Ohm’s Law and Resistance. Resistivity. Today’s agenda: Electric Current. You must know the definition of current, and be able to use it in solving problems. Current Density. You must understand the difference between current and current density, and be able to use current density in solving problems. Ohm’s Law and Resistance. You must be able to use Ohm’s Law and electrical resistance in solving circuit problems. Resistivity. You must understand the relationship between resistance and resistivity, and be able to use calculate resistivity and associated quantities. Temperature Dependence of Resistivity. You must be able to use the temperature coefficient of resistivity to solve problems involving changing temperatures.

Resistance The resistance of a material is a measure of how easily a charge flows through it. Resistance: how much “push” is needed to get a given current to flow. The unit of resistance is the ohm: Resistances of kilohms and megohms are common:

Every circuit component has resistance. This is the symbol we use for a “resistor:” All wires have resistance. Obviously, for efficiency in carrying a current, we want a wire having a low resistance. In idealized problems, we will consider wire resistance to be zero. Lamps, batteries, and other devices in circuits have resistance.

Resistors are often intentionally used in circuits Resistors are often intentionally used in circuits. The picture shows a strip of five resistors (you tear off the paper and solder the resistors into circuits). The little bands of color on the resistors have meaning. Here are a couple of handy web links: 1. http://www.dannyg.com/examples/res2/resistor.htm 2. http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-resistor-color-code-4-band

Newton’s Laws demand; Ohm’s Law suggests. In some materials, the resistance is constant over a wide range of voltages. For such materials, we write and call the equation “Ohm’s Law.” In fact, Ohm’s Law is not a “Law” in the same sense as Newton’s Laws… … and in advanced Physics classes you will write something other than V=IR when you write Ohm’s Law. Newton’s Laws demand; Ohm’s Law suggests.

I V slope=1/R Materials that follow Ohm’s Law are called “ohmic” materials, and have linear I vs. V graphs. I V Materials that do not follow Ohm’s Law are called “nonohmic” materials, and have curved I vs. V graphs.

I V slope=1/R Materials that follow Ohm’s Law are called “ohmic” materials, and have linear I vs. V graphs. I V Materials that do not follow Ohm’s Law are called “nonohmic” materials, and have curved I vs. V graphs.