#AP Physics C: E&M - Unit 1 Study Guide ⚡

Hey there, future physicist! Let's get you prepped for the AP Physics C: E&M exam with a super-focused review of Unit 1. We'll break down the key concepts, highlight what's most important, and get you feeling confident. Let's dive in!

#🚀 Unit 1: Electrostatics - The Foundation

This unit is all about the basics of electric charge, forces, and fields. It's a big chunk of the exam, so let's make sure you're solid on this stuff. Think of it as the bedrock upon which all the other E&M concepts are built. It's like learning the alphabet before writing a novel – crucial!

#Big Ideas of Electrostatics

• Force Interactions: Forces characterize interactions between objects or systems.
  • Why does your hair stand on end after brushing it with a plastic comb? 🤔
• Fields: Fields predict and describe interactions.
  • How does a charged rubber rod bend a stream of water? 🌊
• Conservation: Conservation laws constrain interactions.
  • How are the kinematics of charged particles used in old televisions?
  • Why is it sometimes necessary to shield against electric fields? 🛡️
  • How are maps of voltage and topographical maps related? 🗺️
  • Why can a bird land on a high voltage wire and not be electrocuted? 🐦

#Exam Impact

• Unit 1 is a big deal, covering about 25-33% of the exam. That's a lot!
• Expect to spend around 40 class periods on this material.
• The AP Classroom personal progress check has ~35 multiple-choice questions and 1 free-response question to help you practice. Make sure to use it!

# 💡1.1: Electric Charge & Coulomb's Law

#What is Charge? 🌩️

• Charge is a fundamental property of matter, like mass.
• Two types: positive (protons) and negative (electrons).
• Measured in:
  • Coulombs (C): For larger amounts of charge.
  • Elementary charges (e): For smaller amounts (like on individual particles).
• In AP Physics C, we mostly deal with point charges – infinitely small objects with charge but no size.

#Law of Electrostatics

• Like charges repel, opposites attract. Simple as that!

  Name	Attracted to	Repelled by
  Positive charge	Negative & Neutral	Positive
  Negative charge	Positive & Neutral	Negative
  Neutral object	Both Positive & Negative	Neither

• Check out the PhET simulation to visualize this in action. It's a great way to make the invisible visible!

#Conservation of Charge

• Example: Touching a +4e sphere to a -12e sphere results in two -4e spheres. Total charge before (-8e) equals total charge after (-8e).

#Conductors and Insulators

• Conductors: Allow charge to move freely (e.g., metals).
• Insulators: Restrict charge movement (e.g., rubber, plastic).

• Insulators are used to prevent charge from going where it shouldn't (like in wires).

#Charging and Discharging

• Friction: Rubbing objects transfers electrons. One becomes positive, the other negative.
• Contact: Touching a charged object to a neutral one transfers charge. Both end up with the same type of charge.
• Induction: Bringing a charged object near a neutral one causes polarization. If grounded, the neutral object becomes oppositely charged.

#Electrostatic Force & Coulomb's Law

• Coulomb's Law quantifies the force between two point charges:

  $$F = k \frac{|q_1 q_2|}{r^2}$$

  Where:

  • F is the electrostatic force.
  • k is Coulomb's constant (1/4πε₀).
  • q1 and q2 are the magnitudes of the charges.
  • r is the distance between the charges.

• Key Point:

  • Positive F means repulsion, negative F means attraction.
  • Force is a vector! Use vector addition if multiple forces are acting on a charge.

# Practice Questions

Practice Question

1. Multiple Choice

   

   Answer: Barun is correct. This is charging by induction. The positive rod coming close to the electroscope will cause the positive charge in the electroscope to be repelled, traveling down to the leaves which makes the leaves separate.

2. Multiple Choice

   

   a) What is the direction of the force on the test charge due to the two other charges?

   b) If F is the magnitude of the force on the test charge due to only one of the particles, what is the net force acting on the test charge due to both of the charges?

   Answers:

   a) The net force must be directed towards the bottom left corner of the page. The test charge and both Q charges are positive so they must repel. The test charge is repelled downwards and to the left.

   b) The test charge experiences two forces, both of magnitude F repelling it downwards and to the left. Using the Pythagorean theorem we can determine the resulting net force

   $$F_{net} = \sqrt{F^2 + F^2} = \sqrt{2F^2} = F\sqrt{2}$$

   

3. Multiple Choice

   

   Answer: C = D > A = B > E = F

   Each pair of point charges must have an equal force on them (look at the Newton's 3rd Law tip from earlier)! A&B and C&D have opposite charges so therefore, they must attract. E&F have the same charge and must be repelling. The magnitude of the force is directly proportional to the charges and inversely proportional to the square of the separation distance (💭 think back to the electrostatic force formula for the relationships here). Therefore, the shorter distance in C&D results in a larger force even though the charge in B has a greater magnitude.

   

#Final Exam Focus 🎯

Okay, you've made it through the first unit! Here's what to really focus on for the exam:

• Coulomb's Law: Know the formula inside and out. Practice using it in different scenarios.
• Charging Methods: Understand friction, contact, and induction. Be able to explain how each works.
• Force Vectors: Remember that force is a vector! You'll need to do vector addition to find net forces.
• Conservation of Charge: This is a fundamental principle, make sure you understand it.
• Conductors vs. Insulators: Know the difference and how they affect charge movement.

You've got this! Go ace that exam! 💪