Question

Hi, I have multiple questions on Physics, as I am struggling at physics. These are my questions.
1. A hot block of iron does 50 kJ of work on a cold floor. The block of iron also transfers 20 kJ of heat energy to the air. Calculate the change in energy (in kJ) of the iron block.

2. A chef vigorously stirs a pot of cold water and does 150 J of work on the water. The water also gains 75 J of thermal energy from the surroundings. Calculate the change in energy of the water.

And can you guys please tell me how do you calculate it and what theory applies to me and all the other basics.
Thanks

Answer 1

Answer on Question #72257, Physics / Molecular Physics | Thermodynamics

1. A hot block of iron does 50 kJ of work on a cold floor. The block of iron also transfers 20 kJ of heat energy to the air. Calculate the change in energy (in kJ) of the iron block.

Solution:

The q and w are negative when the system loses heat to the surroundings.

$$\begin{array}{l} \Delta E = w + q \\ w = - 50 \text{ kJ} \\ q = - 20 \text{ kJ} \\ \end{array}$$ $$\Delta E = (- 20 \text{ kJ}) + (- 50 \text{ kJ}) = - 70 \text{ kJ}$$

Answer: - 70 kJ

2. A chef vigorously stirs a pot of cold water and does 150 J of work on the water. The water also gains 75 J of thermal energy from the surroundings. Calculate the change in energy of the water.

Solution:

The q and w are positive when the system receives heat to the surroundings.

$$\begin{array}{l} \Delta E = w + q \\ \Delta E = 150 \text{ J} + 75 \text{ J} = 150 \text{ J} + 75 \text{ J} = 225 \text{ J} \\ \end{array}$$

Answer: 225 J

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