✨ Class 8 Science Notes ✨
Chapter 4: Current Electricity and Magnetism

20 Most Important Words (with Hindi Meaning)

Important Notes for the Chapter

• Atoms have equal numbers of protons and electrons; so normal objects are electrically neutral.
• Electric current is the flow of electric charges (mainly electrons in metals) through a conductor.
• Electric cells (e.g. dry cell, lead-acid cell, Ni-Cd cell) create a constant potential difference to drive current in circuits.
• Potential difference (voltage) is necessary for current to flow. SI unit is volt (V).
• Electric current’s SI unit is ampere (A): \( 1 \ \textrm{Ampere} = 1\ \textrm{Coulomb}/1\ \textrm{second} \)
• Electrical circuit: Combination of wires, battery/cells, switch, and devices forms a complete path for current.
• Dry cells are used in torches, radios, clocks; Lead-acid cells in cars and UPS; Ni-Cd cells and lithium-ion cells in gadgets.
• Batteries are series/parallel combinations of two or more cells to obtain greater potential difference/capacity.
• Magnetic effects of current: A current-carrying wire creates a magnetic field around itself (Oersted’s experiment).
• Electromagnet: A coil wound on iron core carrying current behaves as a strong temporary magnet—used in electric bells, cranes etc.
• Electric bell works by electromagnetism, rapidly making and breaking contact to ring the bell.
• Current in a conductor flows from negative to positive terminal (electron flow); conventional current is considered positive to negative outside the cell.
• SI unit of charge is coulomb (C).
• SI unit of potential is volt (V).
• Electric current is a scalar quantity, not a vector.
• An open circuit breaks the flow of electricity; a closed circuit allows current to pass.
• Lead-acid and Ni-Cd cells are rechargeable; dry cells are not.
• Increasing the number of coils, current strength, or adding an iron core increases the strength of an electromagnet.
• Devices like electric bulbs, bells, heaters, all use different effects of current (magnetic, heating, lighting).

20 Most Important One-Word Answer Questions

1. What is the SI unit of electric current? Ampere (A)
2. SI unit of potential difference? Volt (V)
3. SI unit of charge? Coulomb (C)
4. Instrument to measure electric current? Ammeter
5. Instrument to measure potential difference? Voltmeter
6. Positive terminal of dry cell? Graphite rod
7. What makes the electromagnet strong? Iron core
8. Device converting chemical energy to electrical energy? Cell/Battery
9. Conventional direction of current in circuits? Positive to negative
10. Actual direction of electron flow? Negative to positive
11. Main component for producing current in dry cell? Electrolyte
12. What is a battery? Combined cells in series/parallel
13. Effect when current passes through wire? Magnetism
14. Who first discovered magnetic effect of current? Oersted
15. Unit for resistance? Ohm (Ω)
16. Which cell is rechargeable? Lead-acid/Ni-Cd
17. Electrolyte in dry cell? ZnCl₂ & NH₄Cl paste
18. Which bell uses electromagnet? Electric bell
19. Electric current: vector or scalar? Scalar
20. Effect behind electric heater? Heating effect of current

20 Very Short Answer Type Questions

1. What does a cell produce in a circuit? Potential difference
2. Name two types of cells. Dry cell, lead-acid cell
3. Main function of a cell? To maintain constant potential difference
4. On which principle does electric bell work? Magnetic effect of current
5. SI unit of resistance? Ohm (Ω)
6. What are insulators? Materials not allowing electricity to pass
7. SI unit of electric potential? Volt (V)
8. What flows in a closed circuit? Electric current
9. Name the electrolyte in lead-acid cell. Dilute sulfuric acid
10. Electromagnet: temporary or permanent? Temporary
11. How can you increase current in circuit? By increasing potential difference
12. What happens if circuit is broken? Current stops
13. Two uses of lead-acid battery? Car, UPS
14. What is the SI unit of power? Watt (W)
15. What is an atom? Smallest unit of matter
16. What causes bulb to glow in circuit? Current flow
17. What is the charge of electron? Negative
18. Device running on dry cell? Torch
19. Key application of electromagnet? Electric bells
20. Why is copper used as conductor? Low resistance, good conductor

20 Short Answer Type Q&A (2–3 lines)

1. Explain the concept of potential difference. Potential difference is the difference in electrical potential between two points in a circuit. It determines the direction of current flow.
2. Describe construction of a dry cell. A zinc container acts as negative terminal, with a graphite rod at the center as positive terminal. Electrolyte paste surrounds the rod, and chemical reactions produce voltage.
3. How does an electric bell work? When current flows, the electromagnet attracts an iron strip which strikes the gong. The circuit briefly breaks, magnetism stops, and cycle repeats, causing ringing.
4. What is an electric circuit? An electric circuit is a closed path through which electricity flows, usually consisting of cells, switches, wires, and loads.
5. Difference between conductor and insulator. Conductors allow easy flow of electric current (e.g., copper); insulators block current (e.g., rubber).
6. State use of lead-acid cell. Lead-acid cells are used in automobiles and UPS for supplying large currents and recharging capability.
7. Why are batteries better than single cells? Batteries provide higher potential difference and current than single cells, making them suitable for high-power applications.
8. What is Oersted’s experiment? Oersted showed that a current-carrying wire deflects a magnetic needle, proving electric current creates magnetic field.
9. What are rechargeable cells? Give two examples. Rechargeable cells can be reused by reversing current. Examples: Lead-acid cell, Ni-Cd cell.
10. Uses of electromagnet? Used in cranes, electric bells, MRI scanners, and sorting metallic waste.
11. State difference between static and current electricity. Static electricity is charge at rest, current electricity is charge in motion through conductors.
12. How does a bulb glow in a circuit? When current passes through the filament, it heats up and emits light due to resistance.
13. What is resistance? Property of a material to oppose flow of current; measured in ohms (Ω).
14. Explain convention of current direction. Conventional current flows from positive to negative terminal outside the cell, opposite to electron flow.
15. What increases the strength of an electromagnet? Increasing coil turns, current, or using an iron core increases electromagnet strength.
16. Write relation among ampere, coulomb, and second. \( 1\ \textrm{Ampere} = \frac{1\ \textrm{Coulomb}}{1\ \textrm{second}} \)
17. What is role of electrolyte in cell? Electrolyte allows movement of ions, enabling flow of current within the cell.
18. How are batteries formed from cells? Series or parallel connection of multiple cells forms a battery to suit voltage/current needs.
19. Where is Ni-Cd cell used? Portable devices: drills, gadgets, cameras, emergency lights etc.
20. Difference between dry cell and lead-acid cell. Dry cell is not rechargeable and gives small currents. Lead-acid cell is rechargeable and provides large current.

All Textbook Exercise Questions With Answers

• Water in the waterfall flows from a higher level to the lower level because of ________.gravitational attraction
• In an electric circuit, electrons flow from a point of ______ potential to the point of ______ potential.lower, higher
• The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the ______ of the cell.potential difference
• Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be ______ V.4.5 V
• An electric current flowing in a wire creates ______ around the wire.magnetism
• What is a battery? A battery is a combination of two or more cells connected in series or parallel to give higher voltage or capacity.
• What is the SI unit of electric charge?Coulomb (C)
• What is the SI unit of electric current?Ampere (A)
• What is the SI unit of electric potential?Volt (V)
• 1 ampere equals how many coulombs per second?1 C/s
• When a glass rod is rubbed with silk, what charge does it acquire?Positive charge
• The potential difference between lead-acid cell electrodes is nearly ______.2 V
• The Ni-Cd cell gives potential difference of ______.1.2 V
• Which is a scalar quantity: force, acceleration, electric current, or velocity?Electric current
• Working of electric bell is based on which effect?Magnetic effect of current
• Which is SI unit of charge: current, resistance, potential, or charge?Charge
• Does the SI unit of electric potential = ampere?False (it's volt)
• Does conventional current flow from positive terminal?True
• Does large current flow in lightning?True
• Does electric bell work on heating effect?False (it works on magnetic effect)

• Describe with diagram: construction and working of dry cell. A zinc can is negative terminal; graphite rod central positive terminal. Electrolyte paste (ZnCl₂, NH₄Cl) and MnO₂ react chemically to generate voltage across terminals. Current flows when the cell is connected to circuit.
• Describe construction and working of electric bell (diagram). Electric bell uses an electromagnet to attract an iron arm, causing striker to ring the bell. Circuit breaks and remakes, repeating ringing action as long as push button or switch is pressed.
• Potential difference calculation: If 3 cells of 2V each are connected in series, what's total? Total = \(2\,\textrm{V} + 2\,\textrm{V} + 2\,\textrm{V} = 6\,\textrm{V}\)
• State relation among SI units: current, charge, and time. \(1~\textrm{A} = 1~\textrm{C}/1~\textrm{s}\)

📗 End of Notes: Current Electricity & Magnetism (Class 8) 📗