SNC1D PHYSICS 4/6/2013. THE CHARACTERISTICS OF ELECTRICITY L Electrical Resistance (P ) Electrical Resistance. Electrical Resistance

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1 SNC1D PHYSICS THE CHARACTERISTICS OF ELECTRICITY L Electrical Resistance (P ) Electrical Resistance Have you ever noticed that when you recharge your cellphone, MP3 player, or laptop computer, the adaptor gets warm? The warmth is caused by the resistance experienced by the electric current flowing through the adapter. April 6, DPHYS - Electrical Resistance 1 Electrical 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 (like the nichrome wires in the toaster), the more energy it gains from the electrons that pass through it. The energy gained by the substance is radiated to its surroundings as thermal and/or light energy. In the case of the toaster, a red glow is produced and thermal energy toasts the bread. April 6, DPHYS - Electrical Resistance 2 1

2 Electrical Resistance Electrical resistance is the degree to which a substance opposes the flow of electric current through it. The SI unit for resistance is the ohm (S) and the symbol is R. All substances resist electron flow to some extent. ELECTRIC RESISTANCE(R) degree to which a substance resists the flow of electric current through it SI unit is ohm (S) April 6, DPHYS - Electrical Resistance 3 Electrical Resistance For example, current in a circuit might pass through the filament in a light bulb. The filament is a resistor, which is any material that can slow current flow. The filament's high resistance to the electron s electrical energy causes it to heat up and produce light. April 6, DPHYS - Electrical Resistance 4 Electrical Resistance There are many different types of resistors. For example, lightweight carbon resistors are used in electronics; heavier ceramic resistors are used in larger circuits. Dimmer switches and the volume controls on a stereo are examples of variable resistors resistors that allow you can change their resistance. RESISTOR any material that can slow current flow circuit symbol is April 6, DPHYS - Electrical Resistance 5 2

3 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 is measured using an ohmmeter. As with a voltmeter, an ohmmeter must be connected in parallel with the load. April 6, DPHYS - Electrical Resistance 6 NOTE! The circuit does not need to be closed in order to measure the resistance. An ohmmeter has its own energy source which provides a flow of electrons through the load. April 6, DPHYS - Electrical Resistance 7 OHMMETER device used to measure resistance must be connected in parallel with the load circuit symbol is April 6, DPHYS - Electrical Resistance 8 3

4 PRACTICE 1. A student connected an ohmmeter as shown. Did the student connect the ohmmeter correctly? Explain. yes it is connected in parallel across the load April 6, DPHYS - Electrical Resistance 9 PRACTICE 2. Draw a circuit diagram that shows a two-cell battery in series with a switch and two lamps in parallel. Include an ohmmeter correctly connected to one of the lamps. April 6, DPHYS - Electrical Resistance 10 There are many situations in everyday life where it is necessary to not only measure the resistance in a circuit but the current, and voltage as well. For this task, a multimeter may be used. A multimeter is a device that allows you to measure current, voltage, and resistance by changing its settings. Multimeters are typically small, inexpensive hand held devices. MULTIMETER device that can measure current, voltage, and resistance April 6, DPHYS - Electrical Resistance 11 4

5 All materials have some internal electrical resistance. The greater the resistance, the lower the current, and the warmer the material becomes when current flows through it. The electrical resistance of a material depends on four main factors: cross-sectional area length temperature type of material April 6, DPHYS - Electrical Resistance 12 When you cut through a wire, you can see its cross-section more easily. The diameter of the cross-section gives you a sense of how thick the wire is. Thicker wires have less internal resistance than thinner ones. Electrons flowing through a thicker wire have more room to move freely. This is similar to what happens with water in a pipe. The greater the diameter of a pipe, the greater the water flow will be. April 6, DPHYS - Electrical Resistance 13 As you increase the length of a wire, its internal resistance increases. This happens because electrons have to travel through more material. For example, the longer the extension cord, the more resistance it has, and the warmer it will get while being used. April 6, DPHYS - Electrical Resistance 14 5

6 NOTE! This can be dangerous because the cord can overheat and potentially cause a fire. However, this hazard can be avoided by using a larger-diameter wire which lowers the resistance of the extension cord. April 6, DPHYS - Electrical Resistance 15 PRACTICE 3. An extension cord that you would use for a lamp is much thinner than an extension cord recommended for use with large appliances. If you plugged in a refrigerator using a lamp extension cord, the plastic coating of the cord could melt and perhaps start a fire. Why does this occur? How can this hazard be prevented? the smaller the wire, the more resistance there is, so the wire heats up a heavier or larger diameter extension cord should have been used so it would not heat up as easily April 6, DPHYS - Electrical Resistance 16 When a wire gets warmer, the atoms that make up the wire gain energy and vibrate faster. The increased vibration results in more collisions between the atoms and the free-flowing electrons in the current. Since greater vibrations cause more collisions, resistance increases with temperature. April 6, DPHYS - Electrical Resistance 17 6

7 PRACTICE 4. Explain why your computer needs a cooling fan. so the electronic components do not become too hot and stop working or start a fire April 6, DPHYS - Electrical Resistance 18 The ability of a material to conduct electricity is determined by how freely electrons can move within the material. Copper is used in circuits because it is an excellent conductor. It has a low electrical resistance. Silver is a better conductor because its resistance is even lower. Silver, however, is an expensive metal and is not suitable for low-cost electrical devices. April 6, DPHYS - Electrical Resistance 19 FACTORS THAT AFFECT RESISTANCE(R) cross-sectional area (A) as A R length (L) as L R temperature (T) as T R type of material depends on the material s conductivity April 6, DPHYS - Electrical Resistance 20 7

8 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. current potential difference resistance 12 A 120 V 10 S April 6, DPHYS - Electrical Resistance 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 April 6, DPHYS - Electrical Resistance 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 April 6, DPHYS - Electrical Resistance 23 8

9 4. (a) Draw a circuit diagram that shows a battery connected in series with a switch and three lamps connected in series. April 6, DPHYS - Electrical Resistance (b) Show how the appropriate meters should be connected to test: the 1 st lamp for resistance the 2 nd lamp for current the 3 rd lamp for potential difference April 6, DPHYS - Electrical Resistance Electrons in a circuit can be compared to a group of shoppers who go out to spend money in shops. Use this analogy to explain (i) electric potential, (ii) current, and (iii) resistance in a series circuit. ELECTRIC POTENTIAL is like money: the more money in your pocket, the greater your potential. When you reach the first checkout, you spend some of your money to buy stuff. At the second checkout, you spend your money until your pockets are empty, then you go back to the bank for more. April 6, DPHYS - Electrical Resistance 26 9

10 5. Electrons in a circuit can be compared to a group of shoppers who go out to spend money in shops. Use this analogy to explain (i) electric potential, (ii) current, and (iii) resistance in a series circuit. ELECTRIC CURRENT is like a crowd of people moving through a shopping mall. No one moves unless they are given some money. The more money they get, the quicker they get going! People flow from rich to poor. Everyone who goes into an aisle or a shop comes out again. Checkout counters slow the people down, and take their money. April 6, DPHYS - Electrical Resistance Electrons in a circuit can be compared to a group of shoppers who go out to spend money in shops. Use this analogy to explain (i) electric potential, (ii) current, and (iii) resistance in a series circuit. The checkout RESISTANCE causes people to pile up into lines. If the line gets very long, people are willing to spend more money to get through. April 6, DPHYS - Electrical Resistance 28 10