Week 1. Ohms law, power, Kirchhoff`s law V,I.

Ohm`s law V=IR. So what is a Volt ? Potential energy per unit charge or work done to move a charge against electric field, form one potential to a higher potential.

**Dynamic resistance** R_{dynamic}= ΔV/ΔI it is the local resistance – the tangent to the slope of the device V-I curve.

**Power** P=I×V or derived: P=I²R or **P=V²/R**

**Kirchhoff`s law**: sum of voltages around circuit is zero, sum of currents in and out of node is zero.

**Parallel resistance**: R_{total} = (R1×R2)/(R1+R2)

**Rule of thumb: if two parallel resistors differ by a factor of ten or more, then we can ignore the larger of two.**

**In other words: in a parallel circuit, resistor much smaller than other dominates. Ina series circuits, the larger resistor dominates.**

**Voltage divider**. Rely on fact the I is the same on top and bottom.

_{out}= V

_{in}× R2 / (R1+R2)

**proportional**to the

**resistances**.

**Thevenin`s model.**How to calculate circuit when it gets loaded? Model the actual circuit with the simpler circuit – Thevenin model – which is idealized voltage source with resistor in series.

_{thevenin}is just open circuit, the voltage when nothing is attached ( no load ).

_{thevenin}is V

_{thevenin}/I

_{short-circuit }

_{thevenin}is to see it as the parallel resistance of several resistances viewed from the output.

_{thevenin}describes this property neatly in a single number.

**Design rule of thumb**: when circuit A drives circuit B. Let R

_{out}for A be <= 1/10 R

_{in}for B.

_{outA}is much smaller then R

_{inB}then the divider delivers nearly the all of the original signal.