# 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:

Voltage going in same direction of loop is positive

Voltage going opposite of the loop is negative

Current going with the loop over a resistor is negative

Current going opposite to the loop over a resistor is positive