Density Example
If we have 2 liters of gasoline (2000 cubic centimeters) the mass of the gasoline is
2000\ \textrm{cubic centimeter gasoline} \cdot \frac{0.740\ \textrm{gram gasoline}}{1\ \textrm{cubic centimeter gasoline}} = 1.48\ \textrm{kilograms gasoline}
Volumetric Energy Density Example
How many joules contained in 10 gallons of gas?
10 \textrm{gallon} \cdot \frac{114000 BTU}{1 gas} \cdot \frac{1055 J}{1 BTU} \cdot \frac{GJ}{10^9 J} = 1.2 GJ
Gravimetric Energy Density Example
Suppose I need to create 2 MJ of heat energy and I have some coal that has an energy density of 20 MJ per kg.
How much of that coal do I need to burn?
2 MJ \cdot \frac{kg}{20 MJ}
Efficacy Example
How many gallons of gas will it take to make a 1500 mile road trip. Assume you have a car that gets 35 miles to the gallon.
1500 mi \cdot \frac{gal}{35 mi} = 43 gal
Efficiency Example
When you are doing an efficiency question, ask yourself
- which energy is greater, the input or the output?
- if you divide or multiply by the efficiency will the input or output be greater?
- which side of the equals sign is the energy input and output on?
Efficiency Example
I have a coal plant that can convert 37% of the chemical energy in coal to electrical energy. I want to deliver 50 MWh of electrical energy.
- How many MWh of coal must I burn?
- How many kg of coal is that?