Resistance

Resistance The degree to which a substance opposes the flow of electric current through it. All substances resist electron flow to some extent.

Resistance Conductors, such as metals, allow electrons to flow freely through them and have low resistance values. Insulators resist electron flow greatly and have high resistance values.

Resistance Resistance is measured in ohms (Ω) using an ohmmeter. An ohmmeter is a device for measuring resistance. Ohmmeters are connected in parallel

Resistance When a substance resists the flow of electrons, it slows down the current and converts the electrical energy into other forms of energy. The more resistance a substance has, the more energy gained by the substance is radiated to its surroundings as heat and/or light energy

Resistance in a Circuit A resistor is any material that can slow current flow. In a light bulb, the filament’s high resistance to the electron’s electrical energy causes it to heat up and produce light.

Types of Resistors Resistors can be made with a number of techniques and materials The two most common types are wire-wound and carbon-composition.

Factors that Affect the Resistance of a Wire How Factor Affects Resistance Material Silver has the least resistance but very expensive to use in wires. Most Conducting wires are made from copper

Factors that Affect the Resistance of a Wire How Factor Affects Resistance Temperature As the temperature of the wire increases, its resistance increases and its conductivity decreases. In other words, a colder wire is less resistant than a warmer wire.

Factors that Affect the Resistance of a Wire How Factor Affects Resistance Length Longer wires offer more resistance than shorter wires. If the wire doubles in length, it doubles in resistance

Factors that Affect the Resistance of a Wire How Factor Affects Resistance Cross-sectional Area Wider wires offer less resistance than thinner wires. If the wire doubles in width, its resistance is half as great. Conducting wires that carry large currents need large diameters to lessen their resistance. http://youtu.be/QYacs_GygXo

Check Your Learning 1. The label on the bottom of a toaster says it is a 120 V device that draws 12 A at 10 S. Identify the current, potential difference, and resistance.

Check Your Learning The label on the bottom of a toaster says it is a 120 volts device that draws 12 amps at 10 ohms. Identify the current, potential difference, and resistance. current 12 A potential difference 120 V resistance 10 Ω

Check Your Learning 2. You want to buy a small electric space heater. You consider two different heaters. They both produce the same amount of heat but one has a longer thinner cord while the other has a shorter, thicker cord. Which one should you buy? Why?

Check Your Learning 2. You want to buy a small electric space heater. You consider two different heaters. They both produce the same amount of heat but one has a longer thinner cord while the other has a shorter, thicker cord. Which one should you buy? Why? the one with the shorter, thicker cord – there is less resistance in the cord so it is less likely to warm up and potentially catch fire

Check Your Learning 3. What effect would the following have on a conductor’s resistance? (a) decreasing the diameter of a conductor (b) placing an extension cord outside in the winter (c) plugging two identical extension cords together to make it longer (d) changing from copper to silver (silver is a better conductor)

Check Your Learning 3. What effect would the following have on a conductor’s resistance? (a) decreasing the diameter of a conductor (b) placing an extension cord outside in the winter (c) plugging two identical extension cords together to make it longer (d) changing from copper to silver (silver is a better conductor) (a) diameter cross-sectional area resistance (b) temperature resistance (c) Length resistance (d) conductivity resistance

Check Your Learning 4. (a) Draw a circuit diagram that shows a battery connected in series with a switch and three lamps connected in series.

Check Your Learning 4. (b) Show how the appropriate meters should be connected to test: • the 1st lamp for resistance • the 2nd lamp for current • the 3rd lamp for potential difference

Relating Current, Voltage, & Resistance German physicist Georg Ohm discovered a mathematical relationship between potential difference (V), current (I), and resistance (R). Ohm’s law states that, as long as the temperature remains constant, V = IR. However, Ohm’s findings apply only to certain types of materials.

Relating Current, Voltage, & Resistance For these materials, when you plot a graph of the voltage versus the current, you get a straight-line or linear relationship. The slope of the straight line represents the resistance of the material. Constant slope = Constant Resistance These materials are “ohmic”materials because they obey Ohm’s law.

Practice Which of the following graphs shows a load with a greater resistance? Explain your answer.

Practice Which of the following graphs shows a load with a greater resistance? Explain your answer.

Practice Which of the following graphs shows a load with a greater resistance? Explain your answer. (b) since it is steeper (slope of graph = resistance)

Ohm’s Law OHM’S LAW as long as the temperature stays constant V=IR where V is the potential difference (V) I is the current (A) R is the resistance (Ω)

Using Ohm’s Law

Using Ohm’s Law