Class 7: Electric Current and its Effects

Electric Current and its Effects

Symbols of Electric Components

Questions about circuit symbols and representations

1. What are symbols of electric components? Symbols are simple drawings or signs used to represent electric components like cells, batteries, bulbs, and switches in a circuit diagram.
2. In the symbol of an electric cell, what do the longer and shorter lines represent? In the symbol of an electric cell, the longer line represents the positive terminal and the shorter, thicker line represents the negative terminal.
3. How are wires represented in circuit diagrams? Wires used to connect different components in a circuit are represented by straight lines.

Battery

Questions about battery composition and usage

1. What is a battery? A battery is a combination of two or more cells connected together.
2. How are the cells connected to make a battery? The positive terminal of one cell is connected to the negative terminal of the next cell to make a battery.
3. Where are batteries used? Batteries are used in torches, toys, transistors, TV remote controls, and many other devices.
4. Are cells always placed one after the other in devices? No, in some devices the cells are placed side by side, but even then the positive terminal of one cell is connected to the negative terminal of the next cell by metal strips.
5. How does a device help us place cells correctly? Inside the battery compartment, '+' and '–' symbols are marked to help us place the cells correctly.
6. What is the use of a cell holder? A cell holder helps in holding two or more cells together with proper connections to make a battery.
7. How are wires attached to a cell holder? Wires are connected to the two metal clips of the cell holder, and then the battery is ready for use.

Circuit Diagrams

Questions about circuit functionality and components

1. Why is it easier to use a circuit diagram instead of actual drawings? It is easier because drawing symbols is simple and neat, whereas showing actual cells, wires, and bulbs is difficult.
2. When does a bulb glow in an electric circuit? A bulb glows only when the switch is in the 'ON' position and the circuit is complete.
3. Can the switch be placed anywhere in the circuit? Yes, the switch can be placed anywhere in the circuit.
   A switch can be connected to either positive or negative terminal, it will work the same. But for safety reasons, it is better to connect it on the positive terminal.
4. What do we call a circuit when the switch is in the 'ON' position? When the switch is in the 'ON' position, the circuit is closed and current flows through it.
5. What do we call a circuit when the switch is in the 'OFF' position? When the switch is in the 'OFF' position, the circuit is open and no current flows.
6. Would the bulb glow if its filament is broken? No, the bulb would not glow because the circuit would be incomplete.

Heating Effect of Electric Current

Questions about thermal effects of electricity

1. Who invented the electric bulb? Thomas Alva Edison
2. What is an Incandescent Bulb? It is the old type of bulb we used in homes before LED and CFL. It gives light when electricity passes through a thin wire called filament inside it. It works on the heating effect of electric current.
   🔬 How it works:
   1. When current flows → the filament (made of tungsten metal) resists the flow.
   2. This resistance makes the filament very hot (around 2500–3000 °C).
   3. When it becomes so hot, it glows and produces light.
   4. The bulb is filled with an inert gas (like argon or nitrogen) so the filament doesn't burn quickly.
3. What is the thin wire inside a bulb called? The thin wire inside a bulb is called the filament.
4. Why does the filament glow when current passes through it? The filament glows because it becomes hot when electric current passes through it.
5. Why does a glowing bulb become warm? A glowing bulb becomes warm due to the heating effect of electric current on the filament.
6. What is meant by the heating effect of electric current? When an electric current passes through a wire, the wire becomes hot. This is called the heating effect of electric current.
7. Name some appliances that work on the heating effect of electric current. Electric iron, immersion heater, geyser, hotplate, electric kettle, room heater, hair dryer, and toaster work on the heating effect of electric current.
8. What is the coil of wire inside heating appliances called? The coil of wire inside heating appliances is called the element.
9. Why does the element of heating appliances become red hot? The element becomes red hot because a large amount of heat is produced in it when current passes through it.
10. On what factors does the amount of heat produced in a wire depend? The amount of heat produced in a wire depends on the material, length, and thickness of the wire.
11. Why do ordinary wires of a circuit not get heated much? Ordinary wires of a circuit do not get heated because they are made of materials and thickness that do not allow much heat production.
12. Why is the use of incandescent bulbs not recommended? Incandescent bulbs waste electricity because a large part of the electricity is used to produce heat instead of light.
13. What are better alternatives to incandescent bulbs? Fluorescent tubes, CFLs, and LED bulbs are better alternatives because they use less electricity and are energy-efficient.
14. Why are LED bulbs more preferred nowadays? LED bulbs are preferred because for the same amount of light they use less electricity, are more efficient, and save energy.
15. What is the ISI mark? ISI mark is a standard mark given by the Bureau of Indian Standards, New Delhi. It shows that the product is safe and follows quality specifications.
16. Why is it advised to use ISI marked electrical appliances? It is advised to use ISI marked appliances because they are safe, electricity efficient, and reliable.
17. What is a fuse? A fuse is a safety device that melts and breaks the circuit when the current exceeds the safe limit.
18. Why are fuses important in electrical circuits? Fuses are important because they prevent fire and damage to appliances if too much current flows in the circuit.
19. What are the reasons for excessive current in circuits? Excessive current may happen due to short circuit (direct touching of wires because of damaged insulation) or overload (connecting too many devices in a single socket).
20. What is a short circuit? When the insulation of wires gets damaged and the wires touch directly, a large current flows. This is called a short circuit.
21. What is overload? When many devices are connected to a single socket, more current flows than the safe limit. This is called overload.
22. What are MCBs? MCBs (Miniature Circuit Breakers) are safety devices that automatically turn off the circuit when current exceeds the safe limit.
23. Why are MCBs better than fuses? MCBs are better than fuses because they can be switched on again easily after the fault is repaired, while fuses once blown must be replaced.

Magnetic Effect of Electric Current

Questions about electromagnetic phenomena

1. Who first discovered that a current-carrying wire shows a magnetic effect? Hans Christian Oersted first discovered that a current-carrying wire can deflect a compass needle and behaves like a magnet.
2. What is an electromagnet? A coil of wire wound on an iron object that behaves like a magnet only when current flows through it is called an electromagnet.
3. What happens to an electromagnet when the current is switched off? When the current is switched off, the coil usually loses its magnetism and stops attracting objects.
4. Where are electromagnets used in daily life? Electromagnets are used in cranes to lift heavy loads of iron, in machines to separate magnetic materials from junk, in electric bells, in toys, and by doctors to remove small pieces of magnetic material from the eye.
5. Why are electromagnets better than permanent magnets in some uses? Electromagnets are better than permanent magnets because they can be made very strong and can be switched on or off as needed.
6. What is an electric bell? An electric bell is a device that uses an electromagnet to produce sound when current passes through it.
7. What are the main parts of an electric bell? The main parts of an electric bell are: a coil wound on an iron piece (electromagnet), an iron strip with a hammer, a contact screw, and a gong.
8. How does the electromagnet help in ringing the bell? When current passes through the coil, it becomes an electromagnet and pulls the iron strip. The hammer on the iron strip strikes the gong and produces sound.
9. Why does the ringing of the electric bell continue automatically? The ringing continues automatically because:
   When the iron strip is pulled, the circuit breaks and current stops.
   The coil stops being an electromagnet, so the iron strip comes back to touch the contact screw.
   This reconnects the circuit, the coil becomes an electromagnet again, and the hammer strikes the gong.
   This happens repeatedly and quickly, producing continuous ringing.
10. What effect of electric current is used in an electric bell? The magnetic effect of electric current is used in an electric bell.
11. Write a Short note on the working of electric bells. Working of an Electric Bell:
    When the switch is pressed, current flows through the coil.
    The coil acts as an electromagnet and pulls the iron strip.
    The hammer attached to the strip hits the gong and produces sound.
    Pulling of the strip breaks the circuit, so the coil loses magnetism.
    The strip comes back to touch the contact screw, completing the circuit again.
    This process repeats very quickly, and the bell keeps ringing.

Connection of Cells

Extra questions about cell configurations

1. What are two types of connection of cells? The two types of connection of cells are:
   1. Series connection
   2. Parallel connection
2. What is series connection of cells? In series connection, the positive terminal of one cell is connected to the negative terminal of the next cell.
   In series connection, we get more voltage, so the device works brighter or stronger.
3. What is parallel connection of cells? In parallel connection, all positive terminals are joined together and all negative terminals are joined together.
   In parallel connection, the voltage remains the same, but the device works for a longer time.
4. How cells are connected in inverter battery? In an inverter battery, cells are connected in series.
5. When we use series connection of cells? We use series connection when we need more voltage (more power), such as in torch bulbs, tv remote, toys etc.
6. When we use parallel connection of cells? We use parallel connection when we need the device to run longer time at the same voltage, such as in toys or remote control cars.