MCAS Question - Efficiency

MCAS Question - Efficiency

By Stacy-Michelle Reid

Multiple Choice

When is the Physics MCAS?

• Next week!

• May 7th & 8th

• June 7 & 8

• June 1 & 2

New MCAS Lessons

Over the next couple days you will have some short lessons to teach you about something new that we didn't learn this year. Each of these items will likely have one MCAS question at the end of the year. Remember - you only need to answer about 15 MCAS questions correctly to pass!! Each one you know will help you!

Multiple Choice

REVIEW QUESTION: If you apply 50N of work to move something 2m, how much work did you do?

• 25J

• 100J

• 0.04J

• 50J

Multiple Choice

The law of conservation of energy states that when one form of energy is converted to another:

• Energy is destroyed in the process

• No energy is destroyed in the process

• Energy is created in the process

• Some amount of energy cannot be accounted for

Multiple Choice

In energy transformations - "wasted energy" is usually in the form of:

• heat

• light

• sound

• chemical energy

Today's MCAS Question: Efficiency

For example, if you had to haul some furniture, which of these two trucks would make that easier? Gas is expensive right now - which would get better gas mileage?

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Not all machines do their job equally well

This usually refers to the USEFUL type and amount of energy that you get FROM that machine.

Outputs

This is the type and amount of energy that gets put IN to a machine.

​Inputs

Inputs & Outputs

How does input get used?

• Not all of your work input will be used for what you want

• OUTPUT IS ALWAYS LESS THAN INPUT!!

• Some of your input will always go into friction in the machine itself

• Some of your input will go to useless outputs

• Some of your input will go to the desired output​

Multiple Choice

In a real-world machine, what is true about work in compared to work out?

• work in < work out

• work in > work out

• work in = work out

Multiple Choice

Because of friction, which will always be higher:

• Input Work

• Output Work

EFFICIENCY

Efficiency is the ratio of work output to work input

• In other words - efficiency is how effective your machine is.

• It's always a percentage between 0% and 100%​.

• You can find it using this formula on our formula sheet:

Multiple Choice

Energy Efficiency is calculated as useful energy output divided by the total energy input, and is expressed as a percentage

• True

• False

Multiple Choice

Is it possible to have a machine in the real world with 110% efficiency?

• Yes!

• No!

Because energy out (output) is always less than energy in (input), you will always get a decimal between 0 and 1.00

​

In order to make this a percentage, you can multiply it by 100%

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The higher a machine's efficiency, the less work is lost to heat and friction.

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Multiple Choice

0.89 efficiency is:

• .89% efficient

• 8.9% efficient

• 89% efficient

• 890% efficient

Multiple Choice

0.1 efficiency is:

• 0.1% efficient

• 1.0% efficient

• 10% efficient

• 100% efficient

Multiple Choice

0.05 efficiency is:

• 0.05% efficient

• 0.5% efficient

• 5% efficient

• 50% efficient

This formula is one of our easiest this year!

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• To calculate efficiency, divide the energy out by the energy in.

• ​After you divide, you should get a decimal.

• ​Multiply the decimal by 100% to get the percent effective

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Let's Practice!

Multiple Choice

If you put 50J of work into a machine and get 25J of work out, which is your machine's efficiency?

• 50%

• 200%

• 25%

• 100%

Multiple Choice

As a percentage, what is the efficiency of a machine that uses 100J of energy to put out 90J of work?

• 1.11

• 0.90

• 90%

• 111%

Multiple Choice

Which machine loses more energy to heat?

• a 50% effective machine

• a machine that uses 100J of energy and does 60J of work

Multiple Choice

A new power station is being constructed, and the designers claim that it will be extremely efficient. They claim that for every 1,000,000 J of energy that is put into the station 400,000 J of energy will be produced. How efficient is the station?

• 40%

• 60%

• 4%

• 2.5%

Remember - The only OUTPUT we use for calculations is the one we want from the machine!

Multiple Choice

A team of university students test a new car they are designing. They find that the 8500J of of the energy is transferred to movement (kinetic) energy and 3500J is transferred to heat energy. They put a total of 12,000J of energy in to the car in the form of an electrical battery. How efficient is the car?

• 80%

• 20%

• 70%

• 29%