FOCUSED REVIEW
Focused Review — First Law of Thermodynamics — Foundational
Reinforce the highest-leverage ideas and representative problem-solving tools for First Law of Thermodynamics.
TIME
Approximately 15 minutes
BEST FOR
Targeted reinforcement
FINISH WITH
A readiness check
After this focused review, you'll be able to...
reinforce the key relationships, apply them to representative problems, and identify what still needs work.
Choose how you want to review
Course Alignment
This Physics Sensei Unit Review is an independent learning resource. Use it to reinforce key concepts, prepare for homework, or review before a quiz or exam.
RESOURCE: Physics Sensei Unit Review | UNIT ID: THM-U08 | TOPIC: First Law of Thermodynamics | COURSE LEVEL: Foundational
BEST USED ✓ After learning the unit ✓ Before starting homework ✓ Before a quiz or exam
Your Review Plan
Complete these six stages in order. Each stage builds on the previous one and prepares you for the final readiness check.
6 Stages • Approximately 15 minutes.
Warm-Up Check
Before you begin, take a moment to see what you already remember. Do not worry about getting everything right. This is only a starting point.
ACTIVITY 1
Key Ideas
Energy-transfer signs
Use the convention W = work done by the system. If 200 J of heat enters a gas and the gas does 50 J of work, is Q positive or negative? Is W positive or negative?
Reveal Answers
Q is positive and W is positive.
Why it works: Energy entering as heat makes Q > 0; expansion work done by the system makes W > 0.
ACTIVITY 2
Common Mistakes
Internal energy change
A system receives 300 J of heat and does no work. What happens to its internal energy?
Reveal Answers
The internal energy increases by 300 J.
Why it works: With W = 0, the first law gives ΔU = Q.
ACTIVITY 3
Quick Application
Compression intuition
A gas is compressed while no heat is exchanged. Does its internal energy tend to increase or decrease?
Reveal Answers
Increase.
Why it works: Compression means work is done on the gas, so W by the system is negative and ΔU = -W is positive.
Ready to strengthen your understanding?
You've refreshed what you already know. Next, you'll reinforce the essential concepts that will help you solve problems with confidence. Need to see the learning path again?
Core Concepts
Reinforce the two highest-leverage relationships, then use them in representative situations.
KEY CONCEPT 1
The First Law
Energy conservation for a thermodynamic system is ΔU = Q - W, where W is work done by the system. Heat added to the system is positive Q; heat leaving is negative Q.
ΔU = Q − W
Example: If Q = 500 J and W = 120 J, then ΔU = 380 J.
Sensei note: Choose and state one sign convention before calculating; most sign mistakes come from switching conventions.
KEY CONCEPT 2
Heat and Work Are Transfers
Internal energy is a property of the system. Heat and work are ways energy crosses the system boundary; a system does not “contain heat” or “contain work.”
ΔU = U₂ − U₁
Example: A hot object can have high internal energy, but heat refers to energy transferred because of a temperature difference.
Sensei note: Ask what crosses the boundary and in which direction.
Ready to apply these ideas?
You've reinforced the essential concepts. Now it's time to put them into practice by working through guided examples and building your problem-solving confidence. Need a quick reminder?
Guided Practice
Apply the reinforced ideas to two representative situations, then use the strategy card to check your setup.
PRACTICE 1
Guided Example
A gas absorbs 650 J of heat and does 180 J of work. Find ΔU.
Use ΔU = Q - W. Substitute Q = 650 J and W = 180 J.
Reveal Answers
ΔU = 650 J - 180 J = 470 J.
Why it works: Energy entered as heat, but some left as work.
PRACTICE 2
Independent Check
A gas releases 150 J of heat while it is compressed. The work done by the gas is -400 J. Find ΔU.
Assign signs first, then apply the first law.
Reveal Answers
ΔU = -150 J - (-400 J) = 250 J.
Why it works: Compression transfers more energy into the gas than the heat loss removes.
Ready to check your understanding?
You've practiced the essential skills with guidance. Now it's time to solve a few short problems on your own and confirm you're ready to move forward. Need a quick reminder?
Confidence Check
You've rebuilt the key ideas and practiced them with guidance. Now try these short questions on your own to check your understanding before moving on.
QUICK CHECK 1
Sign of compression
Solve using the first law and the stated sign convention.
An adiabatic compression has Q = 0 and W = -90 J. Find ΔU.
Reveal Answers
ΔU = 90 J.
Why it works: ΔU = 0 - (-90 J).
QUICK CHECK 2
Heat needed
Solve using the first law and the stated sign convention.
A system has ΔU = 250 J and does 75 J of work. Find Q.
Reveal Answers
Q = 325 J.
Why it works: Q = ΔU + W.
How did it go?
You've checked your understanding. Take one final look at the essential ideas before deciding what to do next. Need a quick reminder?
Summary
Before moving on, take one final look at the most important ideas from this review.
KEY TAKEAWAY 1
Energy accounting
Use ΔU = Q - W with a declared sign convention.
KEY TAKEAWAY 2
Boundary transfers
Heat and work describe energy crossing the boundary, not stored quantities.
Ready for your next step?
You've reviewed the essential ideas one last time. Now choose the resource that best matches how confident you feel. Need a quick reminder?
Next Step
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You've completed this review. Choose the next resource that best matches how confident you feel.
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Review the key ideas and examples again.
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