Chapter 11: Current Electricity. Current Electricity To operate electrical devices, you need a steady flow of electrons. Unlike static electricity, a

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<ul><li> Slide 1 </li> <li> Chapter 11: Current Electricity </li> <li> Slide 2 </li> <li> Current Electricity To operate electrical devices, you need a steady flow of electrons. Unlike static electricity, a flow of electrons move continuously as long as 2 conditions are met. 1.The flow of electrons requires an energy source 2.The electrons will not flow unless they have a complete path to flow through (electrical circuit) Current electricity is the continuous flow of electrons in a circuit. </li> <li> Slide 3 </li> <li> Electric Circuits An electric circuit must include: 1.An energy source 2.A conducting wire 3.A load which is a device that converts electrical energy to another form of energy; ex. Light bulb Many electric circuits also include a switch that turns the circuit on or off by closing or opening the circuit. </li> <li> Slide 4 </li> <li> Electric Current Electric Current is the measure of the rate of electron flow past a given point in a circuit; measured in amperes (A). </li> <li> Slide 5 </li> <li> Examples of Loads Converts electricity to light bulblight or heat energy fanmechanical energy doorbellsound energy </li> <li> Slide 6 </li> <li> Electric Current, cont Think of it in terms of a waterfall. If you could count the number of water molecules that drop over the edge every second, you could get the rate at which water is flowing past a particular point. Now think of the water molecules as electrons in a wire running past a particular point in a specific amount of time and you have the concept of current. </li> <li> Slide 7 </li> <li> Measuring Current Current in a circuit is measured using an ammeter. The unit of electric current is the ampere (A). An ampere is a measure of the amount of charge moving past a point in the circuit every second. </li> <li> Slide 8 </li> <li> Measuring Current, cont One ampere of charge in a circuit means that 6.2 x 10 18 electrons pass a particular point every second. </li> <li> Slide 9 </li> <li> Measuring Current, cont When you connect an ammeter or voltmeter to a circuit, they must be on the right terminals. There are two terminals on a meter that you use to connect to a circuit. The negative () terminal is often black, The positive (+) terminal is often red. </li> <li> Slide 10 </li> <li> Measuring Current, cont Always connect the positive terminal of the meter to the positive terminal of the electrical source. Connect the negative terminal of the meter to the negative terminal of the electrical source. </li> <li> Slide 11 </li> <li> Calculating Current I Q t </li> <li> Slide 12 </li> <li> QuantitySymbol Units of measurement ChargeQC (coulomb) CurrentI A (amperes) amps Timets (seconds) Calculating Current, cont </li> <li> Slide 13 </li> <li> I Q t Rearranging formulas - The magic triangle Therefore, 1 Ampere = 1 Coulomb / s Calculating Current, cont </li> <li> Slide 14 </li> <li> Sample Question 1 If 310 C of charge passes a point in a conductor in 10 minutes, what is the current through that point in the conductor? Use GRASP and sig. digits. </li> <li> Slide 15 </li> <li> Sample Question 2 A 6.5 amp vacuum cleaner is used for 34 minutes, how much charge would pass through the vacuum during this time? Use GRASP and sig. digits. </li> <li> Slide 16 </li> <li> Sample Question 3 A 5.3 amp drill has 20 coulombs of charge pass through it, how long was the drill used for? Use GRASP and sig. digits. </li> <li> Slide 17 </li> <li> Potential Difference Example: An apple hanging from a low. The apple has potential energy because of its position above the ground. If the apple falls down, it will convert its stored energy, or potential energy, into motion. Suppose an apple at a higher branch. It would have even more potential energy to convert. Each electron has electric potential energy. Potential energy is the energy stored in an object. </li> <li> Slide 18 </li> <li> Potential Difference, cont A battery has chemical potential energy in the electrolyte in its electrochemical cells. The chemicals in the electrolyte react with the electrodes. This causes a difference in the amount of electrons between the two terminals. </li> <li> Slide 19 </li> <li> Potential Difference, cont One terminal in a battery has mainly negative charges (electrons). The other terminal has mainly positive charges. The negative charges are electrons, which can move, are attracted to the positive charges at the positive terminal. </li> <li> Slide 20 </li> <li> Potential Difference, cont If a conductor, such as a copper wire, is connected to both terminals, then the electrons flow from the negative terminal to the positive terminal. The difference in electric potential energy between two points in a circuit is called the potential difference or voltage (V). </li> <li> Slide 21 </li> <li> Potential Difference, cont This difference causes current to flow in a closed circuit. The higher the potential difference in a circuit, the greater the potential energy of each electron. </li> <li> Slide 22 </li> <li> Measuring Potential Difference The potential difference between two locations in a circuit is measured with a voltmeter. </li> <li> Slide 23 </li> <li> Measuring Potential Difference, cont Example: Connecting wires from a negative terminal on a battery across a voltmeter and then back to the positive terminal. The voltmeter would then display the potential difference of the battery. The SI unit for measuring potential difference is the volt (V). </li> <li> Slide 24 </li> <li> Measuring Potential Difference, cont A voltmeter must be connected in parallel with a load in the circuit in order to compare the potential before and after the load. </li> <li> Slide 25 </li> <li> Electrochemical Cells Batteries A battery is a combination of electrochemical cells. Each electrochemical cell is a package of chemicals that converts chemical energy into electrical energy that is stored in charged particles. A simple electrochemical cell includes an electrolyte and two electrodes. </li> <li> Slide 26 </li> <li> Electrochemical Cells Batteries; cont An electrolyte is a liquid or paste that conducts electricity because it contains chemicals that form ions. An ion is an atom or a group of atoms that has become electrically charged by losing or gaining electrons. Sulphuric acid is an example of an electrolyte. </li> <li> Slide 27 </li> <li> Electrochemical Cells Batteries; cont Electrodes are metal strips that react with the electrolyte. Two different electrodes, such as zinc and copper, are used in a battery. The electrodes and electrolyte react causing one electrode to collect and the other to lose electrons. </li> <li> Slide 28 </li> <li> Wet Cells and Dry Cells An electrochemical cell that has a liquid electrolyte is called a wet cell. Wet cells are often used as an energy source for cars and other motorized vehicles. </li> <li> Slide 29 </li> <li> Wet Cells and Dry Cells, cont An electrochemical cell that uses a paste instead of a liquid electrolyte is called a dry cell. Dry cells are used in flashlights, hand-held video game devices, cameras, and watches. Each electrode in a dry cell or battery can also be called a terminal. Terminals are the end points in a cell or battery where we make a connection </li> <li> Slide 30 </li> <li> Fuel Cells A fuel cell is an electrochemical cell that generates electricity directly from a chemical reaction with a fuel, such as hydrogen. Fuel cells are used in electric vehicles. The cell is not used up like an ordinary cell would be because as the electricity is produced, more fuel is added. </li> <li> Slide 31 </li> <li> Homework Read 434-438 Complete #1-4, page 436 and #1-5, page 438 </li> <li> Slide 32 </li> <li> Recall. Electric current can be thought of as the number of electric charges (electrons) passing a point in the circuit per second. Can also be thought of as a speed of electron flow. </li> <li> Slide 33 </li> <li> Electric Potential (Voltage) Voltage can be thought of as the energy that each electron in the circuit contains. Electric potential refers to the amount of energy that electrons possess in a circuit. </li> <li> Slide 34 </li> <li> Electric Potential, cont A load (ex. light bulb) converts electrical energy into another form of energy (ex. light energy and heat energy). You can compare this to the water flowing past a water wheel. The wheel converts some of the energy of the water into motion. The water has more energy before the wheel than after the wheel. Therefore, there is higher voltage before the load than after the load. </li> <li> Slide 35 </li> <li> Calculating Electric Potential (Voltage) The electrical potential energy for each coulomb of charge in a circuit is called the electric potential (V) aka voltage. V E Q </li> <li> Slide 36 </li> <li> Calculating Potential Difference (Voltage), con`t QuantitySymbol Units of measurement ChargeQC (coulomb) EnergyEJ (Joules) VoltageVV (Volts) </li> <li> Slide 37 </li> <li> Where E is the energy required to increase the electric potential (V) of a charge, Q. Potential difference is often called voltage. Calculating Potential Difference (Voltage), con`t and the formula can be rearranged Energy Charge V E Q </li> <li> Slide 38 </li> <li> Calculating Potential Difference (Voltage), con`t Therefore: One volt (V) is the electric potential difference between two points if one joule of work (J) is required to move one coulomb (C) of charge between the points. Volts = Joules Coulombs </li> <li> Slide 39 </li> <li> Sample Question 1: What is the potential difference across an air conditioner if 72 C of charge transfers 8.5 x 10 3 J of energy to the fan and compressor? Use GRASP and sig. digits. </li> <li> Slide 40 </li> <li> Sample Question 2: A static electric shock delivered to a student from a friend transfers 1.5 x 10 1 J of electric energy through a potential difference of 500 V. What is the quantity charge transferred in the spark? Use GRASP and sig. digits. </li> <li> Slide 41 </li> <li> Sample Question 3 How many joules of energy are produced if a gas powered generator produces 120 V with a charge of 60 C at the negative terminal? Use GRASP and sig. digits. </li> <li> Slide 42 </li> <li> Resistance The degree to which a substance opposes the flow of electric current through it. All substances resist electron flow to some extent. </li> <li> Slide 43 </li> <li> Resistance, cont 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. </li> <li> Slide 44 </li> <li> Resistance, cont Resistance is measured in ohms () using an ohmmeter. An ohmmeter is a device for measuring resistance. </li> <li> Slide 45 </li> <li> Resistance, cont 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. </li> <li> Slide 46 </li> <li> Resistance in a Circuit A resistor is any material that can slow current flow. In a light bulb, the filaments high resistance to the electrons electrical energy causes it to heat up and produce light. </li> <li> Slide 47 </li> <li> Resistors and Potential Difference Resistors can be used to control electric current or electric potential in a circuit. In a circuit, electrons have a higher potential difference as they enter a resistor compared to when they leave the resistor because they use up some energy in passing through the resistor. </li> <li> Slide 48 </li> <li> Example: Imagine electrons entering a resistor as being at the high end of a ramp, where they have a lot of potential energy. In this analogy, electrons leaving the resistor are at the bottom end of the ramp, where their potential energy has been converted to another form of energy. </li> <li> Slide 49 </li> <li> Wire-wound Resistors Resistors can be made with a number of techniques and materials. One common type is the wire-wound resistor. A wire-wound resistor has a wire made of heat-resistant metal wrapped around an insulating core. The longer and thinner the wire, the higher the resistance. They are available with values from 0.1 up to 200 k . </li> <li> Slide 50 </li> <li> Resistance in a Wire comparing water to electricity Longer thinner pipes have more resistance to the flow of water than pipes with a larger diameter. The same idea applies to electricity. The more resistance that you have in a circuit, the more it will decrease current at a given potential difference. Larger, shorter wires provide less resistance for electrons to travel. Temperature and material can also affect resistance. </li> <li> Slide 51 </li> <li> Ohm`s Law Ohms law established the relationship between potential difference (V) and current (I). Ohm`s law refers to the amount of resistance in a circuit. The symbol for resistance is called the ohm () in honour of Georg Ohms work in this field. </li> <li> Slide 52 </li> <li> Ohm`s Law According to Ohm`s law, the amount of voltage (or energy) in a circuit is equal to the current multiplied by the resistance. Ohms law states that, as long as temperature stays the same, V = IR if the resistance of a conductor remains constant, then the current is directly proportional to the voltage. </li> <li> Slide 53 </li> <li> Ohm`s Law and Temperature Ohms law works for most circuits. However, temperature affects resistance. Generally, resistance is lower when a conductor is cooler. As the temperature increases, resistance increases. For example, a filament in an incandescent light bulb often has 10 times more resistance when the bulb is warm. </li> <li> Slide 54 </li> <li> Calculating Resistance Resistance = voltage current V I R and the formula can be rearranged Voltage Current R = V I I = V R V = I x R </li> <li> Slide 55 </li> <li> Calculating Resistance, con`t QuantitySymbol Units of measurement ResistanceR (ohm) CurrentIA (Amphere) VoltageVV (Volts) </li> <li> Slide 56 </li> <li> Sample Question 1 A current of 4.0 A flows through a 40 resistor in a circuit. What is the voltage? Use GRASP and sig. digits. </li> <li> Slide 57 </li> <li> Sample Question 2 A 30 V battery generates a current through a 15 resistor. How much current does the battery generate? Use GRASP and sig. digits. </li> <li> Slide 58 </li> <li> Sample Question 3 An electric stove is connected to a 240-V outlet. If the current flowing through the stove is 20 A, what is the resistance of the heating element? Use GRASP and sig. digits. </li> <li> Slide 59 </li> <li> Circuit Diagrams Engineers and designers of electrical circuits use special symbols that show the components and connections in a circuit. A drawing made with these symbols is called a circuit diagram. </li> <li> Slide 60 </li> <li> Circuit Diagrams, cont Follow these rules when you draw circuit diagrams. 1. Always use a ruler to draw straight lines for the conducting wires. 2. Make right-angle corners so that your finished...</li></ul>