Electric current, Ohm’s law, resistance, and resistivity

Electric Current

1. Electric current is the rate of flow of electric charge through a conductor.
2. The formula is I = Q/t, where I is current, Q is charge, and t is time.
3. It is a scalar quantity and is measured in amperes (A).
4. In metallic conductors, current is due to the motion of free electrons.
5. Electric current can be direct (DC) or alternating (AC).
6. The direction of conventional current is opposite to the motion of electrons.

Ohm’s Law

1. Ohm’s law states that the current I through a conductor is directly proportional to the voltage V across it, provided the temperature remains constant.
2. The formula is V = IR, where R is the resistance of the conductor.
3. Ohm’s law is valid only for ohmic conductors, which have a constant resistance.
4. Materials that do not obey Ohm’s law are called non-ohmic conductors, e.g., diodes and transistors.

Resistance

1. Resistance is the opposition offered by a material to the flow of electric current.
2. The formula for resistance is R = V/I.
3. It is measured in ohms (Ω).
4. The resistance of a conductor depends on its length (L), cross-sectional area (A), and the material.
5. The resistance of a wire increases with an increase in temperature for most conductors.
6. Superconductors have zero resistance below a critical temperature.

Resistivity

1. Resistivity is a material property that measures how strongly a material opposes the flow of electric current.
2. The formula is ρ = R * A / L, where ρ is resistivity, R is resistance, A is cross-sectional area, and L is length.
3. It is measured in ohm-meters (Ω·m).
4. Resistivity depends only on the material and its temperature.
5. Conductors have low resistivity, while insulators have high resistivity.
6. Semiconductors have resistivity values between those of conductors and insulators.

Key Concepts and Applications

1. Resistance in circuits is combined using series and parallel combinations.
2. The total resistance in series is R_total = R1 + R2 + ... + Rn.
3. The total resistance in parallel is 1/R_total = 1/R1 + 1/R2 + ... + 1/Rn.
4. Resistors are used in electrical circuits to control current and voltage.
5. Ohm’s law forms the basis of electrical circuit analysis.
6. Resistivity helps in selecting materials for electrical wiring and components.
7. Low-resistance materials like copper and aluminum are used for wiring.
8. High-resistivity materials like nichrome are used in heating elements.
9. Superconductors have applications in maglev trains and MRI machines.