Energy and the Environment: Division III

Activity 1: Resistors

Integration Notes:
Students will examine the effect that a variety of resistors have on the amount of current flowing through a circuit. (see Electricity Background, Resistance).

This activity leads students to the knowledge they will need to construct their headlamp that uses LEDs as a light source. (see Electricity Background, Sources of light).

Learner outcomes
Students will:

• State the effect that a variety of resistors have on the amount of current flowing through a circuit.
• Determine the amount of resistance provided by a resistor by examining its coloured bands.

Materials:

• variety of radio resistors (10 Ohm to 20 Ohm)
• two C dry cell batteries (Caution: resistors can get hot when a current is applied)
• ammeter
• voltmeter
• connecting wires

Introduction:
George Simon Ohm (1785 - 1854) was the first person to publish the results of experimentation on the resistance of wires of various sizes. Ohm constructed wires of many different lengths and thicknesses. Today, we simply call these devices resistors because all they do is resist the flow of current. Ohm applied a potential difference (voltage) across his wire resistors and measured the current through them. He discovered that for each resistor, no matter what potential difference he applied, the resistance was the same.

Resistors are very important in circuits. (see Electricity Background, Ohm's Law) They are used to control current or potential difference in a circuit. For example, if you connect an LED to a battery (cell) you will likely destroy the LED unless you have a resistor in the circuit to control the potential difference over the LED and most importantly, the current through it.. (see Electricity Background, Sources of Light)

In this activity, students will examine the coloured bands on a variety of resistors and use the chart provided to determine their resistance. The second part of the activity involves incorporating the resistors into a simple circuit and using an ammeter to determine the effect that they have on the amount of current flowing through the circuit.

Activity Instructions:

Part one:
1. Working in groups of two or three, examine a variety of resistors. Look for the colour bands as shown in this diagram.

The first two coloured bands (closest to one end of the resistor) are the first two digits of resistance. The third coloured band indicates the number of zeros that follow the second digit.

Using the chart below, this resistor has a value of 2,700,000 Ohms (2.7 x 10⁶ Ohms). This value is determined by:

• First Digit (Red) = 2
• Second Digit (Violet) = 7
• Third Digit (Green) = 5 therefore, multiply by 100,000
• Total Resistance is 2,700,000 Ohms (2.7 x 10⁶ Ohms)

2. The resistance for the resistor shown on the left can be determined as follows:

First Digit - (Yellow) = 4 Second Digit - (Violet) = 7 Third Digit - (Red) = multiply by 100 Total Resistance is 4,700 Ohms (4.7 x 103 W)

3. Draw the coloured bands that would be used to represent a resistor with a resistance of:

• 320 Ohms
• 7,800,000 Ohms

4. Use the colour bands to determine the resistance of the resistors available.

5. If you have a multimeter, measure the actual value of each resistor and compare it to the stated value. What is the percent error?

Note: The tolerance is the percentage error for that resistor. A resistor with a tolerance of 10 per cent means that that resistor will have an effective resistance of plus or minus 10 per cent of the value represented by the coloured bands.

• A Gold Band has a tolerance of 5%
• A Silver Band indicates a tolerance of 10%.
• No Band indicates a tolerance of 20%.

Part 2: Measuring Resistance

1. Construct a simple circuit with a battery, switch and resistor. You will use a voltmeter to measure potential difference and the ammeter to measure current. Make sure that you use an appropriate resistor for the circuit. The smaller the value of resistance, the more heat generated, so it is safer to initially use larger resistors.

Carefully touch the resistors to determine their temperature before picking them up. You do not want to burn yourself.

2. Make sure you start by setting the meters on the largest scale so that the meter isn't overwhelmed.

3. Connect the voltmeter into the circuit in parallel with the resistor so that you are measuring the potential difference across the resistor. Close the switch and record potential difference (volts) on a data chart with three headings: Number of cells used; Potential Difference (V) Volts; Electrical Current (I) Amps

4. Connect the ammeter to measure current. Close the switch and record the current (amps). Record this value on your data chart.

5. Add the second cell in series to the circuit and repeat steps three and four.

6. Change the resistor and repeat this investigation.

Discussion:
1. Compare the resistance as determined by the colour bands to the experimentally determined resistance.

Resistance = Potential Difference/Current
R = V/I

2. Explain any discrepancies between the calculated value and the value indicated by the coloured bands.

3. Explain what happens when you add a second cell to the circuit.

4. Describe a resistor and its importance in electrical circuits.

JUMP TO: Activity 1 | Activity 2 | Activity 3 | Challenge | Top

Activity 2: Series and Parellel Circuits

Integration Notes:
Students will experiment with alternative ways of setting up simple circuits. Through this exploration, series and parallel circuits will be understood. This activity gives students the knowledge of circuits that they will need in order to be successful at the challenge at the end of the phase.

Learner outcomes
Students will:

• Construct series and parallel circuits. (see Electricity Background, Electric Circuits)
• State the difference between the two basic types of circuits.
• Use the appropriate symbol to represent an electrical device in a circuit diagram.

Materials:

• wire
• two AA batteries
• two 3-volt light bulbs
• switch
• resistors from Activity 1
• multimeter (ammeter and voltmeter)
• index cards
• poster paper
• glue, scissors, ruler, etc.

Introduction:
An electrical circuit requires a source of electrical energy such as a dry cell battery. The circuit must have a closed path to connect the components. Although a circuit will function without a switch, it is convenient to have one. You can open and close the circuit with a switch.

A resistor is used to represent any one of many different components, called loads that convert electrical energy into other forms of energy. For example, motors, toasters, light bulbs, and stereos convert electricity into motion, heat and light. There are so many different electrical devices that it would not be practical to use different symbols for each. The resistor represents something that they all have in common. They all resist the movement of charge through a circuit.

In this activity students will experiment with finding as many ways as possible to construct a circuit. They will be asked to incorporate resistors, a light bulb and switch in a variety of ways and organize their circuits into two main types of circuits - series and parallel. (see Electricity Background, Electric Circuits)

Activity Instructions:

1. Examine the chart below that illustrates the symbols for each component of a circuit. You will be expected to use these symbols in the circuit diagrams that you draw for the rest of this activity.

Component	Symbol
Battery (per cell) - is a combination of cells
Light bulb - is an example of a resistor
Resistor - is anything that resists the movement of charge through the circuit
Switch

2. Collect all of the materials listed above and experiment with arranging them in as many different ways as possible. Each time you construct a new circuit, draw a diagram that represents the circuit on an index card.

3. When you have come up with a few different arrangements and recorded them on index cards, organize the cards into three piles based on the similarities in the arrangement of the components. One pile should contain circuits where all of the components are connected in a linear fashion, or one after another. This pile represents series circuits. Use the ammeter (multimeter) to measure the amount of current that flows through each circuit. Record this information on a data chart.

4. The second pile should contain circuits where one or more components branch off from the circuit at the same point. This pile represents parallel circuits. Use the ammeter to measure the amount of current that flows through each circuit. Record this information on a data chart.

5. The third pile should contain circuits that are a combination of parallel and series circuits. Make a list of the similarities and differences between the series and parallel circuits. Use the ammeter to determine the amount of current flowing through each circuit. Record this information on a data chart.

6. Glue your index cards and list of similarities in the circuits onto a piece of poster paper to make a poster that illustrates the two different types of circuits.

JUMP TO: Activity 1 | Activity 2 | Activity 3 | Challenge | Top

Activity 3: Light Emitting Diodes

Integration Notes:
Students will examine LEDs and how they differ from incandescent bulbs. This activity presents students with the information they will need in order to be successful at the challenge at the end of this phase (construct a headlamp using LEDs).

Learner outcomes
Students will:

• Construct a circuit containing resistors, a battery and four white LEDs.
• Determine the size and amount of resistors needed in a circuit that includes four LEDs.

Materials:

• wire
• three - 1.5 volt batteries in series (a minimum of 3.6 volts is required for most LEDs)
• additional batteries as required
• four white LEDs
• resistor (45 Ohms) - this value is based on providing 4.5 volts of source voltage
• additional resistors

Introduction:
A Light Emitting Diode (LED) (see Electricity Background, Sources of Light) is a special diode that emits light when connected in an electrical circuit. A clear or coloured epoxy case surrounds and encloses a semiconductor chip.

Students will incorporate resistors (see Electricity Background, Ohm's Law) into a simple circuit so that they can use four LEDs to provide light, instead of relying on a relatively power hungry incandescent bulb. If the students have completed the activities from Phase 3 (construct a headlamp), they will have a good understanding of what is required to complete this activity. The knowledge students gained in Activities 1 and 2 of this phase are vital to this activity if the students are going to construct an effective circuit with an LED without melting it.

Activity Instructions:

NOTE: Advise students to be careful about the heat that can be generated in a resistor if a large current is passed through it.

Diagram of an LED, provided by theledlight.com. For more on how LED's work, view the Technical Information from the LED site.

1. Use the knowledge gained in Activities 1 and 2 to construct a circuit using a single LED in a simple circuit. Use a AA battery as a power source and begin with a larger, rather than smaller resistor to prevent melting the LED.

Make sure to insert the LED in the correct orientation since it only works when the current passes through it in a single direction. Experiment to find which way LEDs must be inserted, remembering that the positive is the longer one of the two terminals.

2. Add another LED to the circuit in series and then in parallel and notice the effect on the brightness of the LEDs.

3. Alter the amount of current and the size of the resistor in the circuit to have a working light source with two LED's. Students can work out the size of resistor they will need to protect the LED's by using Ohm's Law. (see Electricity Background, Ohm's Law)

For example: