📚 Calculating Food Energy | 计算食物能量
Food contains stored chemical energy. When we eat food, our bodies release this energy through respiration. In the laboratory, we can estimate the energy content of food by burning it and measuring the heat released. This article explains how to calculate food energy using a simple calorimetry method.
食物含有储存的化学能。当我们吃下食物时，身体通过呼吸作用释放这些能量。在实验室中，我们可以通过燃烧食物并测量释放的热量来估算食物的能量含量。本文将解释如何使用简单的量热法计算食物能量。
1. What is food energy? | 什么是食物能量？
Food energy is the chemical energy stored in the bonds of food molecules such as carbohydrates, fats and proteins. When these molecules are broken down during respiration, the energy is released for life processes.
食物能量是储存在碳水化合物、脂肪和蛋白质等食物分子化学键中的化学能。当这些分子在呼吸作用中被分解时，能量被释放出来供生命活动使用。
Different foods store different amounts of energy. For example, fats store more energy per gram than carbohydrates or proteins.
不同食物储存的能量不同。例如，每克脂肪储存的能量比每克碳水化合物或蛋白质更多。
2. Units of energy: joules and calories | 能量单位：焦耳与卡路里
e food energy values are often large, we usually use kilojoules (kJ), where 1 kJ = 1000 J.
能量的国际单位是焦耳 (J)。由于食物能量数值通常很大，我们通常使用千焦 (kJ)，其中 1 kJ = 1000 J。
You may also see calories (cal) on food labels. 1 calorie is about 4.2 joules. A food Calorie with a capital C is actually 1 kilocalorie (kcal), equal to 4200 J or 4.2 kJ.
你可能还会在食品标签上看到卡路里 (cal)。1 卡路里约等于 4.2 焦耳。食品标签上的大写 C 卡路里实际上是 1 千卡 (kcal)，等于 4200 J 或 4.2 kJ。
- 1 kJ = 1000 J | 1 千焦 = 1000 焦耳
- 1 cal ≈ 4.2 J | 1 卡路里约等于 4.2 焦耳
- 1 kcal = 1000 cal ≈ 4.2 kJ | 1 千卡 = 1000 卡 ≈ 4.2 千焦
3. Why do we calculate food energy? | 我们为什么要计算食物能量？
We calculate food energy to understand how much fuel a food provides. This helps people plan balanced diets and compare different foods.
我们计算食物能量是为了了解某种食物能提供多少燃料。这有助于人们规划均衡饮食并比较不同食物。
In science, calculating energy also helps us test predictions. If a food is high in fat, we expect it to release more heat per gram when burned.
在科学中，计算能量也有助于检验预测。如果一种食物脂肪含量高，我们会预期它燃烧时每克释放更多热量。
4. The burning food experiment | 燃烧食物实验
A simple school experiment burns a small piece of food under a boiling tube of water. The heat from the burning food warms the water. By measuring the temperature rise of the water, we can estimate the energy released.
学校里的简单实验是在装有水的沸腾管下方燃烧一小块食物。燃烧食物产生的热量使水升温。通过测量水温的升高，我们可以估算释放的能量。
The equipment usually includes a mounted needle to hold the food, a boiling tube, a thermometer, a measuring cylinder, and a heat-proof mat. Safety goggles must be worn because burning food can spit or produce smoke.
设备通常包括固定食物的针、沸腾管、温度计、量筒和隔热垫。必须佩戴护目镜，因为燃烧的食物可能会飞溅或产生烟雾。
5. The heat transfer equation | 热传递方程
The heat gained by water is calculated using the equation:
水吸收的热量用以下公式计算：
Q = m × c × ΔT
In this equation, Q is the heat energy in joules (J), m is the mass of water in grams (g), c is the specific heat capacity of water (about 4.2 J/g°C), and ΔT is the temperature rise in degrees Celsius (°C).
在这个公式中，Q 是热量，单位为焦耳 (J)；m 是水的质量，单位为克 (g)；c 是水的比热容，约为 4.2 J/g°C；ΔT 是温度升高的摄氏度数 (°C)。
The specific heat capacity tells us how much energy is needed to raise 1 g of water by 1°C.
比热容告诉我们使 1 克水升高 1°C 需要多少能量。
6. Worked example: calculating energy released | 计算示例：计算释放的能量
rned and heats 20 g of water from 21°C to 36°C. The temperature rise ΔT is 36 − 21 = 15°C.
假设燃烧 0.5 克薯片，使 20 克水从 21°C 升高到 36°C。温度升高值 ΔT = 36 − 21 = 15°C。
Using Q = m × c × ΔT:
使用公式 Q = m × c × ΔT：
Q = 20 g × 4.2 J/g°C × 15°C = 1260 J = 1.26 kJ
This is the energy transferred to the water, not the total energy in the food, because some heat is lost to the air and equipment.
这是传递给水的能量，而不是食物中的总能量，因为一部分热量散失到空气和设备中。
7. Calculating energy per gram | 计算每克能量
To compare foods fairly, we calculate the energy released per gram. Divide the heat gained by the mass of food burned.
为了公平比较食物，我们计算每克食物释放的能量。用水吸收的热量除以燃烧食物的质量。
energy per gram = Q ÷ mass of food | 每克能量 = 热量 ÷ 食物质量
In the crisp example, energy per gram = 1260 J ÷ 0.5 g = 2520 J/g = 2.52 kJ/g.
在薯片示例中，每克能量 = 1260 J ÷ 0.5 克 = 2520 J/g = 2.52 kJ/g。
This value lets us compare different foods even if the sample sizes are different.
即使样品大小不同，这个数值也能让我们比较不同食物。
8. Comparing different foods | 比较不同食物
Typical school results show that high-fat foods such as peanuts and crisps release more energy per gram than high-carbohydrate foods such as bread or sugar.
典型的学校实验结果显示，花生和薯片等高脂肪食物每克释放的能量比面包或糖等高碳水化合物食物更多。
For example, a peanut may release around 25 kJ/g, while bread may release around 10 kJ/g. These values are only estimates because of heat losses.
例如，一颗花生每克大约释放 25 kJ，而面包每克大约释放 10 kJ。由于热量损失，这些数值只是估算值。
| Food | 食物 | Energy per gram (kJ/g) | 每克能量 (千焦/克) |
|---|---|
| Peanuts | 花生 | ~25 |
| Crisps | 薯片 | ~22 |
| Bread | 面包 | ~10 |
| Apple | 苹果 | ~2 |
9. Food labels and daily energy needs | 食品标签与日常能量需求
Food labels usually show energy in kilojoules and kilocalories per 100 g or per serving. The label helps you compare products and track your daily energy intake.
食品标签通常以每 100 克或每份为单位，用千焦和千卡标示能量。标签有助于比较产品并记录每日能量摄入。
Published by TutorHao | KS3 Chemistry Revision Series | aleveler.com
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