Understanding Electric Charge and Current

What is Electric Charge?

Electric charge is a fundamental property of matter that causes objects to experience electromagnetic forces. It comes in two forms, positive and negative, with like charges repelling each other and opposite charges attracting. The unit of charge is the coulomb (C).

Conservation of Charge

One of the basic principles of electromagnetism is the conservation of electric charge. This states that the net charge in a closed system always remains constant - charges can be transferred between objects but cannot be created or destroyed.

Electric Current

An electric current is the rate of flow of electric charge. It arises due to the movement of electrically charged particles, typically electrons in metals and electrolyte solutions. The conventional direction of current is taken as the flow of positive charges, even though electrons, which are negatively charged, are actually the charge carriers in metallic conductors.

The relationship between current I, charge Q, and time t is given by:

I = Q/t

Worked Example

Problem: A wire carries 2.4 C of charge in 30 s. Calculate the current.

Solution:

• Given: Q = 2.4 C, t = 30 s
• Using I = Q/t
• I = 2.4 C / 30 s = 0.08 A

Conductors and Insulators

Materials can be classified as conductors or insulators based on their ability to permit the flow of electric charge:

• Conductors like metals allow charge to flow freely through them when a potential difference is applied. This is due to the mobility of the delocalized valence electrons in the metallic lattice.
• Insulators like plastics and ceramics have very few mobile charges and strongly resist the flow of electric current through them.

Basic Circuit Symbols and Conventions

Circuit diagrams use standardized symbols to represent components like cells/batteries, resistors, ammeters, voltmeters, and more. It's important to understand the direction of conventional current flow and polarity indications for sources and meters.