Electric Charge

What is electric charge?

• A fundamental attribute of matter that determines the intensity of electromagnetic forces (Similar to mass, in the way that it determines the intensity of gravitational forces)

#Charge laws

1. Charges of equal sign repel each other. Charges of different sign attract each other.
2. The sum of all the electric charges in any closed system is constant: charge is neither created nor destroyed, just transferred from one body to another.
3. Charge is quantized: the charge on any macroscopic body is always either zero or an integer multiple (negative or positive) of the electron (proton) charge.

#Basic Structure of Matter: Atoms

How can we explain the previous laws? matter is quantized!

• Basic structure of atoms can be described in terms of three particles: electron (negative charge), proton (positive charge), and neutrons (no charge)
• The charge of the electron has the same magnitude as the charge of the proton.
• Protons and neutrons form a dense core: the nucleus (order of $10^{-15}m$)
• Protons and neutrons are held together thanks to nuclear forces (not relevant in ordinary life)
• Electrons surrounding nucleus by attraction. Typical size $10^{-10}m$ ( 1 angstrom, symbol Å). Five orders bigger than nucleus!!
• The masses of the proton and neutron are nearly equal and are roughly 2000 times the mass of the electron. Over 99.9% of the mass of any atom is concentrated in its nucleus. ![[Pasted image 20240306140923.png]]

#Basic Structure of Matter: Ions

In the International System of Units (abbreviated SI), the unit of charge is called coulomb (1C).

1C is actually a high amount of charge! More common is to use submultiples: $1 mC = 10^{-3} C$, $1 µC = 10^{-6} C$. e has no sign

#Basic structure of matter: insulators, conductors and semiconductors

Basic classification of matter according to conductivity

• Conductors: allow charge to move easily within the material.
  • Examples: copper, iron, gold…
• Insulators: electrons remain bound to the positively charged nuclei.
  • Examples: plastic, paper, rubber…
• Semiconductors: intermediate between good conductors and good insulators.
  • Examples: silicon, germanium…

#Charging

3 ways of charging a body

• By friction: electrons are transferred by rubbing one object on another.
• By conduction: electrons are transferred from an object to a different object by contact. Some of the excess electrons in a certain object flow into a conductor (from an insulator or another conductor).
• By induction: electrons are transfered as they gather in a certain region (usually by polarization). It gives another body a charge of opposite sign without losing any of its own charge.

![[Pasted image 20240306141510.png]] Charging by Conduction

Everyday physics example of friction+conduction: When you use shoes with rubber sole, rubbing yourself against certaing fibers causes charge to build up on you, which will remain because it can’t flow through the air, the insulating fibers or the shoes to the ground. When you then touch a conducting object like a doorknob, there is a sudden transfer of electrons from your fingers, feeling a shock.

![[Pasted image 20240306141535.png]] Charging by Induction