Grade 7 Ch 3 Electricity: Circuits& Components

Grade 7 Ch 3 Electricity: Circuits and their Components

1. Uses of Electricity

•  Electricity powers

1. cooking (e.g., microwave),

2.lighting (e.g., bulbs),

3.transportation (e.g., trains),

4. heating/cooling (e.g., AC),

5.entertainment (e.g., TV),

6.communication (e.g., phone),

7. and others (e.g., computer).

•  Grouping: By location (home, school), function (daily needs, fun), or device type (portable, fixed).

2. Torchlight and Its Components

What is a Torchlight?

A torchlight (also called a torch or flashlight) is a portable device that produces light, commonly used to see in the dark.

Components of a Torchlight:

• Electric Cell: Portable power source with positive (+) and negative (-) terminals.
• Battery: 2 or more cells connected (+ terminal to - terminal) in series for more power.
• Incandescent Lamp: Has a filament that glows when heated.
• LED Lamp: No filament; glows when current flows correctly (longer wire = positive, shorter = negative).

·         Switch: Completes or breaks the circuit to turn the lamp on/off.

How It Works

When the switch is in the "on" position, it connects the cells to the lamp, allowing electricity to flow and make the lamp glow. In the "off" position, the connection is broken, and the lamp stays off.

3. Electrical Circuit

What is an Electrical Circuit?

• An electrical circuit is a complete path that allows electric current to flow from a power source (like a cell) through a device (like a lamp) and back, making the device work.
• Current flows only in a closed circuit. An open circuit (e.g., switch off) stops the flow.
• Direction: Current flows from positive terminal to negative terminal.

A. Electric Cell: An electric cell is a small, portable source of electrical energy, like those used in

                            torchlights, clocks, or remotes.

•     Terminals: Every cell has two terminals:

                Positive Terminal (+): Marked with a "+" sign, usually a metal cap.

                Negative Terminal (-): Marked with a "-" sign, usually a flat metal disc.

•      Function: The cell provides the energy needed to make devices work by allowing cu the positive to the negative terminal.

B. Battery: A battery is a combination of two or more electric cells connected together to provide more                     energy or last longer.

• Connection: In a battery, the positive terminal of one cell is connected to the negative terminal of the next cell, forming a chain. This forms battery.

 

• Example: In a torch, two cells are placed so the positive terminal of one touches the negative terminal of the other, making the lamp glow when connected properly.

Fascinating Fact: The term "battery" is often used for a single cell, like the one in a mobile phone, even though it's technically one cell.

C. Electric Lamp

       1. Incandescent Lamp:

•  Structure: An incandescent lamp has a glass bulb with a thin wire called a filament inside, supported by two thicker wires.
• Terminals: The filament connects to two terminals-one at the metal case of the lamp's base and one at the metal tip in the center.
• How It Works: When electric current flows through the filament, it gets hot and glows, producing light.
• Usage: Found in older torchlights, these lamps glow regardless of which terminal connects to the cell's positive or negative terminal.

       2. LED Lamp

• Structure: A Light Emitting Diode (LED) lamp has no filament. It has two wires-one longer (positive terminal) and one shorter (negative terminal).

• How It Works: LEDs glow only when connected correctly, with the positive terminal (longer wire) to the battery's positive terminal and the negative terminal (shorter wire) to the negative terminal. Current flows in one direction only.

• Usage: Many modern torchlights use LEDs because they are efficient and long-lasting.

4. Making an Electric Lamp Glow Using an Electric Cell or Battery

To make an electric lamp glow, we can use a simple circuit with an electric cell (or battery), an incandescent lamp, a cell holder, and some electric wires. Here's how it works:

•          Electric Cell: This provides the electrical energy required to make the lamp glow. The energy com from the chemical reaction inside the cell.
•          Incandescent Lamp: The lamp is a device that uses electricity to produce light. It has a filament that glows when electricity passes through it.
•          Cell Holder: A holder is used to securely place the electric cell in the circuit. It ensures the proper connection of the battery's terminals to the wires.
•          Electric Wires: Wires are used to connect the components in the circuit. The wires allow the flow of electric current from the cell to the lamp.

5. How the Circuit Works:

·         Connection: The two terminals of the electric cell are connected to the lamp through the electric wires.

·         Current Flow: When the circuit is complete, the electric current flows from the negative terminal of the cell, through the wires, and into the lamp. This causes the filament inside the lamp to heat up and produce light.

·         Prediction of Lamp Glow: Depending on how the circuit is set up, ure lamp may or may not glow. If the circuit is properly connected (with all components in place), the lamp will glow. Otherwise, it will not light up.

6. Electrical Circuit

A circuit is a closed loop that allows current to flow from the positive terminal of a cell, through a device (like a lamp), and back to the negative terminal.

•         Current Flow: Electric current is considered to flow from the positive to the negative terminal of the cell
•         Incandescent Lamp: Glows when the circuit is complete, regardless of terminal connections.
•         LED: Glows only when terminals are correctly aligned with the battery's terminals.

Electric Switch

A switch is a device that controls the flow of current by completing (closing) or breaking (opening) a circuit.

How It Works:

1. ON Position: The switch closes the circuit, allowing current to flow, making the lamp glow.

2. OFF Position: The switch opens the circuit, stopping the current, so the lamp doesn't glow.

Placement: A switch can be placed anywhere in the circuit and still control the current flow.

Example: In a torch, sliding the switch to "on" completes the circuit, and sliding it to "off" breaks it.

Real-Life Switches: Home switches for lights work similarly but are designed differently for safety and convenience.

7. Circuit Diagrams

What is a Circuit Diagram?

A circuit diagram is a drawing that uses standard symbols to represent the components of an electrical circuit, making it easy to understand how the circuit is built.

Electrical Components & their symbols

Fig: Positive & negative terminals of in the symbols of (a) a Cell (b) an LED

Purpose: Circuit diagrams simplify complex circuits, making them easier to draw and understand.

Dive Deeper-Standardization

International organizations like the International Electrotechnical Commission (IEC), American National Standards Institute (ANSI), and the Institute of Electrical and Electronics Engineers (IEEE) create standard symbols used worldwide, ensuring everyone understands the same diagrams.

8. Electrical Conductors and Insulators

Materials are classified based on whether they allow electric current to flow through them.

Conductors Materials that allow electric current to flow easily are called conductors or good conductors of electricity.

Examples: Metals like copper, silver, gold, aluminum, and objects like keys, coins, and sewing needles.

Use in Circuits: Conductors, especially copper, are used for wires because they allow current to flow efficiently.

Dive Deeper - Best Conductors:

Silver, copper, and gold are the best conductors, but copper is used most often due to its lower cost and availability.

Insulators: Materials that do not allow electric current to flow are called insulators or poor conductors of electricity.

Examples: Plastic, rubber, glass, wood, cork, paper, wax, ceramics, and objects like plastic scales, erasers, and candles.

Use in Circuits: Insulators are used to cover wires, make plug tops, and switches to prevent electric shocks and ensure safety.

                   Fig: (a) Conduction tester (b) Using the conduction tester for testing a material

 Results of testing materials for electrical conductivity:

Importance of Both:

Conductors: Used in wires, switches, plugs, and sockets to carry current.

Insulators: Protect users from electric shocks by covering conductive parts.

Caution/Safety: The human body is a conductor, so electric current passing through it can cause severe injury or death. Never touch switches or plugs with wet hands, use devices in wet areas, or handle damaged electrical equipment.

Dive Deeper - Types of Electricity:

Direct Current (DC): Produced by batteries, used in small devices like torchlights and phones.

Alternating Current (AC): Supplied from power plants to wall sockets, used for larger appliances.

Points to Remember

·         An electric cell provides portable electrical energy and has a positive terminal (metal cap, +) and a negative terminal (metal disc, -).

·         A battery is formed by connecting two or more cells, with the positive terminal of one cell touching the negative terminal of the next, to supply more energy or last longer.

·         The term "battery" is sometimes used for a single cell, like in mobile phones.

·         An incandescent lamp has a filament that glows when heated by current, connected to two terminals (metal case and tip), and glows regardless of terminal connections.

·         A "fused" incandescent lamp doesn't glow because its filament is broken, stopping current flow.

·         An LED has no filament, only glows when its positive terminal (longer wire) connects to the battery's positive terminal and negative terminal (shorter wire) to the negative terminal, as current flows in one direction.

·         A switch completes (ON) or breaks (OFF) a circuit, controlling current flow, and can be placed anywhere in the circuit.

·         An electrical circuit is a closed path for current, flowing from the positive to the negative terminal of a cell, making devices like lamps glow.

·         Circuit diagrams use standard symbols (e.g., long/short lines for cell terminals, triangle for LED) set by organizations like IEC, ANSI, and IEEE, making circuits easy to understand globally.

·         Conductors (e.g., copper, silver) allow current to flow and are used for wires, while insulators (e.g.. plastic, rubber) block current and are used for safety coverings.

·         Copper is widely used for wires due to its good conductivity, lower cost, and abundant supply compared to silver or gold.

·         The human body conducts electricity, so handling electrical devices unsafely (e.g., with wet hands) can cause injury or death.

·         Battery-powered devices use Direct Current (DC), while wall sockets supply Alternating Current (AC) for larger appliances.

Difficult Words and Their Meanings

Electricity: A form of energy that powers devices, like lights or fans, by flowing through wires or

circuits.

Circuit: A complete path that allows electric current to flow from a power source (like a cell) through a device and back.

Electric Cell: A small, portable device that produces electrical energy, with positive and negative terminals, used in torchlights or remotes.

Battery: Two or more electric cells connected together to provide more energy or last longer.

Terminal: The ends of a cell or device (positive or negative) where current enters or leaves.

Incandescent Lamp: A light bulb with a filament that glows when heated by electric current, used in older torchlights.

Filament: A thin wire inside an incandescent lamp that glows to produce light when current passes through it.

LED (Light Emitting Diode): A modern lamp that glows when current flows in one direction, with no filament, used in many torchlights.

Switch: A device that controls current by opening (stopping) or closing (allowing) a circuit.

Conductor: A material, like metal, that allows electric current to flow easily, used for wires.

Insulator: A material, like plastic or rubber, that blocks electric current, used to cover wires for safety.

Circuit Diagram: A drawing using standard symbols to show how electrical components are connected in a circuit.

Activities

Activity 3.6: Let us Construct

                                  Fig: (a) An electric cell holder with two wires attached

                                         (b) An electric cell inside the cell holder

                                        (c) Wires connected to an electric cell using electrical tape

 

Table 3.1: Trying to make the lamp glow

Activity 3.7: Making LED glow

Electric Switch:

Activity 3.8 & 3.9: Let us construct & Test

                               Fig: An electrical circuit with a switch in (a) ‘OFF’ position (b) ‘ON’ position

Circuit Diagram

                    

Activity 3.10: Let us Draw

Let Us Enhance Our Learning

1. Choose the incorrect statement.
(i) A switch is the source of electric current in a circuit.
(ii) A switch helps to complete or break the circuit.
(iii) A switch helps us to use electricity as per our requirement.
(iv) When the switch is in ‘OFF’ position, there is an air gap between its terminals.
Answer:  (i) A switch is the source of electric current in a circuit.
Explanation: A switch is a device that completes or breaks a circuit.

2. Observe Fig. 3.16. 

    With which material connected between the ends A and B, the lamp will not glow?

Answer: The lamp will not glow when insulating materials like rubber, plastic, and ceramics are connected between ends A and B.

3. In Fig. 3.17, if the filament of one of the lamps is broken, will the other glow? Justify your answer.

Answer: The current in the circuit flows through a single path, as both bulbs are connected in series. If the filament of one lamp is broken, the circuit becomes open, and the other bulb will not glow.

4. A student forgot to remove the insulator covering from the connecting wires while making a circuit. If the lamp and the cell are working properly, will the lamp glow?
Answer: No, the lamp will not glow because the insulating cover on the wires prevents current from flowing. The insulation needs to be removed from the connecting wires to establish proper contact between the wires.

5. Draw a circuit diagram for a simple torch using symbols for electric components.
Answer:

  

 6. 

   In Fig. 3.18:

(i) If S2 is in ‘ON’ position, S1 is in ‘OFF’ position, which lamp(s) will glow?
(ii) If S2 is in ‘OFF’ position, S1 is in ‘ON’ position, which lamp(s) will glow?
(iii) If S1 and S2 both are in ‘ON’ position, which lamp(s) will glow?
(iv) If both S1 and S2 are in ‘OFF’ position, which lamp(s) will glow?

Answer:
              (i) No lamp will glow because the circuit is broken at S1.
              (ii) No lamp will glow because the circuit is broken at S2.
              (iii) Both L1 and L2 will glow because the circuit is complete.
              (iv) No lamp will glow because the circuit is open at both switches.

7. Vidyut has made the circuit as shown in Fig. 3.19. Even after closing the circuit, the lamp does not glow. What can be the possible reasons? List as many possible reasons as you can for this faulty operation. What will you do to find out why the lamp did not glow?

Answer: Possible reasons why the lamp doesn’t glow:

            (i) The cell is weak or dead.
            (ii) The bulb is fused or faulty.
            (iii) Loose or faulty connections.
            (iv) The safety pin is an insulator or a poor conductor.
            (v) Corroded battery terminals.

Things to do to find the problem:
            (i) Check the battery.
            (ii) Test the bulb.
            (iii) Inspect all connections.
            (iv) Examine the safety pin and replace it with a good conductor material.

8. In Fig. 3.20, in which case(s) the lamp will not glow when the switch is closed?

Answer:

(a) The switch is closed, current flows, and the lamp glows.
(b) This is the same circuit as (a), but the cell is reversed. The lamp will still glow regardless of the polarity.
(c) LEDs allow current to flow in only one direction. Due to the wrong polarity, the current will not flow and the LED will not glow.
(d) The LED is connected with the correct polarity, so current flows and the LED will glow.
Hence, (a), (b) and (c) are the cases in which the lamps will not glow.

9. Suppose the ‘+’ and ‘–’ symbols cannot be read on a battery. Suggest a method to identify the two terminals of this battery.
Answer: The two terminals of a battery can be identified using an LED. Connect the battery terminals to the LED in any order. If the LED glows, the battery terminal connected to the positive terminal (longer wire) of the LED is the positive (+) terminal, and the other is the negative (–) terminal.

10. You are given six cells marked A, B, C, D, E, and F. Some of these are working and some are not. Design an activity to identify which of them are working.
(i) List the items that you require.
(ii) Write the procedure that you will follow.
(iii) With the items, carry out the activity to identify the cells that are working.
Answer:
(i) Items Required: A torch bulb, bulb holder, connecting wires and the six cells (A, B, C, D, E, F)

(ii) Procedure:

1. Connect the bulb to a cell using the wires.
2. Observe if the bulb lights up.
3. Repeat the process for each cell (A to F), testing one at a time.
4. Record which cells cause the bulb to glow.

(iii) Activity:
       If the bulb lights up: The cell is working.
       If the bulb does not light up: The cell is not working.
       Test all six cells and note which are functional based on the bulb’s response.

11. An LED requires two cells in series to glow. Tanya made the circuit as shown in Fig. 3.21. Will the lamp glow? If not, draw the wires for correct connections.

Answer: No, the LED will not glow due to incorrect connections between the two cells which will not allow the current to flow.

To correct it:

(i) Connect the positive terminal of the first cell to the negative terminal of the second cell (series connection).
(ii) Connect the free negative and positive terminals to the LED accordingly.