Voltage, Current, & Resistance Ohm’s Law: Voltage, Current, & Resistance

Voltage The term voltage refers to the difference in electrical potential (∆V) between 2 different locations, or parts of a circuit. Recall that the equation for electrical potential (V) at a distance (r) from an object with charge (q) is *Recall that the constant, k = 9 x 109 Nm2/C2

Voltage We can see from this equation that the further you move from a positive (+) charge (the bigger “r” gets), the lower the electric potential will be. Since positive charges tend to move away from other positives, we can say that positive charges tend to move from locations of high electrical potential to locations of low electrical potential.

Voltage Electrons, and other negatively charged particles do just the opposite; they naturally move from locations of low electrical potential to locations of higher electrical potential.

Voltage Just as balls or other objects tend to move because of differences in height, and heat flows from one location to another because of differences in temperature, electrical current flows through a circuit only when the two ends of the circuit (called terminals) have different electrical potentials. Again, this difference in electrical potential is what we mean when we use the word voltage.

Current When a voltage (potential difference) is set up between the terminals of any electrical circuit, electrons will “flow” through the conducting path (usually metal wire) which connects one terminal to the other. Despite the fact that it is the electrons which move in a conductor, we still define conventional current as the direction of positive charge flow.

Current For this reason, we say that the current flows from the high potential end of the circuit to the low potential end, even though the electrons are actually moving in the opposite direction (from low to high potential).

Current The amount of current is defined as the rate at which charge passes through a conductor; we can calculate it by dividing the amount of charge (q) passing through a circuit by the amount of time (t) it takes to do so.

Resistance Electrical resistance is the characteristic of any part of an electrical circuit to oppose or limit the amount of electrical current (the rate at which charge passes through the circuit). Objects called resistors are often included in electrical circuits for the specific purpose of limiting the amount of current that runs through the circuit.

Resistance The amount of resistance that a particular resistor provides is determined by 3 factors: The length of the resistor The cross-sectional area of the resistor The resistivity of the material

How Length Affects Resistance Think of resistance as a construction zone on a freeway. The longer the construction zone is, the more it will slow down the traffic. Likewise, the longer a resistor is, the more resistance it will provide.

How Area Affects Resistance The cross-sectional area of the resistor is inversely proportional to the amount of resistance. In other words, the wider the resistor is, the less resistance it will provide. In the analogy of the freeway, the more lanes that are open, the faster the traffic will flow.

Resistivity Resistivity has to do with the characteristics of the material of which the material is composed (i.e. ceramic vs. metal). Highly conductive materials (such as metals) will have a low resistivity, will materials (like wood) that are poor conductors, will have a relatively high resistivity.