Chapter 27: Current and Resistance
- Electric current: The amount of charge flowing per unit time,
I =
dQ/dt , or Q = ∫ I(t)
dt ; The unit is the ampere, 1 A = 1 C/s.
- Current density: Defined by I = ∫ J ·
dA , and related to microscopic quantities by J =
nevd.
- Ohm's law: The relationship between electric potential difference
and current,
I = V/R , same as V = IR and R = V/I ; The
unit is the ohm, 1 Ω =
1 V/A.
- Resistivity: Relates the electric field and current density
in a material,
E = ρJ, or J = σE
in
terms of the conductivity σ =
1/ρ.
- Resistance: The resistance depends
on the type of material and the size and shape of the resistor. For a uniform block of length L and cross-sectional area A,
R = ρL/A .
- Power needed to maintain a current: In general, for any device, P = IV.
If the current goes through a resistor and V is the voltage
across that resistor, then the equation can be rewritten as P = I2R = V2/R .
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