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Interpreting Heat Energy line graphs can be frustrating for many students and even parents.

If your child is unable to draw meaningful insights from Heat Energy line graphs, he/she may see himself/herself struggling with Heat Energy graph questions.

In this article, I will teach you how to interpret such line graphs easily and how to answer the corresponding question on the heat conductivity of the materials.

Let’s Try This Question!

Source: St. Margaret’s Primary School – 2018 P4 SA2 Examination Paper [Q38]

Before I move on to explaining this question, please take a moment to pick your choice of material.

Now, explain your choice. Did your answer include the following points?

  1. Choice of material
  2. The data you can see from the graph
  3. The heat conductivity of the material you chose
  4. The link between the heat conductivity of the material and the function of a handle of a spatula

Many of my students who struggle to obtain full marks for such questions either:

  • chose the wrong material because they cannot tell which line depicts a poorer conductor of heat,
  • chose the wrong material because they mistakenly think that a better conductor of heat would be more suitable to make the handle of the spatula, or
  • chose the correct material, but failed to use the data from the graph and/or explain the heat transfer between the user’s hand and the heat source.

If you fall into one of the above groups of students, read on to learn how to score full marks for this type of questions on heat conductivity!

Interpreting The Graph

From the graph, we can see that the temperature of the rod made of material A was higher than that made of material B after 10 minutes. This shows that the temperature of material A decreased more slowly over time as compared to the temperature of material B.

If you have difficulty interpreting the graph, try visualising the two lines on the graph as slides. Line A is a more gradual slide, whereas line B is a steeper slide. Which would allow you to slide down more slowly?

Line A would allow you to slide down slower.

I hope this image is useful in helping you visualise that the temperature of material A decreased slower.

Properties of Good & Poor Conductors of Heat

Next, we need to decide if each line shows the properties of a better or poorer conductor of heat.

Let us recall the properties of good and poor conductors of heat:

Since material A decreased in temperature more slowly, material A lost heat to the cooler surrounding air more slowly and is a poorer conductor of heat.

Conversely, as material B decreased in temperature more quickly, material B lost heat to the cooler surrounding air more quickly and is a better conductor of heat.

In summary,

  • Material A: Decrease in temperature more slowly -> lose heat more slowly -> poorer conductor of heat
  • Material B: Decrease in temperature more quickly -> lose heat more quickly -> better conductor of heat

Choosing The Correct Material

After establishing the heat conductivity of the two materials, we need to consider the function of the handle of the spatula in order to determine the more suitable material.

Picture this: You are in the kitchen at home, trying to fry an egg to go with your dinner.

It is time to flip the egg so that the other side will be evenly cooked as well. You pick up the spatula, slide it under the egg, and you try to slowly dislodge the egg from the bottom of the frying pan so that you can flip the whole piece over successfully.

However, the longer the spatula remains near the hot pan, the hotter the handle of the spatula gets.

Do you want the handle to heat up quickly or slowly?

Well, unless you wish to fry your hand along with the egg, you would want the handle to increase in temperature slowly!

Now, refer to the properties of good and poor conductors of heat once more. Which type of conductor of heat should you pick to make the handle of the spatula?

That’s right – a poor conductor of heat is more suitable than a good conductor of heat! Hence, material A, the poorer conductor of heat, should be our choice for this question.

Phrasing Our Answer

In case you have forgotten, here is the question:

“Which material, A or B, would be suitable for making the handle of the spatula? Explain your answer.”

Since there are two materials to pick from, you need to compare your material of choice to the other material that you did not pick. Additionally, since this is a question on Heat Energy, your explanation needs to include the heat transfer between the heat source and the relevant objects.

Hence, here is what the answer should look like:

Choose:

Material A.

Use data from the graph:

The temperature of material A decreased more slowly.

Explain the data:
This shows that material A lost heat to the cooler surrounding air more slowly and is a poorer conductor of heat.

Link back to the function of the handle:

Thus, a handle made of material A would conduct heat from the flame to the user’s hand more slowly, preventing the user’s hand from getting scalded easily.

I hope that after reading this article, you are now more confident in tackling Heat Energy questions that come with line graphs!