Charge and current

• Electric current, I, is defined as the rate of flow of charge. The SI base unit for current is Amperes (A)
• Current in an electrical circuit can be measured using an ammeter, which is placed in series
• Charge, Q, is a physical quantity that can be positive or negative and is measured in coulombs (C)
• 1 coulomb is defined as the flow of charge in a time of 1 second when the current is 1 ampere
• same charges repel each other, whereas opposite charges attract each other
• Protons have a charge of +1, electrons have a charge of -1, representing multiples of the elementary charge, e, 1.6x10-19 C
• The net charge of a particle is due to the gain or loss of electrons. In an atom, protons equal electrons, resulting in a neutral charge
• Increasing electrons produces a negative ion, removing electrons produces a positive ion
• Net charge on a particle Q = ± ne, where n is the number of electrons added or removed
• Electric current is carried by electrons in metals due to a lattice of positive ions surrounded by free electrons
• Conducting liquids like electrolytes carry charge through positive and negative ions, e.g., water with dissolved salt
• Conventional current is the flow of charge from positive to negative terminal, opposite to electron flow in metals
• Kirchhoff’s first law states the sum of currents into a point equals the sum of currents out of that point, due to conservation of charge
• Mean drift velocity is the average velocity of electrons as they move through a wire, colliding with positive metal ions
• Number density, n, represents the number of free electrons per unit volume in a material
• Conductors like metals have high number densities, while insulators like plastics have lower number densities
• An additional formula for current: I = nAv, where A is the cross-sectional area of the wire and v is the mean drift velocity