# 1.2 Electric Fields & Electric Potential Peter Apps

### Electric Fields

Every charged object has an electric field surrounding it, similar to how every object with mass has its own gravitational field. The more charge (or mass) there is, the stronger the field is. The only difference is that while a gravitational field must be attractive, an electric field can be either attractive or repulsive. By convention, we use the direction that a positive test charge will move to draw our electric fields.
Rules for drawing:
• Field lines are vectors and must be drawn with arrows.
• Lines go away from a positive charge and towards a negative charge.
• The strength of the field can be visually represented by the density of the field lines. Therefore field, lines must never touch or cross. This would represent an infinitely strong field.

### Simple Fields

• Point Charges
• Notice the radial symmetry in the fields. Image from wikimedia.org

• Two Point Charges Image from Ck12.org

• Two Parallel Plates
• Notice how the field is the same everywhere between the plates. Image from researchgate.net

Try using the PhET simulation to create your own fields and notice the how the field strength changes as a function of charge and distance.

### Electric Field Strength

We've seen visually what electric fields look like. Now it's time to mathematically describe them.
The basic idea is to place a test charge at various locations in the field, measure the electrostatic force at that location, then calculate the field strength. The equation off of your reference tables for electric field strength is:  ### Electric Fields in Conductors & Insulators

Suppose you bring a conductor near a charged object. The side of the conductor closest to the charged object will be induced with the opposite charge. However, the charge will only exist on the surface of the conductor. There will NEVER be an electric field inside a conductor. On the contrary, an insulator can store the charge inside and may have an internal electric field.
Here's an animation from Wikipedia showing how this works in a conductor. When an external electrical field (arrows) is applied, the electrons (little balls) in the metal move to the left side of the cage, giving it a negative charge. The remaining unbalanced charge of the nuclei gives the right side a positive charge. These induced charges create an opposing electric field that cancels the external electric field throughout the box. This is the basic idea behind a Faraday Cage. Image from Wikipedia.org

### Practice Questions

1. Ranking the strength (magnitude) of the electric force:  2. Looking at the graph and details below, determine at which point, if any, the electric field strength is zero. Image from apclassroom.collegeboard.org

Point A must have an electric field strength of 0. The point must be closer to the smaller charge (Q) than the larger charge (-4Q), so it can't be D or E. It must also be where the force vectors between the test charge point in the opposite direction so that the net force is 0. Therefore, it can't be point C either. Since the negative charge (-4Q) is 4x greater than the positive charge, the point must be 2x as far from the -4Q charge as it is from the Q charge. That only leaves A as the answer. 3. Image from apclassroom.collegeboard.org

Graph A is correct. At x = 2 and x = 4, the distance from the charges is 0, so the field strength must trend towards infinity. At x = 3, the repulsion from the 2 charges cancels out so the field must be 0 there.

##### 🔍 Are you ready for college apps?
Take this quiz and find out!
Start Quiz
#####  FREE AP physics e m Survival Pack + Cram Chart PDF
Browse Study Guides By Unit
🙏
Exam Reviews
✍️
Exam Skills- FRQ/MCQ
🔋
Unit 2: Conductors, Capacitors, Dielectrics
🔌
Unit 3: Electric Circuits
🧲
Unit 4: Magnetic Fields
⚛️
Unit 5: Electromagnetism
#####  