While digital computers pass a signal through the circuit for every clock cycle, analog computers move the mechanical parts to pass the signals. Here, the law of conservation of energy applies, and to move beyond a set number of parts, we need to supply more energy.

Not only that, while the energy available sets the amount of operations that can be performed, the energy available per unit cycle, or power determines the possible designs of the computer as well as the maximum speed of computation.

Using digital circuits, we can do more operations with less energy as we are using subatomic particles (electrons), and we can also reduce the size of the computer as they can be contained within smaller circuits. So digital circuits are more space and energy efficient.

Additionally, they are also more time efficient because due to the lack of powerful energy sources to move large mechanical devices or monolithic designs which don't break when handling large amounts of energy, we have to split the device into smaller components that are supplied power at different sections, and thus the flow will have to be scheduled, making operations take more time. Although this saves power, it slows the speed well enough that the operations performed are less time efficient as well.

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