Hi everyone! As I’m writing this, it has been raining for the past few days in Singapore and the weather had been chilly. As I kept shivering in the morning, I would often head down to Starbucks or another cafe for a cup of piping hot latte to keep warm.

Whenever I order my cup of hot latte to take away at Starbucks, I realise that it always comes with a cup sleeve. Not only that, I also realise that the types of cup sleeves used are different for different cafes.

Some cup sleeves are made of smooth cardboard, like the one used in Starbucks as shown below on the left. On the other hand, some cup sleeves are corrugated, like the one shown below on the right. As a result, I often wonder why some cafes choose to use corrugated cup sleeves over smooth ones.

What do you guys think is the reason?

To answer this question, I have selected a Heat Energy question from the 2021 Anglo-Chinese School (Junior) (ACSJ) P6 SA2 Examination Paper for discussion.

## Question Analysis

Source: Anglo-Chinese School (Junior) – 2021 P6 SA2 Examination Paper [Q34]

## Thought Process

Before we start, we must first understand the purpose of cup sleeves.

“Why are cups of hot coffee often served with a cup sleeve?”

What do you think would happen if you were holding on to the cup of hot coffee without the cup sleeve?

Would you be able to hold onto the cup for a long time? No.

“Why are we unable to hold on to the cup of hot coffee for a long time without the cup sleeve?”

This is because our hands would get scalded.

What is the Science concept tested here that causes our hands to get scalded? It is, “Heat travels from a warmer region to a cooler region”.

“How do we explain why our hands would get scalded without the cup sleeve?”

When we hold onto the cup of hot coffee without the cup sleeve, too much heat is conducted from the hot coffee to our cooler hands, causing our hands to be scalded.

## Let’s Analyse The Question

“Sally found that she could hold onto the cup of hot coffee with sleeve Y longer than sleeve X. Explain why.”

For Sally to hold onto the cup of hot coffee with sleeve Y longer than sleeve X, should sleeve Y conduct heat from the hot coffee to her cooler hands faster or slower?

Sleeve Y must conduct heat from the hot coffee to her hands slower than sleeve X. How is sleeve Y able to do so?

⭐️ Tip: When given 2 set-ups or more in the question, spot the difference between the set-ups and you will have a better idea on the Science concept to link to!

Using the picture above, what is unique about sleeve Y, as compared to sleeve X? We can see that sleeve Y has a corrugated (jagged) surface while sleeve X has a smooth surface. By having a corrugated surface for sleeve Y, how is sleeve Y able to conduct heat at a slower rate?

We have to link to the Science concept, which are the factors that affect the rate of heat transfer.

Do you recall what are the factors affecting the rate of heat transfer? Let’s list them below!

🔥 Factors affecting the rate of heat transfer:

🧯 Heat conductivity

🌡 Temperature difference

☕️ (Exposed) surface area

Using the factors above, let’s figure out which factor is responsible for causing sleeve Y, with a jagged surface, to conduct heat from the hot coffee to Sally’s hands at a slower rate.

### Factor 1: Heat Conductivity Of Sleeves X & Y

Does a better or poorer conductor of heat conduct heat from one region to another at a slower rate? I am sure you will agree with me that it should be the poorer conductor of heat.

“Is it true that sleeve Y is a poorer conductor of heat than sleeve X?”

Since we have previously discussed that sleeve Y conducts heat slower from the hot coffee to Sally’s hands, many students are inclined to assume that sleeve Y must be a poorer conductor of heat.

However, for sleeves X and Y to have different heat conductivity, must they be made of the same material or different material? Different material.

Let’s take a look at the question again and check the materials used to make sleeves X and Y:

As seen from the question above, both sleeves X and Y are made of the same material, cardboard.

As such, is it possible for sleeve Y to be a poorer conductor of heat than sleeve X? The answer is no. Both sleeves X and Y should have the same heat conductivity.

### Factor 2: Temperature Difference Between The Hot Coffee & Sally’s Hands When Using Sleeves X & Y

Does a larger or smaller temperature difference between 2 objects cause heat to be conducted between them at a slower rate?

From what you have been taught in school, it should be a smaller temperature difference between 2 objects.

“Is it then true that the temperature difference between the hot coffee in the cup with sleeve Y and Sally’s hands is smaller than that for sleeve X?”

To answer the question above, we need to find out the temperature of both the hot coffee and Sally’s hands. When the hot coffee is made in a cafe, it is immediately transferred to the cup.

“Will the temperature of the hot coffee be the same or different when it is immediately poured into the cups with sleeves X and Y respectively?”

The temperature of the hot coffee would be the same. Similarly, regardless of whether sleeve X or sleeve Y is used, I am sure you will agree with me that the temperature of Sally’s hands would also be the same, which is at her body temperature of about 37 oC.

“As a result, will the temperature difference between the cups of hot coffee and Sally’s hands be the same or different?”

The temperature difference between the cups of hot coffee and Sally’s hands would be the same for both sleeves X and Y.

As a result, does the factor, temperature difference, explain why sleeve Y conducts heat from the hot coffee to Sally’s hands slower? The answer is no.

### Factor 3: Surface Area Of Sleeves X & Y In Contact With Sally’s Hands

Does a larger or smaller surface area in contact between 2 objects allow heat to be transferred between them at a slower rate? It should be a smaller surface area.

Referring to the above diagram and recalling our previous discussion, sleeve Y has a corrugated surface while sleeve X has a smooth surface. Does the corrugated surface of sleeve Y result in a smaller surface area of sleeve Y in contact with Sally’s hands?

To answer the question above, it will be easier to visualise by drawing a diagram of Sally’s hand touching cardboard sleeves X and Y respectively, where the sections highlighted in red represent the surface area of Sally’s hand in contact with cardboard sleeves X and Y (refer to the diagram below).

“Based on the diagram above, does cardboard sleeve Y have a larger or smaller surface area in contact with Sally’s hands?”

We can see that there is a smaller surface area of sleeve Y in contact with Sally’s hands, which means that the correct factor to explain why sleeve Y is able to conduct heat at a slower rate is surface area.

⭐️ Tip: Using the first letter of each factor, you can easily memorise the 3 factors affecting the rate of heat transfer below as “THE”!

🌡 Temperature difference

🧯 Heat conductivity

☕️ (Exposed) surface area

Now that we understand why Sally can hold onto the cup of hot coffee with sleeve Y longer than with sleeve X, we are ready to write our answer!

Since we have to make use of the diagram (data) to explain why sleeve Y allows Sally to hold onto the cup of hot coffee for a longer time, we have to use the “CUE” template structure to write our answer:

C: Choose

U: Use Data

E: Explain Data

However, as the question already “chose” sleeve Y as the one that allows Sally to hold onto the cup of hot coffee for a longer time, we do not have to apply “C” anymore and only have to apply “U” and “E” in our answer.

Try applying the “U” and “E” to your answer and see if you can get the same answer as me!

Use data: There is a smaller surface area of cardboard sleeve Y in contact with Sally’s hands.

Explain data: Thus, cardboard sleeve Y will conduct heat from the hot coffee to Sally’s hands slower, allowing Sally to hold onto the cup of hot coffee with cardboard sleeve Y for a longer time.