Circuits!
- Power companies try to save the amount of copper needed for power lines by using thinner wires, which makes R quite high.
To minimize P dissipated by the wires, they minimize I by maximizing V. This is why power lines transfer electricity at high voltage.
- Current is the rate of charge flow through the cross-section of a conductor (wire).
Traditionally, the direction of current is taken as the flow of positive charges.The unit for current is Coulombs per second, C/s.
- Resistors
All resistors in series share the same current.Vseries = V1 + V2 + V3Voltage drop among resistors in series is split according to the resistance - greater resistance, greater voltage drop (V = IR).Vparallel = V1 = V2 = V3All resistors in parallel share the same voltage.Iparallel = I1 + I2 + I3Current among resistors in parallel is split according to the resistance - greater resistance, less current (I = V/R).
- Resistivity (ρ = RA/L)
Resistivity is the inverse of conductivity.Greater resistivity, greater resistance of the material.Rearranging the above equation to get R = ρL/A. To make a wire of low resistance, select a material that has low resistivity, keep the wire short, and keep the diameter of the wire large.Extension cords are made really thick to keep the resistance down, so it doesn’t heat up and cause a fire.
- Conductivity theory
- Conductivity is affected by electrolyte concentration:
No electrolyte, no ionization, no conductivity.Optimal concentration of electrolyte, greatest conductivity due to greatest mobility of ions.Too much electrolyte, ions are too crowded, less ion mobility, less conductivity.- Conductivity is affected by temperature:
In Metals, Conductivity Decreases as Temperature Increases.In Semiconductors, Conductivity Increases as Temperature Increases.At extremely low temperatures (below a certain critical temperature typically a few degrees above absolute zero), some materials have superconductivity - virtually no resistance to current flow, a current will loop almost forever under such conditions.- Conductivity (σ) is the inverse of resistivity (ρ).
Place a capacitor inside a solution, the solution will conduct a current between the plates of the capacitor, thus you can measure the conductivity of a solution using a capacitor.
