Intro to Electricity and circuits pg. 47 This lesson introduces the topic of electricity in general, and its practical applications. It includes the concepts of electric current and its unit, electric circuits, and schematic diagrams for circuit elements, and distinguishes between open circuits, closed circuits, and short circuits.

Objectives Physics terms Define electric current. electric current Identify the electric symbols for basic circuit elements. Use a multimeter to test continuity. Identify and construct open, closed, and short circuits. electric current ampere (A) electric circuit open circuit closed circuit short circuit Tell students they will be learning some of the basic, practical information that is used by real electricians and electrical engineers.

Objectives Physics terms Define voltage source. Characterize materials as conductors or insulators based on their electrical properties. Demonstrate the use of multimeters, power supplies, and resistors. State and apply Ohm’s law to calculate current, voltage or resistance in an electric circuit involving a single resistor. voltage volt (V) battery Voltmeter resistance resistor ohm (Ω) electrical conductor electrical insulator

Important People in History Prior to 1800, no one knew how to create a steady supply of electric current. Alessandro Volta was the first to succeed. He put two different metals in an acid or salt solution & produced an electric current between the terminals. He connected multiple voltaic piles together to create a battery of them. Legend has it that Benjamin Franklin flew his metal key on his kite in a lightning storm & learned about what electricity was. Benjamin Franklin suggested this experiment, but it is unclear from his notes whether he ever attempted some version of it. To try this in an actual lightning storm would be to risk death by electrocution. If he performed this experiment, he was lucky to survive it.

What is electricity & how is it used Electricity is the flow of electric charges, typically through wires, conductors, and electrical devices. Sometimes we can see the effects of electricity in nature. Many household appliances and personal devices use electricity. The room you are in right now is probably full of devices that use electricity. Ask the students for a couple of examples of static electricity in their everyday lives. When scientists first started learning about electricity, they did not know how to create currents in the laboratory. They could only create static charges.

Key Terms in Electricity There are 3 key terms that we’ll use throughout our discussion of electricity: Current Voltage Resistance

What is electric current? Electric current is the flow of charged particles, usually through wires and circuits. Current is measured in amperes or amps (A). Its symbol is I See page 472 for this mouse-over animated illustration. Water current can be measured as the amount of water that flows past a certain point in a pipe every second. Electric current is a measure of the amount of electric charge that flows through a certain point in a wire every second. The charges are always in the wires, but there is only a current if the charges flow along the wire. Animated illustration, page 472

Electric circuits & conserving charge An electric circuit is a conducting path through which electric current can flow. Electric current (I) must be the same at all points in a simple, single-loop circuit. Charge can never be created or destroyed. All the charge that flows through one point in the circuit must flow through the next point also.

Open and closed circuits An open circuit does not have a complete path so no current flows. A closed circuit has a complete path, allowing current to flow. Point out the direction of the current arrow in the closed circuit, which shows current flowing from the positive battery terminal toward the negative terminal. This is called the “conventional current” direction, and is the direction that positive charges would flow through the circuit. Students will use this convention throughout the unit. (In fact, it is actually the electrons that flow, and they flow in the opposite direction)

Common circuit elements Draw the students’ attention to the electrical symbol for each component. Point out the long and short slashes on the battery symbol, representing the + and – terminals. A battery can be represented by one or more sets of these slashes. Reassure them that they will be learning about resistors soon, but not in this lesson.

What is a short circuit? The current flows much more easily through a simple wire than it does through the bulb and resistors. So when the switch is closed the current takes the easy way around and almost no current at all will flow through the other three paths.

What is a battery? A battery is a device that converts stored chemical energy into electrical energy. Batteries come in many different kinds – even lemons! chemical energy electrical energy The multimeter in this figure is reading 0.41 Volts. But the lemon by itself is not creating this voltage. The juice inside a lemon is being used in place of Volta’s acid or salt solution. Point out to the students that the nails in the lemon must be made of two different metals.

Reading a battery Batteries come in many sizes and voltages: 1.5, 6, 9, 12 V, for example. Voltage is the electric potential difference between the two terminals of a battery. Voltage is measured in volts (V), named after Volta, the inventor of the battery. Ask the students the voltage of a typical car battery (12 V). The voltage of a wall socket? (120 V).

Going further: What’s inside a battery Advanced students may be curious about how the battery works. Refer them to their e-Book, which provides additional information.

Volts and amps Current and voltage are the two most important physical quantities for understanding electric circuits. Current measures the flow of charge through a wire. Voltage measures the change in electrical potential energy per unit of charge. In other words: current is the actual amount of “stuff” (charges) that flows through wires & voltage is the measure of how much energy charges have. Voltage is one of the most difficult concepts for students to grasp. It is the difference in electric potential energy per coulomb of charge between two points. But for students, it is important for them to grasp that current is the actual amount of “stuff” (charges) that flows through the wires, and voltage is a measure of the energy of those charges,

A useful analogy Charges in a wire are analogous to water in a pipe: Electric current is analogous to the flow of the water. The battery is analogous to the water pump. Voltage is analogous to the change in gravitational Ep.

Analogy: water and electricity Water flows because there is an energy difference, measured in height. Current flows because there is an energy difference, measured in volts. Mouse over this slide to make the animation bar visible. If this animation does not function on your setup, use the one in the e-Book. Water always flows from a position of high gravitational potential energy to a position of lower potential energy. Charges always flow from a position of high electric potential to a position of lower electric potential. r View this short video on page 475 of your e-Book.

Electrical conductors Electricity is all around us, yet we are not electrocuted on a daily basis. Why not? Some materials are good electrical conductors, while others are good insulators. Air, plastic, glass, wood, and rubber are all good insulators. Prompt the students for more examples of insulators. How about conductors?

Electrical wire A typical electrical wire has copper on the inside and plastic insulation on the outside. Copper is a good conductor because it has many electrons that are free to move through the material. The plastic covering on the wire protects us from getting electrocuted. To have a practical, working circuit, we need a complete conducting path, protected by a insulator, to prevent short circuits.

Water is conductive too! Ordinary tap water may conduct electricity which is why you should never use electrical devices in or near a bathtub or shower! But also think about it, what is the cord to all your electronics made of? Ask “what happens at the beach or pool when the lifeguards hear thunder?”.

How much current? This lamp conducts electricity. When the lamp is connected to a 10 V source, 2 amps of current flows through it. But why? What determines how much current will flow? The lamp itself helps determine the current flow. Is the lamp filament a good conductor, or does it resist the flow of current?

Resistance Some materials are better conductors than others. How do we quantify this difference? Electrical resistance (R) is a measure of how hard it is for electricity to flow through a material. Resistance is measured in ohms (Ω). [Ω is the Greek letter “Omega.”] An ohm is a volt per amp In this lesson the students will measure resistance indirectly by measuring current and voltage, and applying Ohm’s law.

Resistance: an analogy This simulation is available in the e-Book.

Resistors Electric circuit elements that have resistance are called resistors. Resistors control the flow of current in a circuit. Light bulbs have resistance, too. Resistors come in many sizes, but these resistors have the typical cylindrical shape and colored resister code found in commonly-used devices.

Reading resistors A resistor has three bands on one end, giving its value using two digits and a multiplication factor. The easy way to begin reading a resistor’s code is to find the tolerance band. Once they find it, students should start at the opposite end of the resistor and read off the colors. Go through a couple examples with them. For example: (green brown red) equals 51 x100 ohms = 5100 ohms. An even simpler way to explain it is that the third band tells them how many zeros to add to the first two digits.

How do we measure current, voltage & resistance? Electric current ammeter Voltage voltmeter Resistance ohmmeter More commonly we use a device that can measure all of them, a multimeter. In other words, the multimeter must be connected in series.

Equations Ohm’s law relates current to voltage and resistance in electrical circuits. Ohm’s “law” is not really a law: it’s a mathematical relationship closely obeyed by most conducting materials. Label what each stands for & their units. There is only one key equation for this lesson. It is presented as part of the introduction to the lesson. Its meaning will be explained later.

Three ways to use Ohm’s law Write the equation (rearranged if necessary) that you would use to do each of the following. Determine the current through a resistor. Determine the voltage drop across a resistor. Determine the resistance of an unknown resistor. Point out to students that these three equations are just a single equation, rearranged. In fact, Ohm’s law is even more useful than the slide implies, since it can be used for a single resistor, or for any arrangement of resistors.

Applying Ohm’s law What is the current through a 10 Ω resistor when a voltage of 5 V is applied across it? Preview: In a few minutes, the students will set up this circuit and create a current of about 0.5 amps.

Practice What is the current through a 10 Ω resistor when it is connected to a 5 V battery? To reduce the current, should resistance be increased or decreased? If the resistance increases by a factor of two, how does the current change? What if resistance is halved? What is the voltage drop across a 50 Ω resistor when a current of 0.10 A flows through it? Work through a few example problems like these with the students. Answers are on subsequent slides.

Homework Which statement below is a correct definition of electric current? Electric current is . . . the energy of moving electrical charges. the flow of electric charges through a wire or circuit. the number of electrons in a circuit element. the attraction between charged particles in a wire. Remind students of the analogy between water and electricity made earlier in the presentation.

Homework Label each of these electrical symbols with the name of the electrical component it represents: battery; resistor; lamp; switch; or wire. The shape of the electrical symbol usually helps the students remember its function.

Homework 3. Identify the following circuits: Ask the students which circuit will have the highest current. Which will have the lowest (or no) current? Why?

Homework 4. Yuri connected his power supply (pictured below) to his circuit using the red plug. The lamp did not light. What did he do wrong? Circuit diagrams like these are standard everywhere. By the end of this lesson the students will know how to draw these diagrams, and what these symbols represent.

Homework 5. Which statement below provides a correct practical definition of a voltage source? A voltage source . . . creates an electric potential difference between two points in a circuit, such as the two ends of a battery. provides the electrical potential energy needed for a circuit to operate. is analogous to the pump in a system of circulating water. all of the above. Encourage the students to take their time and read the options carefully. All are true.

Homework 6. What does the “1.5 V” label on the battery mean? The voltage of the positive terminal is 1.5 V. The voltage of the negative terminal is 0 V. The voltage between the positive and negative terminals is 1.5 V. All of the above Get the students to focus on the batteries in the circuit, not the resistors. Students may become distracted by the compound arrangement of resistors that appear in the circuit on the left. Assure them that later in the course they will learn how these resistor arrangements work.

Homework 7. Which materials listed below are conductors? aluminum rubber copper gold diamond Ask the students to fill in the blank: all of these materials are either conductors or _________?

Homework 8. Which equation below is incorrect? D. 𝑽=𝑰𝑹 9. What is the voltage drop across a 50 Ω resistor when a current of 0.1 A flows through it? 10. What is the current through a 10 Ω resistor when it is connected directly to a 15 V battery? Give students time to work through the four possibilities.