process. For a particle and antiparticle of rest energy E0, we can calculate the minimum energy and minimum frequency fmin that the photon must have to produce this particle-antiparticle pair; minimum energy of photon needed= hfmin= 2E0

What is an interaction?
When 2 objects interact, they exert equal and opposite forces on each other. Momentum is transferred between the objects by these forces, if no other forces act on them. For example, if 2 protons approach other they repel each other and move away from each other. The electromagnetic force between 2 charged objects is due to the exchange of virtual photons. What types of interaction are there?

*Neutron-neutrino interaction
*Proton-antineutrino interaction
*Beta minus decay
*Beta plus decay
*Electron capture
The weak nuclear force is responsible for beta decay.
What makes charged particles attract or repel each other?
The electromagnetic force
What is an exchange particle?
They are used to determine the strength of interaction and range of interaction. What are strange particles?
All hadrons have strangeness of +1, 0, -1 or -2. Protons, neutron and pions have 0, kaons have +1 or -1 How can we classify particles?
Particles are either hadrons (These have larger masses and experience the strong force. They’re sub-divided into Baryons or mesons. All hadrons, except the proton, are unstable and decay into...