# Electricity and circuits

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Electricity and circuits
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.

What is electricity? Electricity is the flow of electric charges, typically through wires, conductors, and electrical devices. Sometimes we can see the effects of electricity in nature. 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.

What is electric current?
Electric current is the flow of charged particles, usually through wires and circuits. Current is measured in amperes (A), also called amps. 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

How do we measure electric current?
Electric current is measured using an ammeter or, more commonly, a digital multimeter. In order to properly measure current, all the current must flow into the meter and back out again, into the circuit being tested. In other words, the multimeter must be connected in series.

What is an electric circuit?
An electric circuit is a conducting path through which electric current can flow. The symbol for electric current is I. In this circuit, I = 0.3 amps.

Charge is conserved 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. I I To get them ready for the next slide, ask “What kinds of parts might we need to have to build a circuit in the classroom?”

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.

Knife blade switches A knife blade switch uses a mechanical lever to open/close a circuit connection. The handle is an insulator, so you won’t get a shock when you touch it! When attaching a knife blade switch to a circuit, do so with it in the open position. 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 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 breadboard? A circuit breadboard is a device used for mounting and prototyping electric circuits. Power and ground are connected from an external source to strips on the edge of the breadboard. Many years ago, before these breadboards were invented and made commonly available, people needed an easy way to create and test their prototype electric circuits without having to solder all the connections. They solved the problem by hammering rows of nails into an actual piece of wood such as a breadboard, and connected their circuit elements via these nails. These solderless breadboards are now also referred to as prototyping boards, or protoboards.

Exploring the ideas In Investigation 17A you will build electric circuits using a circuit breadboard. In this investigation, the students will find out how a breadboard works, create a simple circuit, and then modify that circuit to create an open circuit, closed circuit, and short circuit.

Investigation Part 1: Exploring the connections on a breadboard
Use the continuity setting on a multimeter to test whether the probes are connected to each other. Identify 3 kinds of connections: power (red); ground (blue); and terminal (black). Draw a diagram showing the connections between the holes in the breadboard. These solderless plastic breadboards have hidden metal connections between some, but not all, of the socket holes. Students should begin testing the breadboards to discover the hidden connections that make these breadboards such a common and useful piece of equipment.

Investigation Questions
Where should you connect the positive and negative terminals of a battery? Where should you connect other circuit components, such as lamps and resistors? How are the holes in a circuit board connected to each other?

Investigation Draw a diagram showing the connections between the holes in the breadboard on your student assignment sheet. This question is included in the student assignment so students have a place to record their answer. The teacher should circulate through the lab groups assist students who may need help with the equipment.

Investigation Part 2: Wiring a simple circuit
Connect power and ground from a +3 V voltage source (or 2 D-cells) to the breadboard. Connect a simple circuit with one lamp. One lead connects to the breadboard's power strip; the other to the ground strip. If students can’t get a bulb to light, they should try a different bulb or different set of wires in case a component is broken. The figure on the next slide may also be of help.

If not, check all your connections. The two ends of the lamp must be in different rows. You can use ANY rows. This example uses rows 9 and 10. Students can use ANY rows to set up their circuit, but the two ends of the lamp must be in different rows, as shown in this figure. Wires connecting the power source (or ground) to one “leg” of a lamp must be inserted into the SAME row as that leg.

Create a simple switch How can you use wires to create a switch?
Does your lamp go on and off as you connect/disconnect the switch? Using two wires as a simple switch will let them experiment with open and closed circuits.

Short circuit How can you use a wire to modify your circuit to create a short circuit?

Short circuit How can you use a wire to modify your circuit to create a short circuit? Hint: use a wire to bypass the lamp! What should happen to the lamp if you create the short circuit correctly?

Assessment 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.

Assessment wire switch battery lamp resistor
Label each of these electrical symbols with the name of the electrical component it represents: battery; resistor; lamp; switch; or wire. wire switch battery lamp resistor The shape of the electrical symbol usually helps the students remember its function. 22

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

Assessment Short circuit Open circuit Closed circuit
Identify the following circuits: Short circuit Open circuit Closed circuit Ask the students which of these circuits can be dangerous, and why. (Answer: a short circuit can lead to overheating, which can cause a fire.)

Going further How to strip the plastic insulation off the ends of a wire A small amount of plastic insulation must be removed from the ends of wires to use them in circuits. If your wires are not “pre-stripped,” then you will need to use a wire stripper to remove a small amount of the insulation. If students run out of jumper wires, the next figure will show them how to create new ones.

Remind the students to select the right size notch, and not remove too much plastic. Otherwise, uninsulated wires could touch on the breadboarding area.