Physics 8 Lesson 26: Heat capacity of fuel
1. Theoretical summary
1.1. Fuel
Some common fuels: Coal, firewood, oil, gas are fuels
Eg: Coal, firewood, oil, gasoline, alcohol, gas…
1.2. Heat capacity of fuel
VD: The calorific value of kerosene is \(44.10^6(J/kg)\)
1.3. The formula for calculating the amount of heat released by the burned fuel
– The heat released when completely burning m kg of fuel is calculated by the formula: \(Q=mq\)
– In there:

Q: heat released – unit J

q: heat capacity of fuel – unit J/Kg

m: mass of fuel completely burned – unit Kg
2. Illustrated exercise
2.1. Form 1: Find the amount of heat released when burning objects
Calculate the heat released when completely burning 15kg of firewood and 15kg of coal. How many kilograms of kerosene must be burned to obtain the above heat?
Solution guide:
The heat released when completely burning 15kg of firewood is:
\(Q_1=q_1.m_1=10^7.15=15.10^7J\)
The heat released when completely burning 15kg of coal is:
\(Q_2=q_2.m_2=27.10^6.15=4.05.10^8J\)
The amount of kerosene needed to burn the heat \(Q_1\) is:
\(m=\frac{Q_1}{q}=\frac{15.10^7}{44.10^6}=3.41kg\)
The amount of kerosene needed to burn the heat \(Q_2\) is:
\(m=\frac{Q_2}{q}=\frac{4,05.10^8}{44.10^6}=9.2kg\)
So,
The amount of kerosene required to burn the heat \(Q_1\) is: \(m=3.41kg\)
The amount of kerosene needed to burn \(Q_2\) is: \(m= 9.2kg\)
2.2. Form 2: Find the heat capacity of the gas
Can you tell me how many times the heat capacity of gas is equal to the heat capacity of peat?
Solution guide:
The heat capacity of the gas is \(44.10^6(J/kg)\)
The heat capacity of peat is \(14.10^6(J/kg)\)
⇒ The heat capacity of gas is 3.14 times of peat.
3. Practice
3.1. Essay exercises
Question 1: Why is it better to use a charcoal stove than a wood stove?
Verse 2: Calculate the heat released when completely burning 15 kg of firewood, 15 kg of coal. How many kilograms of kerosene are needed to obtain the above heat? Knowing the heat capacity of firewood, coal and kerosene are 10,106 J/kg, 27,106 J/kg, and 44,106 J/kg, respectively.
Question 3: A motorcycle traveling at 54 km/h must produce a power of P = 5 kW. The efficiency of the machine is 60%. How many kilometers can 1 liter of gasoline travel? Knowing the density and heat capacity of gasoline is D = 700 kg/m^{3}, q = 46.106 J/kg.K.
Question 4: On a distance of 90 km, a car moving with a speed of 54 km/h consumes 5 liters of gasoline. What is the efficiency of a car engine? Given that the engine power is 12 kW and the heat output, the density of gasoline is 46.10^{6} J/kg and 700 kg/m^{3}.
3.2. Multiple choice exercises
Question 1: Using a wood stove to boil water, after a while the pot and water heat up. Which object has the capacity to give off heat?
A. Water is heated
B. The pot is heated
C. Firewood is burned
Verse 2: When it is said that the heat capacity of coal is 27.10^{6} J/kg, that means:
A. When burning 1 kg of coal, the heat is 27.10^{6} J.
B. When burning 1 g of coal, the heat is 27.10^{6} J.
C. When completely burned 1 kg of coal gives off a heat of 27.10^{6} J.
D. When completely burned 1 g of coal gives off a heat of 27.10^{6} J.
Question 3: Which of the following statements using the phrase “heat output” is correct?
A. Heat engine capacity.
B. Heat capacity of the power source.
C. Heat capacity of the fuel.
D. The heat capacity of an object.
Question 4: Knowing the heat capacity of coal is q = 27.10^{6} J/kg. The heat released when completely burning 12 kg of coal is:
A. 324 kJ B. 32.4.10^{6} J C. 324.10^{6} J D. 3.24.10_{5} J
4. Conclusion
Through the lecture on the calorific value of this fuel, students need to complete some of the objectives given by the lesson, such as:

The statement defines the calorific value of a fuel.

Write a formula to calculate the amount of heat released by the fuel being burned.

State the names and units of the quantities in the formula.
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