What is the change in internal energy and work to be done in the following case:
1. A gas expands when 849 J of energy is added to it by heat. The expanding gas does 702 J of work on its surroundings. What is the overall change in the internal energy of the gas in Joules?
2. If the work done by the gas is 890.6 J, how much energy in Joules must have been added as heat in order to keep the internal energy at the end of the process equal to the initial internal energy?
According to the law of conservation of energy the total energy in a system is constant. Any work done to introduce energy into the system increases the potential energy of the system and any work done in a process that expels energy out of the system decreases the internal energy in the system.
1. Here we have 849 J of energy added to the gas in the form of heat. This makes the gas expand and it does 702 J of work on its surroundings. The change in the internal energy is the difference 849 - 702 = 147 J
The internal energy of the gas increases by 147 J
2. When the gas does work equal to 890.6 J, the same amount of energy must be added to it, else there will be a fall in the internal energy.
To keep the internal energy at the end of the process the same as the internal energy at the start of the process 890.6 J of energy has to be added.