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PHYS 1120 / 2020 Midterm 2 Study Guide

Voltage


Voltage from a point particle:


Voltage from an electric field:


* The only distance Δd that matters is the distance that is parallel to the electric field *


How voltage is related to work/energy:


If you need to find a particle’s speed from its voltage or energy, use the work-energy theorem from Physics 1:


Capacities


For a parallel-plate capacitor, THE ONLY THING THAT CHANGES CAPACITANCE IS A CHANGE IN THE AREA OF THE PLATES OR THE DISTANCE BETWEEN THE PLATES!!!


A capacitor is related to the charge on its plates and the voltage between its plates by

* Q and V can change each other but they can’t change C!!! *


Resistors + Simple Circuits


Resistance of a resistor is determined by its length and cross-sectional area:


To add resistors in parallel:


To add resistors in series:


In series voltage adds and current is constant!




In parallel voltage is constant and current adds!




Brightness of Bulbs


Highest brightness means most power


Choose the equation with the most stuff that’s constant (i.e. brightness of bulbs in parallel are P=V^2/R since voltage is constant in parallel)


Real quick going back to energy in capacitors cuz I forgot:


Again, use the equation that has the most stuff that’s constant: connected to a battery => constant voltage, not connected to a battery => constant Q


Kirchoff’s Law


The node law: At a 3-way intersection in a circuit, current entering the intersection equals current leaving the intersection!


so here:



The loop law: If you trace any loop around a circuit the total voltage has to equal zero!

Example:



+ It doesn’t matter the direction of your loop, but you have to follow these rules:

  1. Voltage going in same direction of loop is positive

  2. Voltage going opposite of the loop is negative

  3. Current going with the loop over a resistor is negative

  4. Current going opposite to the loop over a resistor is positive



Penji Team

January 11, 2020

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