Electric Resistance

• Measurement of the value of resistance
• Role played by voltage and current in a simple circuit

An electron travelling through a wire or loads of a circuit encounters resistance. Resistance is the hindrance to the flow of charge. For an electron, the journey from terminal to terminal is not a direct route. Rather, it is a zigzag path which results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance which discourages it. The rate at which charge flows from terminal to terminal is the result of the combined effect of these two quantities.

The flow of charge through wires is often compared to the flow of water through pipes. The resistance to the flow of charge in an electric circuit is analogous to the frictional effects between water and the pipe surfaces as well as the resistance offered by obstacles which are present in its path. It is this resistance which hinders the water flow and reduces both its flow rate and its drift speed. Like the resistance to water flow, the total amount of resistance to charge flow within a wire of an electric circuit is affected by some clearly identifiable variables.

As an equation, this serves as an algebraic recipe for calculating the current if the electric potential difference and the resistance are known. It indicates the two variables which would affect the amount of current in a circuit. The current in a circuit is directly proportional to the electric potential difference impressed across its ends and inversely proportional to the total resistance offered by the external circuit.

The electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material.

Discovered by Georg Ohm in the late 1820s, electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm, symbol Ω. Resistance's reciprocal quantity is electrical conductance measured in Siemens, symbol S.

Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material using the following formula.

• Digital multimeter — 2275.10
• Resistors (100, 220, 470 Ω ) — 4763.18-20-24
• Resistors (1, 4.7, 10 K Ω ) — 4763.26-32-34
• Ammeter — 4762.62
• Stackable plug lead, 50 cm, black colour, plug 4mm — 2522.08
• Stackable plug lead, 50 cm, red colour, plug 4mm — 2522.03
• Board for electricity and electronics systems — 4762.02
• Two cell holder — 4762.28
• Toggle switch, single pole (2 plugs) — 4762.72
• Crocodile clips — 2531.52
• Bridging plug — 4762.20

• What can we say about the current in the circuit if we increase (or decrease) the electric resistance?
• How can we find the value of resistance just using the plot of I vs V?