Conservation of Charge
Joseph Brown
7 min read
Listen to this study note
Study Guide Overview
This study guide covers charge conservation, elementary charge, net charge, and electric current for the AP Physics 1 exam. It explains the two types of charge, how charge is quantized, and the difference between static and current electricity. It also includes practice questions with an answer key covering calculations of current, charge transfer, and the number of electrons transferred.
AP Physics 1: Electricity & Charge - Your Last-Minute Guide ⚡
Hey there, future physicist! Let's get you prepped and confident for the AP Physics 1 exam. Remember, you've got this! We're focusing on the core concepts you need, and we'll make sure they stick.
🎯 Core Concepts: Electricity & Charge (Units 1-7 Focus)
🔗 Enduring Understanding 5.A & 1.B - Conservation is Key
Conservation Laws: Certain quantities, like charge, are conserved. This means the total amount remains constant within a closed system, even if it changes forms or locations. Think of it like a bank account - you can move money around, but the total amount stays the same unless you deposit or withdraw.
-
Charge Conservation: The total electric charge in an isolated system never changes. It's a fundamental principle! This is crucial for solving problems involving interactions between charged objects.
Charge is a property of matter that affects how it interacts with other charged objects. It's like mass, but for electrical interactions.
🔌 Essential Knowledge 1.B.1 - Net Charge
- The net charge of a system is the sum of all individual charges. Positive and negative charges cancel each other out. If you have +5 and -3, the net charge is +2. -
When solving problems, always start by calculating the total charge before any interaction.
➕➖ Essential Knowledge 1.B.2 - Two Types of Charge
-
There are only two types of electric charge: positive (like protons) and negative (like electrons).
Neutral objects have equal amounts of positive and negative charge. They're not "charge-less," just balanced!
Don't confuse "neutral" with "no charge." Neutral objects still have charges, just in equal amounts.
👶 Essential Knowledge 1.B.3 - Elementary Charge
-
The elementary charge (e) is the smallest unit of charge that can be isolated. This is the charge of a single proton (+1e) or electron (-1e).
1e = 1.6 × 10⁻¹⁹ Coulombs (C). Remember this conversion for calculations!
Think of the elementary charge as the 'atom' of charge - you can't break it down further.
🤔 What is Charge, Really?
-
Charge is a fundamental property of matter, just like mass. It's what causes electrical interactions.
-
The smallest amount of charge is the elementary charge (e), found on protons and electrons.
Charge is quantized, meaning it comes in discrete units of 'e'. You can't have half an electron's worth of charge.
-
We use Coulombs (C) for larger amounts of charge. 1 Coulomb is a massive amount of charge! Remember: 1e = 1.6x10^-19 C
Don't forget that even though Coulombs are large, they still represent an integer number of elementary charges.
⚡ Static vs. Current
-
Static charge is when charge remains in one location. Think of static cling in the winter. 🥶
-
Electric current is the flow of charge. It's what powers your devices! 💡
Current (I) is the rate of flow of charge (q) over time (t):
Remember the current equation by thinking "I Quit" (I = q/t). The charge (q) is 'quitting' its location over a time (t) interval.
-
Current is measured in Amperes (A). 1 Ampere = 1 Coulomb per second.
-
Check out Unit 9 for more on circuits and current if you have time. 😉
Image: Visual of electric current flow
⚖️ Conservation of Charge
Charge cannot be created or destroyed. It can only be transferred from one object to another. This is a fundamental law of nature.
- In any interaction, the total charge before must equal the total charge after. It's like a balance scale. ⚖️
Image: Visual of charge conservation during an interaction
🚀 Final Exam Focus
Top Priority Topics:
-
Conservation of Charge: This is huge! Expect to see it in various contexts.
-
Elementary Charge: Understand its value and its role in charge quantization.
-
Current: Know the definition and the relationship between current, charge, and time.
-
Net Charge: Be able to calculate net charge for a system of multiple charged objects.
Common Question Types:
-
Multiple Choice: Conceptual questions about charge conservation and the nature of charge.
-
Free Response: Problems involving charge transfer and calculations of current.
Last-Minute Tips:
-
Time Management: Don't get bogged down on a single question. Move on and come back if needed.
-
Common Pitfalls: Watch out for sign errors (positive vs. negative) and unit conversions (e.g., elementary charge to Coulombs).
-
Strategies: Always start by writing down the given information and the relevant equations. Draw diagrams to visualize the problem.
📝 Practice Questions
Practice Question
Multiple Choice Questions
-
A neutral object gains 100 electrons. What is the net charge of the object? (A) +100e (B) -100e (C) +1.6 x 10^-17 C (D) -1.6 x 10^-17 C
-
Two identical metal spheres carry charges of +3Q and -Q, respectively. They are brought into contact and then separated. What is the charge on each sphere after separation? (A) +2Q and -2Q (B) +Q and +Q (C) +4Q and -2Q (D) +3Q and -Q
Free Response Question
Two small, identical conducting spheres, A and B, are initially uncharged. Sphere A is given a charge of +4.0 × 10⁻⁹ C. The spheres are then brought into contact and separated.
(a) What is the charge on each sphere after separation? (2 points)
(b) If sphere A is then connected to a third identical, uncharged sphere C, what is the final charge on sphere A? (2 points)
(c) Calculate the number of electrons that were transferred to sphere A to give it its initial charge. (2 points)
(d) If a current of 2.0 x 10^-6 A flows through a wire, how long would it take to transfer the charge from sphere A to an uncharged sphere? (3 points)
Answer Key:
Multiple Choice:
- (D) -1.6 x 10^-17 C
- (B) +Q and +Q
Free Response:
(a) Each sphere will have half of the total charge, which is (+4.0 x 10^-9 C + 0 C)/2 = +2.0 x 10^-9 C. (2 points: 1 for correct magnitude, 1 for correct sign)
(b) The total charge of the system is now +2.0 x 10^-9 C. When sphere A is connected to sphere C, the charge will be divided equally between them. Each will have +1.0 x 10^-9 C. (2 points: 1 for correct magnitude, 1 for correct sign)
(c) The number of electrons is the total charge divided by the elementary charge: (4.0 x 10^-9 C) / (1.6 x 10^-19 C/electron) = 2.5 x 10^10 electrons. (2 points: 1 for correct calculation, 1 for correct units)
(d) First, find the charge on sphere A: 4.0 x 10^-9 C. Then, use I = q/t to solve for t: t = q/I = (4.0 x 10^-9 C) / (2.0 x 10^-6 A) = 0.002 s. (3 points: 1 for using the correct equation, 1 for correct substitution, 1 for correct answer and units)
Multiple Choice Questions
Alright, you've got this! Go ace that exam! 💪
How are we doing?
Give us your feedback and let us know how we can improve