10/21 - Kirchhoff's Rules

Ideal Meters do not affect the circuit

• Voltmeter has infinite resistance (have no voltage drop)

• Ammeter has zero resistance (have no voltage drop)

• Ohmmeter uses its own voltage and current (not even part of the circuit)

Circuits USE ___ NOT ___

Voltage, Current

Current is made of

electrons, which cannot be created or destroyed

Voltage is like work, which can be used to do…work

Current carries electrons that carry the energy to do work

V =

IR, used for almost every part of the circuit

Voltage difference over the 

whole circuit equals the voltage of the source 

Any resistance will use voltage based on

Ohm’s Law (Voltage drop of that resistance) Ohms / Current

Current flows

clockwise in the loop

Too little resistance creates

a short circuit

Junction Rule - 

the sum of the current entering an intersection (junction), must equal the sum of the current leaving (In = Iout) 

Bulb’s resistance is roughly constant

Increases as the wire heats up (hidden way to increase resistance)

Brightness is proportional to current or Power (P=IV)

Brighter light means more current or more energy per seconds

P =

I²R

e

Equivalent Resistance (Req) is equal to

a single resistance replacing 2 more resistances

Current is the same for every

resistance in the same series

Equivalent resistance is a simple sum.

More resistance, less current

Kirchhoff’s Junction Rule

Itotal = Itotal (This means current is constant)

Voltage drop of each branch is the same (Equipotential!)

VdropEq equals the voltage drop of each branch 

Equivalent Resistance is the reciprocal of a

reciprocal sum. More resistance in parallel, more current (LESS equivalent resistance)