Electricity: Circuits and their Components | CBSE Class 7 Science Notes

This note covers uses of electricity, torchlights, electric cells and batteries, incandescent lamps, light emitting diodes, complete and broken circuits, switches, circuit symbols, testing materials, conductors, insulators and electrical safety.

How do we use electricity and explore a torchlight safely?

Electricity operates many devices around us. Its uses include lighting, cooking, heating, cooling, communication, entertainment and transportation. Grouping these uses helps us recognise how many everyday activities depend on an electrical supply.

Where does the electrical supply come from?

Electricity can be generated using wind, sunlight, falling water, natural gas or coal. The supply reaches homes and factories through wires. A hydroelectric power house uses the force of falling water to generate electricity, as at the Bhakra Nangal Dam.

A torchlight, also called a torch or flashlight, offers a portable example to examine. Its lamp is the part that produces light. Sliding its switch, the device that completes or breaks its electrical path, makes the lamp glow in one position and stop glowing in the other.

Inside a torch, you may find two or more electric cells. An electric cell is a portable source of electrical energy. Each cell has two terminals, or connection points, called positive and negative.

A battery is formed by joining two or more cells with the positive terminal of one connected to the negative terminal of the next. These cells supply the energy needed by the torch lamp.

Note: Use only cells or batteries, such as those used in torchlights, clocks, radios or remotes, for these investigations. Never experiment with the power supply at home or school. Electricity from portable generators can also be dangerous.

What are electric cells and how do they form a battery?

Every electric cell has two terminals, the connection points through which it is joined to other parts of a circuit. An electrical circuit is an arrangement providing a path for electric current, the flow of electricity, through its connected parts.

How can the two cell terminals be identified?

One terminal is called positive and is marked with the plus sign, +. The other is called negative and is marked with the minus sign, −. These signs identify the two terminals when making connections.

On the cylindrical cell used in the torch activity, the small projecting metal cap is the positive terminal. The flat metal disc at the other end is the negative terminal. Looking at both ends helps identify the cell's connections before placing it in a holder.

The cell illustration labels the projecting cap as positive and the opposite flat end as negative. Separate arrangements show two cells and four cells joined end to end, with unlike terminals next to each other.

See Figs. 3.2 and 3.3 in your NCERT textbook

How must cells be connected?

Definition: A battery is a combination of two or more cells connected with the positive terminal of one cell joined to the negative terminal of the next.

In a torch using two cells, the lamp glows when the cells are arranged in this order. Connecting more than one cell provides energy to the circuit for a longer time and/or more energy. This does not mean that every device uses the same number of cells.

Cells may also lie side by side in a device. Inside such a battery compartment, you will usually see a thick wire or metal strip connecting the positive terminal of one cell to the negative terminal of the next. Positive and negative signs are typically printed inside to guide placement.

Note: The word battery is also used for a single cell, including the single cell that powers a mobile phone. Distinguish this usage from the connected combination of cells being constructed here.

How does an incandescent lamp produce light?

An incandescent lamp produces light when a thin wire inside it becomes hot and glows. This thin wire is the filament. Many old torchlights still use incandescent lamps. A glass bulb surrounds the filament, allowing its glow to be seen.

What connects the filament to the outside?

Two thicker wires support the filament. One connects to the metal case at the base of the lamp. The other connects to the metal tip at the centre of the base. The metal case and metal tip form the lamp's two terminals.

The two terminals are fixed so that they do not touch each other. The lamp also contains insulators, materials through which electric current cannot pass, which keep the metal parts apart. The supporting wires and separate terminals provide a route through the filament when the lamp is connected correctly.

The labelled lamp image and simplified drawing show the glass bulb, filament, two thick supporting wires, insulating support, metal case and metal tip. The case and tip are labelled as the two separate terminals.

See Fig. 3.4 in your NCERT textbook

What makes the filament glow?

When current passes through the filament, the filament gets hot and produces light. The whole glass bulb is not the filament. Similarly, the thicker supporting wires and the thin glowing filament have different roles, even though they are connected inside the lamp.

For an incandescent lamp, either lamp terminal can connect to the positive terminal of the cell. The other lamp terminal must connect to the negative terminal of the cell. The lamp glows as long as the circuit is complete and current flows through the filament.

A fused lamp is one that no longer glows, usually because its filament is broken. A broken filament interrupts the current's path. Connecting wires correctly outside the lamp cannot restore the missing connection inside a broken filament.

How is an LED different from an incandescent lamp?

LED stands for Light Emitting Diode, a lamp component that produces light when current passes through it in the permitted direction. Many torches in use today have LED lamps. A torch may use one or more LEDs, sometimes of different shapes.

Unlike an incandescent lamp, an LED has no filament. The small LEDs used in the activity have two attached wires of unequal lengths. The longer wire is the positive terminal and the shorter wire is the negative terminal.

Why does connection direction matter?

Current can pass through an LED in one direction only. In the two-cell activity, connect the battery's positive terminal to the LED's longer wire and the battery's negative terminal to its shorter wire. The LED glows with these connections.

If the wires connected to the LED are interchanged, it does not glow. The connections must therefore match the terminal signs. This is different from an incandescent lamp, for which interchanging the connections to the lamp's two terminals does not prevent it from glowing in a complete working circuit.

How is the two-cell LED activity arranged?

A cell holder holds cells in position and provides connections to them. Place each cell with its negative terminal towards the spring in the holder. The holder's connection to the positive end of one cell is the battery's positive terminal; its connection to the negative end of the other is negative.

The panels show an empty holder, the holder with two cells, and two LED connections. The LED is shown glowing in panel (c), with matching positive and negative connections, and not glowing with the reversed connections in panel (d).

See Fig. 3.10 in your NCERT textbook

How can a cell and an incandescent lamp form a working circuit?

To make the torch lamp glow outside its torch, connect the cell and lamp using wires. A lamp holder supports the lamp and provides places to attach its connecting wires. Using holders helps keep the components in position during the investigation.

How are the components prepared?

1. Collect an electric cell, an incandescent torch lamp, a cell holder, a lamp holder and four lengths of electric wire.
2. Remove about 1 centimetre, written as 1 cm, of plastic covering from both ends of each wire to expose the metal.
3. Attach two wires to the ends of the cell holder. Insert the cell with its negative terminal towards the holder's spring.
4. Attach two wires to the screws of the lamp holder. Turn the lamp into the holder so that it is fixed in place.
5. Connect one lamp terminal to one cell terminal and the other lamp terminal to the other cell terminal, completing the path through the lamp.

If holders are unavailable, electrical tape can attach wires to the two cell terminals and the two lamp terminals. The exposed metal at each wire end must make the intended connection. Leaving plastic covering between the metal parts prevents that connection.

What should be predicted and observed?

A prediction is the expected result before testing; an observation is what is actually noticed. First predict whether a proposed connection will make the lamp glow. Then assemble that arrangement and record whether the lamp glows.

Compare arrangements that work with arrangements that do not. A working path connects both cell terminals through the lamp's two terminals and filament. An incomplete set of connections does not provide this path through the filament.

A cell in its holder is connected by wires to a lamp in its holder. The wires reach the two sides of the lamp holder, forming a complete path, and the lamp is shown glowing.

See Fig. 3.9 in your NCERT textbook

The useful comparison is between the connections, not the appearance of the wire arrangement. Trace where each wire starts and ends. This shows whether the current can pass through the lamp and return to the other terminal of the cell.

When does current flow and why might a lamp fail to glow?

A complete circuit, also called a closed circuit, provides an unbroken path for current through the connected components. The lamp glows only when current passes through the circuit. In a working incandescent lamp circuit, the path includes the filament, rather than simply the lamp's outer glass surface.

Which direction is used for electric current?

The direction of electric current in a closed electrical circuit is taken to be from the positive terminal to the negative terminal of the cell. This direction is used when tracing the external path through the wires and lamp.

For an incandescent lamp, changing which lamp terminal connects to the positive cell terminal does not prevent current from passing through an intact filament. For an LED, the positive and negative connections must be correct because it allows current through in one direction only.

How do different faults interrupt the path?

These conditions should be distinguished when explaining a result. A broken connection outside an incandescent lamp and a broken filament inside it both interrupt the path, but they occur in different places. Reversed LED connections involve the direction in which that component permits current.

Note: Do not call every non-glowing lamp fused. An incandescent lamp may have fused, usually because of a broken filament, but an incomplete external circuit also prevents a working lamp from glowing.

How does a switch open or close a circuit?

A switch is a simple device that completes or breaks a circuit. It controls whether the connected path allows current to pass. A switch does not supply the electrical energy: the cell or battery is the source in these simple circuits.

How can a simple switch be made?

1. Collect two drawing pins, a safety pin or paper clip, two wires and a small piece of cardboard.
2. Pass one drawing pin through the ring of the safety pin and fix it to the cardboard. Leave the safety pin free to rotate.
3. Fix the second drawing pin where the free end of the safety pin can reach and touch it.
4. Connect one wire to each drawing pin. Join this switch with the cell and lamp in the circuit.

When the safety pin touches both drawing pins, it bridges the gap. The circuit is closed, meaning that the path is complete. This is the switch's ON position. Current flows through the complete working circuit and the lamp glows.

When the free end of the safety pin does not touch the second drawing pin, a gap remains. The circuit is open, meaning that its path is broken. This is the switch's OFF position. The gap prevents current flow and the lamp does not glow.

The safety-pin switch is shown with its free end away from the second drawing pin and then touching it. The corresponding cell-and-lamp circuits show the lamp unlit with the switch OFF and glowing with the switch ON.

See Figs. 3.11 and 3.12 in your NCERT textbook

Must the switch occupy a particular position?

A switch can be placed anywhere in the simple circuit. Its task remains to complete or break the path. The switches used for lights and other devices at home operate on the same principle, although their designs differ from the cardboard-and-safety-pin arrangement.

The words open and closed refer to the electrical path. A closed circuit lets current flow; an open circuit interrupts it. Linking each term to the presence or absence of a gap helps avoid confusing the two states.

How do symbols make circuit diagrams easier to understand?

A circuit diagram represents an electrical circuit using symbols for its components. A symbol is a simple mark or drawing that stands for a component. Using symbols makes circuits easier to draw and understand than detailed pictures of their physical parts.

What does each symbol represent?

The signs + and − continue to mean positive and negative when used beside component symbols. For the LED symbol, identifying the permitted current direction is essential. The two arrows are associated with light emission; they do not represent two separate connecting wires.

The cell's long and short lines are marked positive and negative respectively. The LED triangle points towards the negative side, and two arrows point outwards above the symbol to indicate light emission.

See Fig. 3.13 in your NCERT textbook

How is a connected arrangement represented?

Identify the cell or battery, lamp, wires and any switch in the arrangement. Replace each component with its symbol while keeping the same connections. Check the terminal signs and show a gap if the switch is OFF.

Diagram (a) shows a cell, an incandescent lamp and an open switch connected by straight lines. Diagram (b) shows an LED connected to a battery in a complete path, with the LED symbol directed consistently with the battery connections.

See Fig. 3.14 in your NCERT textbook

Standard symbols allow people from different countries and industries to understand the same drawing. The important information is how the components connect and whether the path is complete. A circuit diagram therefore preserves the electrical arrangement without copying every physical detail of a holder or wire.

How can a tester distinguish conductors from insulators?

A conductor of electricity is a material through which electric current can flow easily. Conductors are also called good conductors. Insulators are materials through which current cannot pass; they are also called poor conductors of electricity.

A conduction tester is a cell-and-lamp arrangement with two free wire ends used to check materials. An object placed between these ends becomes part of the path. Whether the lamp glows helps identify whether current passes through the object.

How is the tester used?

1. Connect an electric cell and a lamp with wires, leaving two wire ends free so that there is a gap in the path.
2. Briefly touch the two free ends together. If the lamp glows, the tester is ready to use.
3. Collect objects made from different materials, such as a metal spoon, a plastic scale, a wooden block, a rubber eraser and a glass bangle.
4. Touch the free ends to different parts of one object so that the object bridges the gap. Make sure the wire ends do not touch each other directly.
5. Observe whether the lamp glows and record the object, its material and the result. Repeat with the other objects.
6. Use the observations to decide which materials allow current to pass and which do not.

Panel (a) shows a cell and lamp joined with wires that leave two ends free. Panel (b) shows these ends touching a metal spoon, which completes the path, and the lamp is shown glowing.

See Fig. 3.15 in your NCERT textbook

What do the observations mean?

The lamp glows for some materials only. Metals conduct electricity, while plastic, rubber and ceramics are insulators. In the recorded example of a wooden stick, the lamp does not glow. The material matters, rather than simply the name or shape of the object.

The preliminary check with the wire ends touching is important: it establishes that the tester lights before objects are compared. During testing, keep the wire ends separated by the object. If they touch each other directly, the current has a connection that does not test the object.

Record observations before conclusions. “The lamp glows” describes the observation. “The tested material conducts electricity” gives the conclusion from that observation. Keeping these distinct makes the investigation and its explanation clear.

Why do electrical devices need both conductors and insulators?

Conductors and insulators perform different useful roles. A conducting path allows current to reach the parts that need it. Insulating covers help protect people from electric shocks. The use of one does not make the other unnecessary.

Which materials are used and why?

Metals are used to make electrical wires because they conduct electricity. Silver, copper and gold are the best electrical conductors. Mainly copper is used for wires because of its comparatively lower cost and abundant supply. Different uses require different types of electrical wire.

Conducting parts occur in electrical wires, switches, plug connectors and sockets. Rubber, plastics and ceramics are used as insulating materials to cover wires, plug tops and switches. Insulation means this protective use of insulating material around conducting parts.

The human body also conducts electricity. Current passing through it may cause severe injury or even death. Never touch switches or plugs with wet hands, use electrical devices in wet areas, or handle equipment with damaged insulation or broken plugs.

How do portable sources serve different devices?

Cells and batteries come in different shapes and sizes. Cylindrical batteries are used in torchlights, clocks, remotes and toys. Button cells are used in watches and hearing aids. Rechargeable batteries are used in mobile phones, laptops and electric vehicles.

Electricity from batteries usually powers small devices and is called Direct Current (DC). Electricity supplied from power plants to wall sockets is called Alternating Current (AC) and can run larger appliances. These names distinguish the two types of supply.

The word usually matters when describing batteries and small devices: rechargeable batteries are also used in electric vehicles. For the activities here, use the specified cells and batteries. The ability of a wall socket to supply appliances is not a reason to use it for experiments.

Glossary

• Electric cell — A portable source of electrical energy with two terminals, one positive and the other negative.
• Terminal — A connection point by which a cell, lamp or another component joins an electrical circuit.
• Battery — Two or more cells connected with the positive terminal of one joined to the negative terminal of the next.
• Filament — The thin wire inside an incandescent lamp that becomes hot and glows when current passes through it.
• Incandescent lamp — A lamp that produces light through a filament which gets hot and glows.
• LED — Light Emitting Diode, a component without a filament that glows when current passes through in its permitted direction.
• Electrical circuit — An arrangement of connected components providing a path for electric current to flow.
• Closed circuit — A circuit with a complete path through which electric current can flow.
• Open circuit — A circuit containing a gap that interrupts the path and prevents current flow.
• Switch — A device that controls current flow by completing or breaking an electrical circuit.
• Circuit diagram — A representation of an electrical circuit using symbols for the components and their connections.
• Conductor — A material through which electric current flows easily, allowing it to form a conducting path.
• Insulator — A material through which electric current cannot pass, used to cover or separate conducting parts.
• Conduction tester — A cell-and-lamp arrangement with free wire ends used to identify materials through which current passes.

Common errors and misconceptions

• Misconception: The switch supplies electricity to the lamp. Correct: The cell or battery supplies electrical energy. The switch controls current by completing or breaking the path through the connected circuit.
• Misconception: A closed circuit stops current because it is closed. Correct: Closed means the path is complete. An open circuit has a gap, which prevents current from flowing through it.
• Misconception: The glass bulb is the filament. Correct: The filament is the thin wire inside the glass bulb. It becomes hot and glows when current passes through it.
• Misconception: An LED can be connected either way like an incandescent lamp. Correct: In the LED activity, the longer positive wire must connect to battery positive and the shorter negative wire to battery negative.
• Misconception: Putting two cells together in any direction makes the torch work. Correct: Connect the positive terminal of one cell to the negative terminal of the next when forming the battery.
• Misconception: Every lamp that fails to glow has fused. Correct: An external gap or an insulating covering left between connecting metal parts also prevents a working incandescent lamp from glowing.
• Misconception: Insulators have no useful role because they stop current. Correct: Their coverings help protect people from electric shocks, while conducting parts provide the required electrical connections.

Exam-style questions with model answers

Q1. A cylindrical torch cell has a projecting metal cap at one end and a flat metal disc at the other. Identify its positive and negative terminals. [2 marks]

1. The projecting metal cap is the positive terminal of the electric cell, marked with the plus sign.
2. The flat metal disc at the opposite end is the negative terminal, marked with the minus sign.

Q2. A working cell, working incandescent lamp and conducting wires form a circuit containing a safety-pin switch. All connections except the switch gap are secure. Explain what happens when the pin touches both drawing pins and when it is moved away from the second pin. State the switch's role. [3 marks]

1. When the safety pin touches both drawing pins, it closes the gap. The circuit is complete, current passes through the lamp and it glows.
2. When the safety pin moves away from the second drawing pin, the gap opens the circuit. Current cannot flow and the lamp does not glow.
3. The switch completes or breaks the circuit. It controls current flow; the cell, rather than the switch, supplies the electrical energy.

Q3. A working LED with a longer positive wire and a shorter negative wire is connected to a working two-cell battery suitable for it. All connections are secure, but the longer wire goes to battery negative and the shorter wire to battery positive. Predict the result, explain it and give both corrected connections. [4 marks]

1. The LED does not glow with the connections described, even though the wires are securely attached to the battery and LED.
2. An LED allows current to pass through in one direction only. The stated connections reverse the direction required for it to glow.
3. Connect the LED's longer positive wire to the positive terminal of the battery.
4. Connect the LED's shorter negative wire to the negative terminal of the battery, completing the correct arrangement.

Q4. A working cell and working incandescent lamp are connected with wires, but plastic covering has been left between the wire metal and the terminals. No exposed wire metal touches those terminals. Will the lamp glow? Explain the reason and the correction. [3 marks]

1. The lamp will not glow, although the cell and incandescent lamp themselves are working, because the intended electrical connections have not been made.
2. Plastic is an insulator. The covering between the wire metal and the terminals prevents current from passing through a complete path that includes the filament.
3. Expose the metal at the connecting wire ends and attach it to the appropriate terminals, completing the circuit through the lamp.

Q5. You have a working cell, a working incandescent lamp, connecting wires, a metal spoon, a plastic scale and a rubber eraser. Describe a conduction test: give the setup, readiness check, testing method, expected comparison and the reason for that comparison. [5 marks]

1. Join the cell and incandescent lamp with connecting wires, leaving two wire ends free. These ends make a gap where an object can be inserted.
2. Briefly touch the two free wire ends together. The working tester's lamp glows, showing that it is ready to compare the supplied objects.
3. Touch the ends to different parts of each object in turn, making the object bridge the gap. Keep the ends from touching each other directly.
4. The lamp glows with the metal spoon. It does not glow when the plastic scale or rubber eraser is placed between the free ends.
5. Metal conducts electricity and allows current through the completed path. Plastic and rubber are insulators, so they do not allow the required current to pass.

Q6. Describe an incandescent torch lamp in five points: identify the light-producing part, state how it produces light, describe its supports, identify its two terminals and explain why a broken filament prevents it from glowing. [5 marks]

1. The light-producing part is the filament, a thin wire inside the glass bulb. The filament is distinct from the glass surrounding it.
2. When electric current passes through the filament, it becomes hot and glows. This glow produces the light from the incandescent lamp.
3. Two thicker wires support the filament. They also connect it to the separate metal parts at the base of the lamp.
4. One supporting wire connects to the metal case and the other to the central metal tip. The case and tip form the two terminals.
5. A broken filament interrupts the path through the lamp, stopping current flow. A fused incandescent lamp usually has a broken filament and cannot glow through that broken path.

Q7. A circuit drawing represents a cell by a long line and a short parallel line. Its LED symbol has a triangle and two outward-pointing arrows. Explain the meaning of the two cell lines, the triangle and the arrows. [3 marks]

1. The longer line represents the positive terminal of the cell, while the shorter parallel line represents its negative terminal. These identify the source connections.
2. The triangle in the LED symbol points in the direction in which current can pass through the LED, so its orientation matters.
3. The two outward-pointing arrows indicate that the LED emits light. They are part of its symbol rather than additional connecting wires.

Q8. A metal wire has an insulating plastic covering. State why the metal is useful inside and why the plastic is useful outside. [2 marks]

1. The metal conducts electricity, allowing current to pass through the wire as part of the circuit.
2. The plastic is an insulator. Its covering helps protect people from electric shocks when conducting parts are enclosed.

Key takeaways

• An electric cell is a portable source of electrical energy with positive and negative terminals.
• Connect the positive terminal of one cell to the negative terminal of the next to form the battery arrangement.
• An incandescent lamp produces light when current passes through its filament, making the thin wire hot enough to glow.
• An LED has no filament and allows current through in one direction only, so its terminal connections matter.
• A switch closes a circuit to permit current flow or opens it by creating a gap.
• Circuit diagrams use component symbols, with the long and short cell lines representing positive and negative terminals respectively.
• Metals provide conducting paths, while insulating materials such as plastic and rubber help protect people from electric shocks.
• Use cells or batteries for these investigations, and never experiment with the power supply at home or school.

Test yourself

What are the two terminals of an incandescent torch lamp?

They are the metal case at the base and the metal tip at the centre of the base.

Which wire of the LED used in the activity is positive?

The longer attached wire is positive; the shorter wire is the LED's negative terminal.

In which direction is current taken to flow through the external circuit?

Current is taken to flow from the positive terminal of the cell towards its negative terminal.

What does an open switch introduce into a circuit?

It introduces a gap that breaks the path and prevents current from flowing through the circuit.

Why is the lamp checked before objects are tested with a conduction tester?

Touching the free wire ends together and seeing the lamp glow establishes that the tester is ready for use.

Why is copper mainly used for making electrical wires?

Copper is among the best electrical conductors and has a comparatively lower cost and abundant supply.

Does the term battery always mean several cells?

No. The word battery is also used for a single cell, including the single cell powering a mobile phone.

Why must damaged insulation not be ignored?

Insulation helps protect people from electric shocks. The human body conducts electricity, and current through it may cause severe injury or death.