11.4 Measuring Electrical Resistance (Pages 462-467)

Learning Goals I can solve problems involving Ohm’s Law. I can explain how factors such as material, length, thickness and temperature affect the resistance of wires. I can explain how superconductors and non-ohmic conductors work.

11.4 Measuring Electrical Resistance KEY CONCEPTS: Ohm’s Law states that, for most conductors, the ratio of potential difference (V) to current (I) is a constant called the resistance (R). A resistor is an electrical component with a specific resistance. The unit of electrical resistance is the ohm (Ω). One ohm is equivalent to one volt per ampere (V/A).

11.4 Measuring Electrical Resistance ( KEY CONCEPTS: Four factors affect the resistance of a wire: the type of material; the length (a longer wire has greater resistance); the diameter (a wire with a larger diameter has lower resistance); the temperature (a hotter wire has greater resistance). A super conductor is a material through which electric charge can flow with no resistance. A non-ohmic conductor does not obey Ohm’s law

VIDEO: Electrical Resistance - Georg Ohm Georg Ohm, a German physicist, measured the current and potential difference in circuits that contained metal wires. Ohm found that the ratio of potential difference to current is a constant called resistance. VIDEO: Electrical Resistance - Georg Ohm

Ohm’s Law Ohm’s law: the ratio of potential difference to current is a constant called resistance

The Units of Resistance Ohm (Ω): the unit for resistance, equivalent to one volt per ampere (V/A)

Factors That Affect the Resistance of Wires (Page 465) Factors that affect the resistance of a wire are: type of material (different metals have different resistances) length of wire (resistance increases proportionally with length) diameter or gauge of wire (thicker wire has less resistance) temperature (resistance increases with temperature) Higher gauge equals thinner wire, so 12 is thicker than 16. Copyright © 2010 McGraw-Hill Ryerson Ltd.

Superconductors (Page 466) A superconductor is a material through which electrical charges can flow with little or no resistance. Many metals lose their electrical resistance when they are cooled to very low temperatures. The superconducting magnet in the Large Hadron Collider at CERN uses a current of 21 000A but has a potential difference of only 9 volts. Because of its lack of resistance, a superconducting wire is very efficient at transporting electricity. If superconductors could be made that functioned at room temperature, enormous amounts of electrical energy could be conserved. Copyright © 2010 McGraw-Hill Ryerson Ltd.

Non-ohmic Conductors (Page 466) Conductors that do not follow Ohm’s Law are said to be non-ohmic. Changes in current and potential difference cause changes in resistance. Light bulb filaments behave in a non-ohmic manner. Their resistance increases as temperature increases, as shown in the graph below. Copyright © 2010 McGraw-Hill Ryerson Ltd.

Sample Problem: Determining Resistance

Sample Problem: Determining Resistance - Solution

Sample Problem: How Many Cells?

Sample Problem: How Many Cells? - Solution

VIDEO: Superconductors VIDEO: Levitating Superconductor on a Möbius strip

Practice Page 464 # 1-6 Page 467 # 1-8