Resistance and Conductance To change the image on this slide, select the picture and delete it. Then click the Pictures icon in the placeholder to insert your own image. Ch. 5, pg 153-154

What is resistance in an electrical circuit? In an electrical circuit, each part has a specific job, or function The function of resistors is to transform electrical energy into another form of energy. In an electrical circuit, resistors are described as a force that hinders the flow of the circuit. The higher the resistance of the material, the more energy it takes for the current to flow.

Electrical Resistance Electrical Resistance: the ability of a material to hinder the flow of electrical current Resistors are also known as insulators. The opposite of a resistor or insulator is a conductor. A good conductor is a poor resistor and a poor conductor is a good resistor Resistors/Insulators Conductors Materials which slow down or stop the flow of electrons Materials which allow the flow of electrons Examples: plastic, glass, ceramic, tungsten, wood and distilled water Examples: metals, acids, salt water and bases

Factors effecting resistance and conductance Description Nature Poor conductors = good resistors, good resistors = poor conductors Length Longer = greater resistance Diameter Smaller = greater resistance Temperature Warmer = greater resistance Describe what properties a wire would have to be a good conductor Describe what properties a plastic would have to be a good resistor

Example: There are six electrical wires made of the same substance and having the same length : three have a diameter of 1.5 mm while the other three have a diameter of 3.0 mm. They are placed either end to end to increase the length of the wire or parallel to one another to increase the surface area of the wire. Which three wire arrangement offers the least resistance to the flow of electric current? A) B) C) D)

Measuring Resistance Resistance is measured in ohms, which is symbolized by the Greek letter omega: Ω One ohms is equal to: Ω = V A V= volts (potential difference) A = amps (current intensity)

Ohms Law Defn: For a given resistance, the potential difference in an electrical circuit is directly proportional to the current intensity V = RI V = potential difference (in volts) R = resistance (in Ω) I = the current intensity (in amps) * Ohms law applies only to conductors (not insulators)

Sample Questions Finding the resistance when the resistance graph is given 1- The following graph illustrates the change in potential difference (voltage), V, as a function of the current intensity, I, in a circuit. What is the resistance of this circuit? A) 0.1 Ω B) 3.0 Ω C) 10.0 Ω D) 15.0 Ω

2- The following graph illustrates the change in the current intensity, I, in a circuit element as a function of the potential difference (voltage), V, across its terminals: What is the resistance of this circuit element? A) 0.20 Ω B) 5.0 Ω C) 7.2 Ω D) 28.8 Ω

Finding resistance when conductance graph is given 1- The following graph illustrates the change in the current intensity, I, in a circuit as a function of the potential difference (voltage), V, across its terminals What is the resistance of this circuit? A) 0.33 Ω B) 0.75 Ω C) 1.33 Ω D) 3.00 Ω

2- The following graph illustrates the change in electric current intensity, I, as a function of potential difference (voltage), V, for a given resistor. According to this graph, what is the resistance of this resistor? A) 0.15 Ω B) 5.4 Ω C) 6.7 Ω D) 21.6 Ω

Finding resistance using a provided data table 1- Use the table to make a resistance graph and find the resistance of the resistor Voltage (V) 1 2 3 4 5 Intensity (A) 0.2 0.4 0.6 0.8 1.0

2- Use the table to make a resistance graph and find the resistance of the resistor Voltage (V) 2 3.5 4.3 5.1 6.5 Intensity (A) 0.3 0.8 1.4 1.9 2.2