THE CHARACTERISTICS OF ELECTRICITY  Electrical Resistance SNC1D PHYSICS THE CHARACTERISTICS OF ELECTRICITY  Electrical Resistance

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 ___________________ experienced by the __________________flowing through the adapter. September 18, 2018 1DPHYS - Electrical Resistance

Electrical Resistance When a substance ______________ the flow of electrons, it _______________the current and converts the _________________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 _______________ to its surroundings as _____________and/or ____________ energy. In the case of the toaster, a red glow is produced and thermal energy toasts the bread. September 18, 2018 1DPHYS - Electrical Resistance

Electrical Resistance Electrical ______________ is the degree to which a substance opposes the flow of electric current through it. The SI unit for resistance is the ohm ( ) 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 ( ) September 18, 2018 1DPHYS - Electrical Resistance

Electrical Resistance For example, current in a circuit might pass through the filament in a light bulb. The filament is a ____________, which is any material that can slow current flow. The filament's high resistance to the electron’s electrical energy causes it to _____________and produce ___________. September 18, 2018 1DPHYS - Electrical Resistance

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 September 18, 2018 1DPHYS - Electrical Resistance

What’s the point of a Resistor? The main function of resistors in a circuit is to control the flow of ______________ to other components. Take an LED (light) for example. If too much current flows through an LED it is ______________. So a resistor is used to _________ the current. September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance Measuring Resistance Conductors, such as metals, allow electrons to flow freely through them and have __________ resistance values. Insulators resist electron flow __________ and have high ______________ values. Resistance is measured using an ____________________. As with a voltmeter, an ohmmeter must be connected in parallel with the load. September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance Measuring Resistance 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. September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance Measuring Resistance OHMMETER device used to measure resistance must be connected in parallel with the load circuit symbol is September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance Measuring Resistance 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 September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance Measuring Resistance 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. September 18, 2018 1DPHYS - Electrical Resistance

Factors That Affect Electrical Resistance All materials have some internal electrical resistance. The ____________ the resistance, the ____________ the current, and the ______________ the material becomes when current flows through it. The electrical resistance of a material depends on four main factors: cross-sectional area: Thicker wires have less internal resistance than thinner ones. Electrons flowing through a thicker wire have more room to move freely. length: If you increase the length of a wire, its internal resistance increases. This happens because electrons have to travel through more material. temperature: When a wire gets warmer, the atoms that make up the wire gain energy and vibrate ____________. 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 ____________ with temperature. type of material September 18, 2018 1DPHYS - Electrical Resistance

Factors That Affect Electrical Resistance PRACTICE 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 September 18, 2018 1DPHYS - Electrical Resistance

Factors That Affect Electrical Resistance 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 September 18, 2018 1DPHYS - Electrical Resistance

Factors That Affect Electrical Resistance FACTORS THAT AFFECT RESISTANCE (R) - SUMMARY 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 September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance  Check Your Learning 5. 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 September 18, 2018 1DPHYS - Electrical Resistance

1DPHYS - Electrical Resistance  Check Your Learning 6. (a) Draw a circuit diagram that shows a battery connected in series with a switch and three lamps connected in series. (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 September 18, 2018 1DPHYS - Electrical Resistance