Question

Hiking Imagine that on a cold day you are planning to take a thermos of hot soup with you on a hike. Explain why you might fill the thermos with hot water first before filling it with the hot soup.

Short answer

Filling the thermos with hot water first helps to preheat the container so that the temperature difference between the hot soup and the thermos walls decreases. This reduced temperature difference slows down the rate of heat transfer, helping to maintain the soup's warmth for a longer period during the hike.

Step-by-step solution

Understanding heat transfer and thermal equilibrium

Heat transfer is the process by which energy in the form of heat moves from a warmer object to a cooler one. This process continues until the two objects reach a state called thermal equilibrium, where their temperatures are the same, and no more energy is transferred.

Understanding the purpose of a thermos

A thermos is an insulated container that slows down the rate of heat transfer between the contents of the container and the environment. By keeping hot substances hot and cold substances cold, it helps maintain the temperature of the contents for a longer period of time.

Effects of filling the thermos with hot water first

When we fill the thermos with hot water first, the hot water transfers its heat energy to the walls of the thermos. This in turn increases the temperature of the walls of the thermos before we pour out the hot water. As a result, the walls of the thermos are now closer to the temperature of the hot soup that we are going to pour in.

Benefits of preheating the thermos with hot water

By preheating the thermos with hot water, we reduce the temperature difference between the walls of the thermos and the hot soup. As a consequence, the rate of heat transfer between the soup and the thermos walls decreases. This helps to slow down the cooling of the hot soup, ensuring that it stays warm for a longer duration during the hike. In conclusion, filling the thermos with hot water first works as a preheating process that helps to maintain the temperature of the hot soup for a longer period by reducing the rate of heat transfer between the soup and the thermos walls. This ensures that you can enjoy hot soup even on a cold day during your hike.

Key concepts

Thermal Equilibrium

When you place a mug of steaming coffee on your desk, you'll notice that, after a while, it cools down to room temperature. This process is a classic example of heat transfer, which is the movement of thermal energy from a region of higher temperature to a region of lower temperature. This exchange continues until the temperatures of both regions are equal, leading to what scientists call thermal equilibrium. At this point, there's no net flow of heat energy since the objects involved have the same temperature.

Imagine you're holding a snowball with bare hands. Initially, the snowball begins to melt because heat from your warmer hand transfers to the colder snowball. Gradually, your hand feels colder, and the snowball becomes less solid. Eventually, a point is reached when your hand and the remains of the snowball feel similarly cold – they have reached thermal equilibrium. Understanding equilibrium is crucial for several practical applications, including food storage, climate control in buildings, and yes, even packing a thermos with soup for a hike.

Thermos Insulation

Now, consider that thermos you're planning to take on your hike. The thermos is specially designed with insulation to slow down the process of heat transfer. But how does this work? A thermos typically has a double-walled container, where the space between the two walls is a vacuum. This vacuum acts as an excellent insulator because there are very few air molecules to facilitate heat transfer through conduction or convection. Moreover, the inner surfaces of the walls often have a reflective coating that reduces heat loss by radiation.

Think of insulation like wrapping yourself in a blanket on a chilly day. The blanket-trapped air reduces the heat transfer from your body to the cold air in the room, keeping you warm. Similarly, the thermos insulation keeps the soup's heat from escaping, ensuring that the soup remains hot for a much longer period compared to if it were in a regular, non-insulated container.

Preheating Process

Why pour hot water into the thermos before adding the soup, you might wonder? This strategy involves a preheating process. By filling the thermos with hot water, you're effectively warming up the container's walls. This serves to precondition the container to a temperature closer to that of the hot soup that you'll later introduce. Doing so minimizes the temperature difference between the soup and the flask, thus reducing the rate at which heat will transfer from the soup to the thermos walls.

It's similar to warming your dinner plate in the oven before serving hot food; the food stays warmer longer on a heated plate than it would on a cold plate. By preheating the thermos, you ensure the walls don't immediately absorb the heat from your hot soup, keeping it pleasantly warm throughout your adventures on a cold hiking day. This simple but effective step is a game-changer for keeping your soup or any hot beverage at the desired temperature for longer periods.