The electron beam inside an old television picture tube is 0.40 mmmm in diameter and carries a current of 50 μA. This electron beam impinges on the inside of the picture tube screen. What is the current density in the electron beam and how many electrons strike the screen each second? The electrons move with a velocity of 3.0×107m/s. What electric field strength is needed to accelerate electrons from rest to this velocity in a distance of 5.0 mm? Each electron transfers its kinetic energy to the picture tube screen upon impact. What is the power delivered to the screen by the electron beam?

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Microscopic view of current

In physics, the term current is expressed as the rate of flow of electric charges across the area per unit time (I=q/t). It propagates along in the direction of the motion of positive charges. When the current is caused by the motion of electrons, the direction of the current is opposite to the flow.