Circuits and their Components

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CLASS VII Science ~6 marks/year Ch 3 of 12
Circuits and their Components

Class 7 · Science · NCERT chapter notes · Akanksha Classes

Snapshot
  • An electric circuit is a complete, closed path along which electric current can flow from one terminal of a cell, through the components, back to the other terminal.
  • A cell has two terminals: a positive (+) terminal and a negative (−) terminal; it provides the energy that makes current flow. A battery is two or more cells joined together.
  • An electric bulb glows when current passes through its thin filament; the filament is connected to the bulb's two terminals.
  • A switch is used to open or close a circuit — an open (incomplete) circuit has no current; a closed (complete) circuit allows current to flow.
  • Conductors (most metals) allow electric current to pass; insulators (rubber, plastic, wood, glass) do not.
  • Current produces a heating effect (used in heaters, irons, geysers) and a magnetic effect (used in electromagnets).
  • We must always use cells/batteries in school experiments, never the mains, and follow safety rules to avoid electric shock.
  • Weightage: ~6 marks/year — circuit symbols/diagrams, open vs closed circuits, conductors/insulators, and effects of current.
Detailed Notes

1. Electric Current and the Electric Cell

Electrical appliances such as torches, fans and bulbs work because of electric current — the flow of electric charge through a material. To make current flow, we need a source of electrical energy. In our experiments, this source is an electric cell.

An electric cell is a device that provides electrical energy. It has two terminals — a positive terminal (+) (the metal cap) and a negative terminal (−) (the flat metal disc).

Inside a cell, chemicals store energy. When the cell is connected in a complete circuit, this stored chemical energy is converted into electrical energy, which drives the current. A cell stops working when its stored chemicals are used up.

A battery is a combination of two or more cells joined together to provide more energy. In a battery, cells are usually joined so that the positive terminal of one cell touches the negative terminal of the next.

2. The Electric Bulb

An electric bulb gives light when current passes through it. Inside the glass case is a very thin coiled wire called the filament.

  • The filament is the part that glows and gives light when current flows through it.
  • The filament is connected to the two thick wires that lead to the bulb's two terminals (the metal tip at the base and the metal case at the side).
  • For a bulb to glow, current must flow through the filament; if the filament is broken (fused), the bulb does not glow even in a complete circuit.
The filament of a bulb glows because the current heats it until it becomes white-hot and gives off light. A bulb has two terminals, both connected to the filament.

3. The Electric Circuit

To light a bulb, we connect a cell and a bulb with wires so that current can flow. This complete path is called an electric circuit.

An electric circuit is a complete (closed) path for the flow of electric current. By convention, current flows from the positive terminal of the cell, through the wires and components, to the negative terminal.

3.1 Open and closed circuits

  • Closed circuit: the path is complete; there are no gaps; current flows and the bulb glows.
  • Open circuit: there is a break or gap somewhere; current cannot flow and the bulb does not glow.

A bulb may fail to glow for several reasons: the cell is used up, the connecting wires are loose, the filament is broken (fused), or a switch is open. A good circuit needs firm, clean connections.

NCERT activity — Lighting a bulb with a cell

Connect one end of a wire to the positive terminal of a cell and the other end to one terminal of a bulb. Connect a second wire from the other terminal of the bulb to the negative terminal of the cell. The bulb glows because the circuit is complete (closed). Now remove one wire end — the bulb goes off because the circuit is now open (broken). This shows that current flows only in a complete path.

4. The Switch

A switch is a simple device that opens or closes a circuit, so we can turn current on or off without disconnecting wires.

A switch "on" closes the circuit (current flows, bulb glows); a switch "off" opens the circuit (current stops, bulb is off).

A switch works by joining or separating two ends of the circuit. When the two ends touch (through a conducting strip), the circuit is closed; when they are separated, the circuit is open. Switches in homes work on the same principle.

NCERT activity — Making a simple switch

Fix two drawing pins on a small wooden board, a little apart. Connect a wire from each pin into the circuit. Take a metal paper clip or safety pin and place it so it can swing to touch both pins. When the clip touches both pins, the circuit is closed and the bulb glows; when it touches only one pin, the circuit is open and the bulb is off. You have built a switch.

5. Circuit Diagrams and Symbols

Drawing every component as a picture is slow and untidy. Scientists use simple symbols to draw a circuit diagram. This makes circuits easy to draw and understand.

Component What it does
CellLong line = positive terminal, short thick line = negative terminal.
BatteryTwo or more cell symbols drawn side by side.
BulbA circle with a cross inside it.
Switch (open)A gap in the line — current cannot flow.
Switch (closed)A closed line — current can flow.
Connecting wireA straight line joining components.
In a cell symbol, the longer line is the positive (+) terminal and the shorter, thicker line is the negative (−) terminal.

6. Conductors and Insulators

Not all materials let electric current pass through them. We test materials by putting them in a gap in a circuit (a tester) and seeing if the bulb glows.

  • Conductors: materials that allow electric current to pass through. Examples: copper, aluminium, iron, gold, silver and most metals; the human body and water with dissolved salts also conduct.
  • Insulators: materials that do not allow electric current to pass. Examples: rubber, plastic, dry wood, glass, paper, cloth and air.
Conductors allow current to flow; insulators do not. The metal parts of wires are conductors, while their outer covering is an insulator (usually plastic).

Both are useful: conductors carry the current where it is needed, and insulators keep current from leaking and protect us from electric shock. This is why electric wires have a metal core (conductor) covered with plastic (insulator).

NCERT activity — Testing materials with a tester

Make a circuit with a cell and a bulb, but leave a gap with two free wire ends. Touch the two ends to different objects — an iron nail, a copper coin, a rubber band, a plastic scale, a wooden stick, a pencil lead. The bulb glows for the metal objects and the pencil lead (conductors) but does not glow for the rubber, plastic and wood (insulators). This lets you classify materials into conductors and insulators.

7. Effects of Electric Current

When electric current flows, it can produce useful effects. Class 7 focuses on two of them.

7.1 Heating effect of current

When current passes through a wire, the wire becomes hot. This is called the heating effect of current. A thin wire of special material (like nichrome) heats up a lot and is used as a heating element.

  • Appliances using the heating effect: electric heater, iron, geyser, toaster, hair dryer, electric kettle.
  • In an electric bulb, the heating effect makes the filament white-hot so it gives light.
The heating effect: current flowing through a wire produces heat. The amount of heat depends on the material, length and thickness of the wire.

7.2 Magnetic effect of current

When current flows through a wire, it behaves like a magnet — this is the magnetic effect of current. A coil of wire wound on an iron piece becomes a strong magnet when current flows; this is called an electromagnet.

  • An electromagnet works only while current flows; switch off the current and the magnetism disappears.
  • Uses: electric bells, cranes that lift heavy iron and steel, loudspeakers, and many machines.
NCERT activity — Making an electromagnet

Wind insulated copper wire many times around an iron nail. Connect the two ends of the wire to a cell through a switch. When the switch is on, current flows and the nail can attract small iron pins — it has become an electromagnet. Switch off, and the pins fall off because the magnetism vanishes. This demonstrates the magnetic effect of current.

8. Safety with Electricity

Electricity is very useful but can be dangerous if handled carelessly. Following safety rules prevents electric shocks and fires.

  • In school experiments, always use cells or batteries, never the mains supply.
  • Do not touch electrical switches, plugs or wires with wet hands — water conducts electricity.
  • Never connect the two terminals of a cell directly with a wire only (a "short circuit") — the wire heats up and the cell drains quickly.
  • Replace damaged wires with broken insulation; the bare metal can cause shock.
  • Do not insert fingers or metal objects into sockets.
Water conducts electricity, so wet hands and wet floors greatly increase the danger of electric shock. Always keep electrical things dry.
Practice MCQs
1. An electric cell has:
  1. Only one terminal
  2. Two terminals: positive and negative
  3. Three terminals
  4. No terminals
Answer: (B) A cell has a positive (+) terminal (metal cap) and a negative (−) terminal (flat disc).
2. A battery is:
  1. A single cell
  2. Two or more cells joined together
  3. A type of bulb
  4. A kind of switch
Answer: (B) A battery is a combination of two or more cells joined together to give more energy.
3. The part of a bulb that glows and gives light is the:
  1. Glass case
  2. Filament
  3. Metal base
  4. Switch
Answer: (B) The thin filament inside the bulb glows white-hot when current passes through it.
4. A complete path along which electric current flows is called a/an:
  1. Insulator
  2. Electric circuit
  3. Conductor
  4. Terminal
Answer: (B) A complete (closed) path for current is an electric circuit.
5. In which type of circuit does the bulb glow?
  1. Open circuit
  2. Closed circuit
  3. Broken circuit
  4. Circuit with a gap
Answer: (B) The bulb glows only in a closed (complete) circuit; an open circuit has a gap and no current flows.
6. A switch is used to:
  1. Increase the cell's energy
  2. Open or close a circuit
  3. Make the bulb brighter only
  4. Store electricity
Answer: (B) A switch opens or closes the circuit to turn the current off or on.
7. Which of the following is a good conductor of electricity?
  1. Rubber
  2. Plastic
  3. Copper
  4. Dry wood
Answer: (C) Copper, like most metals, is a good conductor. Rubber, plastic and dry wood are insulators.
8. Materials that do NOT allow electric current to pass through them are called:
  1. Conductors
  2. Insulators
  3. Cells
  4. Filaments
Answer: (B) Insulators (like plastic, rubber, glass) do not allow current to pass.
9. Electric wires are covered with plastic because plastic is a/an:
  1. Conductor
  2. Insulator
  3. Magnet
  4. Cell
Answer: (B) Plastic is an insulator that prevents current from leaking and protects us from shock.
10. The heating effect of electric current is used in a/an:
  1. Electric bell
  2. Electric iron
  3. Magnetic crane
  4. Loudspeaker
Answer: (B) An electric iron, heater, geyser and toaster use the heating effect of current.
11. A coil of wire wound on an iron nail that becomes a magnet when current flows is called a/an:
  1. Permanent magnet
  2. Electromagnet
  3. Insulator
  4. Filament
Answer: (B) Such an arrangement is an electromagnet; it is magnetic only while current flows.
12. In a cell symbol, the longer line represents the:
  1. Negative terminal
  2. Positive terminal
  3. Switch
  4. Bulb
Answer: (B) The longer line is the positive (+) terminal; the shorter, thicker line is the negative (−) terminal.
13. A bulb in a complete circuit does NOT glow. A possible reason is:
  1. The filament is broken (fused)
  2. The wires are made of copper
  3. The cell is fully charged
  4. The switch is closed
Answer: (A) If the filament is broken, current cannot pass through it and the bulb does not glow even when the circuit looks complete.
14. Why should we not touch electrical switches with wet hands?
  1. Water is an insulator
  2. Water conducts electricity and can cause a shock
  3. Wet hands make switches work faster
  4. It saves electricity
Answer: (B) Water (especially with dissolved salts) conducts electricity, so wet hands increase the risk of electric shock.
15. An electromagnet differs from a permanent magnet because it:
  1. Is always magnetic
  2. Is magnetic only while current flows
  3. Cannot attract iron
  4. Needs no electricity
Answer: (B) An electromagnet shows magnetism only while current flows; switch off the current and the magnetism disappears.
Important Questions
Q1. What is an electric circuit? When does a bulb glow in a circuit? (2 marks)
Answer: An electric circuit is a complete (closed) path along which electric current can flow, starting from one terminal of a cell, through the connecting wires and components, and back to the other terminal. A bulb glows only when the circuit is closed (complete) and current is able to flow through its filament. If the circuit is open (has a gap), the bulb does not glow.
Q2. Differentiate between an open circuit and a closed circuit. (2 marks)
Answer: In a closed circuit, the path is complete with no gaps, so current flows and the bulb glows. In an open circuit, there is a break or gap in the path (for example, the switch is off or a wire is loose), so current cannot flow and the bulb does not glow.
Q3. Distinguish between conductors and insulators with two examples each. (3 marks)
Answer: Conductors are materials that allow electric current to pass through them; examples are copper and iron (most metals). Insulators are materials that do not allow electric current to pass through them; examples are rubber and plastic (also glass, dry wood). Both are useful: conductors carry current, while insulators prevent leakage of current and protect us from shock, which is why wires have a metal core covered with plastic.
Q4. Name and explain any two effects of electric current with one use of each. (3 marks)
Answer: (1) Heating effect — when current flows through a wire, the wire becomes hot. It is used in electric heaters, irons and geysers. (2) Magnetic effect — when current flows through a coil of wire, it behaves like a magnet (an electromagnet). It is used in electric bells and cranes that lift heavy iron objects. The electromagnet works only while the current flows.
Q5. A bulb does not glow even though the circuit looks complete. List three possible reasons. (3 marks)
Answer: (1) The filament of the bulb may be broken (fused), so current cannot pass through it. (2) The cell may be used up (dead), so it cannot drive a current. (3) The connecting wires may be loose or the connections dirty, breaking the path. (Other reasons: a switch may be open, or the wires may be connected to the wrong terminals.)
Q6. State any four safety rules to follow while using electricity. (2 marks)
Answer: (1) Use only cells or batteries in school experiments, never the mains supply. (2) Do not touch switches, plugs or wires with wet hands, because water conducts electricity. (3) Never connect the two terminals of a cell directly with only a wire (short circuit), as it heats up and drains the cell. (4) Replace wires with broken or damaged insulation, and never insert fingers or metal objects into sockets.
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