# 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 ot 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

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