What limits current flow, or what opposes the movement of electrons?

Current flow in an electrical circuit is limited or opposed by a property called resistance. Resistance is a measure of how much a material or an object opposes the flow of electric current through it. It's akin to how friction opposes the movement of objects in motion. The unit of resistance is the ohm (Ω).

The resistance in a circuit determines how much current will flow for a given voltage, according to Ohm's Law, which states that the current (I) passing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) of the conductor. Ohm's Law is expressed with the formula I = V/R.

Several factors affect the resistance of a material:

1. Nature of the material: Different materials have different inherent resistivities. Metals, for instance, typically have low resistivity, making them good conductors, while materials like rubber have high resistivity, making them good insulators.

2. Cross-sectional area: A larger cross-sectional area of a conductor will have lower resistance because there are more pathways for the electrons to travel through.

3. Length of the conductor: Longer conductors have more resistance because the electrons have to travel a greater distance through more of the material's resisting atoms.

4. Temperature: In most conductors, as temperature increases, resistance also increases, because the atoms in the material vibrate more at higher temperatures, making it harder for electrons to pass through.

Additionally, other components specifically designed to provide resistance, called resistors, are used in circuits to control the flow of current, divide voltages, or provide heat.