Two copper wires with different diameter are joined end to end. If a current flow in the wire combination, what happens to electrons when they move from the large diameter wire into the smaller diameter wire? Does their drift speed increase, decrease, or stay the same? If the drift speed change, what is the role the force that causes the change? Explain your reasoning.

The term drift speed is defined as the mean velocity attained by the charged electron in the object or conductor due to electric field.

The changes in drift speed is measured by the relation

$v_d=\frac{l}{n\left|q\right|A}$

Here, $v_d,J,I,n\left|q\right|$and $A$are the changes in drift speed, current density, current intensity, charges concentration, charge of charge carries and cross section area respectively.

From the expression it is clear that the drift velocity and the area have the inverse relationship.

So the drift velocity increases as the area increases.

Hence, the drift velocity inversely proportional to the area so the drift velocity increase.

Consider the electric force expression is:

$F=\frac{\mathbb{q}|PI}{A}$

Here, the electric force inversely proportion to the cross-section area. So the force will increase with the decrease in area.

Hence, the electric force causes the change in the drift speed.