Ohm's Law flashcards

$V=IR$, so $I=\frac{V}{R}$ and $R=\frac{V}{I}$. V is volts across the part, I is amperes through it, R is ohms. Example: 36 V across 9 $\Omega$ gives 4 A.

R is V divided by I, not I divided by V (12 V and 3 A gives 4 $\Omega$, not 0.25). Ohm's law is a product, never a sum: 6 V and 2 $\Omega$ never combine to 8.

Volts divided by kilohms gives milliamperes: 12 V / 4 k$\Omega$ = 3 mA. Volts divided by megohms gives microamperes: 5 V / 1 M$\Omega$ = 5 $\mu$A. Otherwise convert to base units first.

Current is proportional to voltage, so it doubles. Power goes with the square ($P=\frac{V^2}{R}$), so it becomes 4 times larger. 25 $\Omega$ at 10 V uses 4 W; at 20 V it uses 16 W.

Series parts share the same current, and their resistances add ($R_{eq}=R_1+R_2$). Parallel branches share the same voltage; branch currents add, and the equivalent resistance is smaller than any branch.

In proportion to resistance, because both carry the same current. 4 $\Omega$ and 8 $\Omega$ across 24 V: I = 2 A, drops of 8 V and 16 V. Equal drops occur only with equal resistors.

With V vertical and I horizontal, the slope is R (a steeper line means more resistance). With I vertical and V horizontal, the slope is $\frac{1}{R}$ in siemens (a steeper line means less resistance).

Nonohmic parts: a tungsten lamp filament (resistance rises a lot when hot, so switch-on current surges) and a diode (conducts one way after about 0.6 to 0.7 V for silicon). Their V-I graphs curve.