22.2
Sources and Properties of Electric Charge
All objects we see around us consist of atoms, which combine to form molecules. The lightest element in the universe is hydrogen, and a hydrogen atom…
All matter is made up of atoms. An atom consists of three subatomic particles: electrons, protons, and neutrons.
The atomic nucleus consists of positively charged protons and electrically neutral neutrons bound by nuclear force.
The SI unit of charge is coulomb. The magnitude of the charge of an electron is the same as that of a proton, which is 1.602 times 10-19 coulomb.
This magnitude is the fundamental unit of charge, as all other charges in nature are its integral multiples. Thus, charges are quantized.
Since electrons are negatively charged, an atom is electrically neutral when it has an equal number of protons and electrons; most objects around us are electrically neutral.
The total charge in the universe is found to be conserved. It can neither be created nor destroyed.
Moreover, the electric charge is locally conserved. The amount of charge in a region changes only because of the flow of charge into it or away from it.
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Q1: What are the three main subatomic particles that make up atoms?
Atoms consist of electrons, protons, and neutrons. Protons are positively charged and located in the nucleus, while electrons are negatively charged and orbit the nucleus. Neutrons are electrically neutral and also reside in the nucleus. Protons and neutrons have nearly identical masses, approximately 2000 times greater than an electron's mass.
Q2: Why is the fundamental unit of charge important in physics?
The fundamental unit of charge is 1.602 × 10⁻¹⁹ coulomb, the magnitude carried by both protons and electrons. All charges in nature are integral multiples of this value, meaning charges are quantized. This quantization is a fundamental property of matter and explains why charge cannot exist in arbitrary amounts.
Q3: How does an atom become electrically neutral or charged?
An atom is electrically neutral when it has equal numbers of protons and electrons, since their charges cancel. If an electron is removed, the atom becomes positively charged and is called a cation. If an electron is added, the atom becomes negatively charged and is called an anion. Such charged atoms are known as ions.
Q4: What force binds protons and neutrons together in the nucleus?
The strong nuclear force binds protons and neutrons together in the nucleus. This force is distinct from electrical and gravitational forces. It overcomes the electrical repulsion between positively charged protons and holds the nucleus together, allowing most of an atom's mass to be concentrated in this tiny region.
Q5: Is electric charge created or destroyed in physical processes?
Electric charge is conserved and can neither be created nor destroyed. Charge can only be transferred from one object to another or flow from place to place. The total charge in the universe remains constant. This conservation applies globally and locally—if charge in one region changes, it must flow into or out of that region.
Q6: What does it mean when two charges are said to cancel each other?
When two objects with equal and opposite charges are close together, the forces they exert on other charged objects nullify, resulting in zero net force. However, the charges themselves do not disappear. This verbal shorthand describes force cancellation, not charge annihilation. The charges remain on their respective objects.
Q7: Why are electrons freer to move than protons in an atom?
Electrons are freer to move because of the relative mass difference between the nucleus and electrons. Protons and neutrons are bound tightly in the nucleus by the strong nuclear force and have much greater mass. Electrons, being much lighter and orbiting outside the nucleus, experience weaker binding and can move more readily between atoms.