AP Physics – 1 st Law Continued

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# AP Physics – 1 st Law Continued - PowerPoint PPT Presentation

AP Physics – 1 st Law Continued. Collect and read the handout on Heat Engine Efficency from the front of the room When finished work your way through the worksheet provided In 20 minutes, we will explore PV diagrams and the Carnot Cycle Next Class : The 2 nd Law of Thermodynamics.

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AP Physics – 1st Law Continued

Collect and read the handout on Heat Engine Efficency from the front of the room

When finished work your way through the worksheet provided

In 20 minutes, we will explore PV diagrams and the Carnot Cycle

Next Class: The 2nd Law of Thermodynamics

PV-Diagrams

Pressure/Volume graphs are of tremendous value in analyzing the performance of heat engines.

Essentially, the amount of work produced by an engine depends on the path it takes (changes in pressure and volume)

PV-Diagrams

Confused…?

Expected, let’s start slow

• Engines follow a path, in this case from 1 2, 23, 34, and 41
• Each affects the Work done by, or done on the System
Net Work of a Machine

Let’s study the engine to the right…

Q: is the direction of the arrows important?

Net Work of a Machine

Let’s study the engine to the right…

Q: is the direction of the arrows important?

YES! Challenges if work is done ON the system or BY the system

Note: Please write done the work done for each of the next for steps

From a to b, the pressure remains constant – its value is P1.

The volume decrease from V1 to V2.

This represents work done on the system.

How can you calculate the W?

From a to b, the pressure remains constant – its value is P1.

The volume decrease from V1 to V2.

This represents work done on the system.

How can you calculate the W?

Step 2: Process b c is an isochoric compression

Isochoric because the volume does not change but the pressure increases.

What is the work for this step?

Step 3: Process cdis an isobaric expansion.

The gas expands from V1 to V2, doing work as it expands. The amount of work is equal to the area under the curve.

Step 4: Process da is an isochoric expansion

Volume stays constant. So no work is done. Pressure decreases.

Complete Cycle

Based on what you have recorded for the work on each step, what is the total work done on the system?

Complete Cycle

Based on what you have recorded for the work on each step, what is the total work done on the system?

OR just the area of the enclosed area!

Whiteboarding!

Form a group of 3 or 4 no more no less

Collect a Whiteboard and pens, and work your way through the following two problems

Make sure you are contributing to your group, if you are not then start a new group!

Question 1:

A heat engine’s cycle is shown in the PV diagram to the right.

P1= 345 kPa, P2 = 245 kPa, P3 = 125 k Pa, and P4= 225 kPa. V1= 35.0 L and V2 = 85.0 L.

What is the net work done during one cycle of the engine?

Note: 1 L = 0.001 m3

Question 1:

A heat engine’s cycle is shown in the PV diagram to the right.

P1= 345 kPa, P2 = 245 kPa, P3 = 125 k Pa, and P4= 225 kPa. V1= 35.0 L and V2 = 85.0 L.

What is the net work done during one cycle of the engine?

Note: 1 L = 0.001 m3

Ans: 6.00 kJ

Question 2:

A substance undergoes a cyclic process shown in the graph. Heat transfer occurs during each process in the cycle.

• What is the work output during process ab?
• How much work input is required during process bc?
• What is the net work done during the cycle

Note: 1 atm = 1.013 x 105 Pa

Note: 1 L = 0.001 m-3

Question 2:

A substance undergoes a cyclic process shown in the graph. Heat transfer occurs during each process in the cycle.

• 1.22 x 104 J (Expansion process, so work is done on surroundings)
• -4.05 x 103 J
• -8.15 x 103 J
Carnot Cycle – An Ideal Engine