Unit 1 Cycle 2: Interactions and Energy



Name:________________________________ Date:_______________ Group: ______

Investigating Conservation of Energy

In this homework you will check conservation of energy for the two systems you investigated in the homework for Cycle 7 Activity 1. Open up the simulator for Cycle 7 Act 2 Homework.

Go to the simulator index page and open Cycle 1 Activity 9 Homework Setup.

Battery and Buzzer

Set up a circuit with a battery and buzzer. Then add energy bar graphs for both the battery and the buzzer.

[pic]

Run the simulator for about 30 seconds, then fill in the following two energy bar graphs.

|Battery |

|ENERGY INPUT |

|Type of Energy = |Amount = |

|ENERGY OUTPUT |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|ENERGY CHANGES IN SYSTEM |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|Buzzer |

|ENERGY INPUT |

|Type of Energy = |Amount = |

|ENERGY OUTPUT |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|ENERGY CHANGES IN SYSTEM |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

How does the electrical energy output from the battery compare to the electrical energy input to the buzzer? Why does this make sense?

Use the energy conservation equation to show explicitly that energy is conserved in the battery system. (That is, write down the equation, and then substitute all the values from the battery energy bar graph, and show that the equation balances.)

Use the energy conservation equation to show explicitly that energy is conserved in the buzzer system.

Calculate the efficiency of the buzzer.

Battery and two bulbs in parallel Parallel circuit with Battery and Two Bulbs

Go to the Simulator Index page and open Cycle 1 ActChapter 1 Activity 7 HW Setup. Set up the following circuit, and then set up energy bar graphs for the battery and each of the two bulbs.

[pic][pic]

[pic]

Run the simulator for about 30 seconds, and then fill in the following energy bar graphs.

| |

|BBattery |

|ENERGY INPUT |

|Type of Energy = |Amount = |

|ENERGY OUTPUT |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|ENERGY CHANGES IN SYSTEM |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|Bulb 1 |

|ENERGY INPUT |

|Type of Energy = |Amount = |

|ENERGY OUTPUT |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|ENERGY CHANGES IN SYSTEM |

|Type of Energy = |Amount = |

|Bulb 2 |

|ENERGY INPUT |

|Type of Energy = |Amount = |

|ENERGY OUTPUT |

|Type of Energy = |Amount = |

|Type of Energy = |Amount = |

|ENERGY CHANGES IN SYSTEM |

|Type of Energy = |Amount = |

How does the electrical energy output from the battery compare to the total electrical energy input to the two bulbs? Why does this make sense?

[pic]

Use the energy conservation equation to show explicitly that energy is conserved in the battery system.

[pic] Choose one of the two bulbs. Use the energy conservation equation to show explicitly that energy is conserved in the a bulb system.

[pic] How does the electrical energy output from the battery compare to the total electrical energy input to the two bulbs?

Generator and Motor

In Activity 9 you examined the energy relationships in a generator-motor circuit.

[pic]

Consider the generator (with whatever is turning the handle) and motor as a single system. Draw an I/O energy diagram for this system in its transient state.

Would this be an open or closed system? How do you know?

Use the computer simulator to set up this circuit, then select the entire circuit and place an energy bar graph. Run the simulator for 3 seconds and PAUSE it.

Check to see if your I/O energy diagram included all the appropriate energy transfers and changes. If not, re-draw it correctly on the next page. Do not erase or change your original diagram.

Check to see if energy is conserved for this system. Show your work below.

Soccer player kicking a ball

|In Activity 2 you drew a Source/Receiver energy diagram for a soccer player and |[pic] |

|a soccer ball, during the time when the ball is being kicked. Now consider the | |

|player and the ball as a single system. | |

| | |

|Complete the energy diagram for this system during the kick. (Assume the effects| |

|of friction are negligible.) | |

| | |

|During the kick, is this an open or closed system? How do you know? | |

| | |

Heat Energy, Useful Energy and Efficiency

Look over all the energy bar graphs you displayed when analyzing electric circuits in Activity 7, as well as the energy bar graphs earlier in this assignment.

UP TO HERE

[pic] What type of energy output is common for all the devices when they operate?

[pic]Why do you think this is?

It turns out that essentially all devices will become warmer as they function, and consequently will transfer heat energy to the surroundings. With a space heater, that’s desirable, since the purpose of the space heater is to provide warmth. But the purpose of many other devices is to provide some other type of energy (e.g. light, electrical, mechanical), not heat energy. The type of energy output that corresponds to the intended purpose of a device is called the “useful energy” output. For example, the useful energy output for a fan is mechanical energy (since the purpose of the fan is to move as much air as possible away from it towards somewhere else in the room).

[pic] Consider the various devices you examined in this Aaactivity 7 and earlier in this assignment. In the second column of the Table belowon the next page, indicate the useful type of energy output for that device. (For the moment, ignore the other columns in the Table.)

The purpose of most devices is to transform one type of energy into another, usually from a type of energy input into a useful type of energy output. ( (In the case of the battery, the purpose is to transform chemical potential energy into a useful type of energy output.) For most devices, the space heater being an exception, the useful energy output is not the same as the total energy output.

[pic] Fill in the third column of the Table below with the type of energy input.

Go over the data you collected from the simulator in Activity 7 and record values (in joules) for the energy inputs and useful energy outputs in the fourth and fifth column of the Table.

Table: Useful types of Energy and Efficiencies for Electrical Devices

DeviceUseful type of energy outputType of energy inputAmount of useful energy output during a certain time (joules)Amount of energy input during a certain time (joules)Efficiency of device (%)Generator

Space HeaterBulb

Buzzer

Motor (fan)

[pic] For each device, is the amount of useful energy output less than, equal to, or greater than the amount of energy input? Why do you think this is?

[pic] Do you think it would ever be possible, in any device, for the amount of useful energy output to be greater than the amount of energy input (or decrease in chemical energy)? Explain your thinking.

Scientists have a measure of how efficient a device is at converting its energy input to useful energy output. This measure expresses the amount of useful energy output as a percentage of the energy input and is called the efficiency of the device.

For example, suppose a device has an energy input, over a certain period of time, of 200 J, but over that same time period has a useful energy output of 150 J. In this case only three-fourths of the input is transformed to useful output, so the device is said to be 75% efficient.

In general, efficiency can be calculated from the following formula:

[pic]

[pic] Calculate the efficiencies of the devices in the Table and record those values in the last column.

[pic] Which is the most efficient device (according to the simulator)?

Which is the least efficient device?

Block sliding across table

At the end of Activity 4 your drew a Source/Receiver energy diagram to describe the chain of interactions involved when a block slows down as it slides along a table after being pushed. You assumed the block and surface both warmed up during the interaction, and you took that information into account when drawing the S/R diagram. (You should go back and review the diagram now.)

[pic] Now consider the block and table as a single system. Draw an I/O energy diagram for the block + table system during the same slowing down and warming up process.

[pic] Is this an open or closed system? How do you know know?

[pic] Suppose we redefined the system as block + table + surroundings. Would this system be open or closed? Why?

Summarizing Qquestions

S1. Do you think it would ever be possible, in any device, for the amount of useful energy output to be greater than the amount of energy input (or decrease in chemical energy)? Explain your thinking.

S52. You probably noticed that the efficiency of the small bulb used in the simulator (which matches the one used in your electric circuits experiments) has a very low efficiency. Regular (incandescent) light bulbs that people use in their homes have efficiencies of only about 5% to 8%. (Most of the energy output is in the form of heat energy, since the filament has to be at a very high temperature (greater than 4000 °F) to glow brightly and produce a reasonable amount of light energy as an output.) Fluorescent bulbs, on the other hand, work in a different way, and although they cost more to purchase they typically have efficiencies around 25% (approximately four times greater than a regular bulb). It is often suggested by those concerned for the environment that the public should purchase energy-efficient light bulbs. Consider the following comment from a student:

“I don’t understand all this fuss about making sure the light bulbs we use have a high efficiency. Since we know energy is conserved, no energy gets destroyed, so why worry?”

How would you respond to this student?

Summarizing Questions

S1: How does the total energy output from the battery compare to the total electrical energy input to the two bulbs? Why does this make sense?

S2: How does the electrical energy output from the battery compare to the total electrical energy input to the two bulbs? Why does this make sense?

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