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Common Mistakes on Energy Conversion

This article aims to help your child explain energy concepts clearly.

Let’s recall the MUST-KNOW energy conversion concepts first before we work on the common mistakes.

Light energy

Light energy is a form of energy that enables us to see.

Fuels such as coal and wood give off light as they burn.

Heat energy

Heat energy is a form of energy that makes things warm or hot.

Heat always flow from a hotter region to a cooler region until the same temperature is reached.

Electrical energy

Electrical energy is a form of energy that can be carried along wires.

Energy stored in batteries and solar cells can be converted into electrical energy.

Electrical energy is a useful form of energy as it can be converted to many other forms of energy.

Kinetic energy

Kinetic energy is a form of energy that moving objects have.

The amount of kinetic energy the object has depends on:

  • Mass of the object.

Example: If a train and a car are moving at the same speed, the train has more kinetic energy. Train has a bigger mass than the car.

 

  • Speed of the object.

Example: If the train is moving faster than the car, the train has more kinetic energy.

Chemical potential energy

Stored energy that can be released from food by respiration.

Stored energy in fuels that are released by burning.

Gravitational potential energy

Any object that is above ground, hanging or on the top of a slope has gravitational potential energy.

Gravitational potential energy is determined by two factors:

  • Position of the object above the ground.Example: The higher the object is from the ground, the greater the gravitational potential energy.
  • Mass of the object.Example: An iron ball and a rubber ball are held 100cm above ground. Both balls are of the same size. Iron ball has a greater mass than the rubber ball. The iron ball has a greater gravitational potential energy.

 

Elastic potential energy

When the spring is stretched or compressed, it has elastic potential energy.

 

Take note:

If the spring is at its original length (neither stretched nor compressed), the spring does not have elastic potential energy.

Take note of these common mistakes

Study the question below.

Ann placed the water wheel at a fixed position below the tap as shown below. She connects the tap at position D and turns on the tap. She observed the speed at which the water wheel turned. She repeated the experiment by placing the tap at positions C, B and A.

Energy conversion potential energy converted to kinetic energy

What happens to the speed of the water wheel as Ann repeated the experiment by placing the tap at position D compared to position A? Explain your answer.

 

Wrong answer:

The water wheel spins faster when the tap is at D compared to A.

The gravitational potential energy is converted to kinetic energy and the kinetic energy is transferred.

 

Why is the above answer wrong?

The answer is confusing.

The student did not write out the situation where energy conversion has taken place.

 

Teachers’ comments:

1) You have to describe the situation where energy conversion takes place.

Do not combine all the energy conversion into one long sentence.

Examiner must know at which situation does energy conversion takes place.

2) The student did not compare.

 

Correct answer:

The water wheel spins the fastest when the tap is at D compared to A.

At D, the tap is at a highest position compared to A.

As the water flows down from D, most gravitational potential energy is converted to the most kinetic energy.

As the water hits the water wheel, most kinetic energy is transferred to the water wheel, causing it to spin the fastest.

 

Take note:

Those highlighted in yellow are the description of a situation.

Use superlative words such as highest, fastest, most

Describe the situation and give the correct energy conversion in that situation.

Tom gave a push to a ball resting on a table.

Using energy conversion concepts, give a reason why does the ball come to a stop eventually?

 

Wrong answer

The ball comes to a stop as all the kinetic energy has used up.

 

Teachers’ comments:

The student has committed a huge error.

The laws of energy conversion states:

  • Energy can neither be created nor destroyed.
  • Energy can be converted from one form to another.

By writing the word “used up”, it could mean the energy is destroyed,

That is wrong. It is against the laws of energy conversion.

 

Apply the rule of  “Energy can be converted from one form to another” into your answer.

 

Correct answer

The ball comes to a stop as all the kinetic energy of the ball is converted to heat energy and sound energy.

David placed a wooden block on a ramp as shown below. He lets the wooden block slides along the ramp.

Energy conversion - potential energy to kinetic energy

Explain why does the wooden block slows down and come to a stop?

 

Wrong answer 1

The kinetic energy of the ball is converted to frictional force.

Frictional force slows down the ball.

 

Teachers’ comments (3 pointers to take note):

Another huge error commited by the student.


1.
Energy is not converted to force. Energy and force are two different scientific concepts.

2. Energy is possessed in the object.

You have to know these:

– Energy possessed in the object is converted from one form to another as the object moves.

– Force is an interaction between two objects.

 

What is an interaction?

(Interacting with one another)

 

Similar to talking with people, there is involvement between two or more objects or persons.

Force is an interaction because two objects are acting / exerting on each other.

Example: Frictional force is acting between the wooden block and the ramp.

 

3. You have to describe the situation where energy conversion takes place.

Description of the situation is highlighted in yellow.

 

Correct answer

As the wooden block slides down the slope, the gravitational potential energy of the wooden block is converted to kinetic energy.

Eventually, the wooden block comes to a stop as all the kinetic energy of the wooden block is converted to heat energy and sound energy.

Tom drops a ball 1metre above the ground.

Wrong answer

Gravitational potential energy acting on the ball at A is converted to kinetic energy.

 

Teachers’ comments:

In the topic of energy conversion, energy is possessed in the object.

For energy conversion, we cannot write “acting on / exerting on”

 

Why?

Energy possessed in the object is converted from one form to another as it moves.

Instead, when we mention forces, we have to write “acting on / exerting on”.

 

Why?

Force is an interaction. Force is acting between two or more objects.

 

Correct answer 1

Gravitational force acting on / exerting on the ball is pulling the object to the ground.

 

Correct answer 2

As the ball drops, gravitational potential energy is converted to kinetic energy.

The diagram shows a roller cart moving along the path starting from position A. The journey of the roller cart ends at position E.

 

 

Explain, in terms of conversion of energy, why must the height of hill Y be shorter than Hill X.

 

Not recommended answer:

If the hill Y is taller than hill X, the cart must have more gravitational potential energy at D than at A for the cart to roll over hill Y.

 

Teachers’ comments:

You can see the student has given an alternative situation.

 

Potential problems your child might face when he or she writes alternative situations:

1) You are giving a teacher an additional scenario. The teacher needs to understand what scenario you are describing. You need to describe the alternative scenario clearly. That could be a problem for some students.

2) After explaining the alternative scenario, you have to compare that scenario with the scenario as stated in the question. This is the challenging part which many students attempted did not do well.

As you can see based on the answer above, the student did not relate his alternative situation back to the question of why “hill Y is shorter than hill X”.

As the time given in the exam is limited, we recommend you to write straight to the point.

Answers have to be clear and concise so that you can move on to the next question.

 

Take note:
Please do not construe “not recommended” as “wrong”.

We accept some students’ preference in their writing. If your child can explain well using the correct scientific keywords, marks are awarded.

 

Answer 1

If the hill Y is taller than hill X, the cart must have more gravitational potential energy at D than at A for the cart to reach D and roll over hill Y.

That is not possible as some of the kinetic energy is converted to heat and sound energy when the cart rolls down from A to B.

There is not enough kinetic energy to convert to the same amount of gravitational potential energy for the cart to reach D if hill Y is the same height as X.

 

 

Correct answer 2

As the roller cart moves down the roller coaster, gravitational potential energy is converted to kinetic energy.

Some kinetic energy is converted to heat energy and sound energy.

Hence there is lesser kinetic energy when the cart is at C.

Lesser kinetic energy is converted to lesser gravitational potential energy to reach the top of hill Y as the height is shorter than hill X.

 

What is the difference between Answer 1 and Correct answer 2?

In Answer 1, we do an additional step of comparing the alternative scenario with the scenario stated in the question.

The explanation is very long and complicated. A student with good command of english and scientific concepts may attempt this way.

In Correct answer 2, the explanation is clear and it goes straight to answering the question. The answer is consistent with the energy conversion sentence structures.

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Talk again soon!

Andy Ling

 

Helping your child overcome the open-ended questions

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