If you will brush as recommended by your dentists, you should do it for two whole minutes and twice each day. However, an average person would rarely do this properly, therefore leaving behind plaque that will definitely cause tooth decay.
The big difference between a manual toothbrush and an electric toothbrush is the number of strokes that they can deliver. Manual brushing only yields 300 strokes per minute while electronic brushing will deliver 50,000 strokes per minute! Also, most of these electric toothbrushes have a timer to let you know if you have brushed the recommended amount of time. Modern ones will even have a sensor to let you know that you are brushing too hard.
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