In a circuit, the drift velocity of electrons is indeed constant in a given wire. However, the potential difference (voltage) across components can still change because different components in the circuit offer resistance to the flow of electrons.
When an electric current flows through a circuit, it encounters resistance, which causes a drop in voltage across the components. This drop in voltage is related to the energy loss in the circuit due to the resistance.
According to Ohm's law, the voltage across a component (V) is equal to the current (I) multiplied by the resistance (R): V = I * R. So, as the current flows through a component with resistance, the voltage across that component decreases.
This voltage drop occurs because some of the electrical energy is converted into other forms of energy, such as heat or light, depending on the component. For example, in a resistor, the energy is dissipated as heat due to the resistance of the material.
In circuits with multiple components, such as resistors, capacitors, or diodes, each component offers a different resistance to the flow of current, resulting in different voltage drops across them.
It's important to note that the total energy of the electrons, including their kinetic energy, remains constant throughout the circuit. However, the distribution of that energy can change as the electrons encounter resistance and voltage drops across the components.
In summary, even though the drift velocity of electrons remains constant, changes in potential difference (voltage) across components occur due to the resistance encountered by the current flow, resulting in voltage drops across those components.