Science and Technology Class 9 Important Questions Chapter 2 Maharashtra Board

Work and Energy

Short Questions

1. What is electric potential?

• It is the electric level that determines the flow of charge between two points.

2. What is potential difference?

• It is the difference in electric potential between two points.

3. Which way do electrons flow in a circuit?

• Electrons flow from lower potential (negative) to higher potential (positive).

4. What is the unit of potential difference?

• The unit of potential difference is Volt (V).

5. What causes potential difference in a cell?

• Chemical reactions inside the cell cause the potential difference.

6. What are free electrons?

• They are weakly attached outermost electrons in a metal that can move freely.

7. What is electric current?

• It is the flow of electrons through a conductor.

8. What is the formula for electric current?

• Current (I) = Charge (Q) / Time (t), or I = Q/t.

9. What is the unit of electric current?

• The unit of electric current is Ampere (A).

10. What does Ohm’s Law state?

• If the physical state is constant, current (I) is directly proportional to potential difference (V).

11. What is the formula for Ohm’s Law?

• V = IR, where V is potential difference, I is current, and R is resistance.

12. What is the unit of resistance?

• The unit of resistance is Ohm (Ω).

13. What is resistance in a conductor?

• It is the hindrance to the flow of electrons caused by collisions with atoms and ions.

14. What is resistivity?

• It is a property of a material that determines its resistance, depending on length and area.

15. What is the unit of resistivity?

• The unit of resistivity is Ohm-meter (Ωm).

16. What are conductors?

• Substances with low resistance that allow current to flow easily, like copper.

17. What are insulators?

• Substances with high resistance that do not allow current to flow, like rubber.

18. What happens to resistors in series?

• The effective resistance is the sum of individual resistances (Rs = R1 + R2 + R3).

19. What happens to resistors in parallel?

• The inverse of effective resistance is the sum of inverses of individual resistances (1/Rp = 1/R1 + 1/R2 + 1/R3).

20. Why is a fuse wire used in circuits?

• It melts to break the circuit if current increases too much, protecting appliances.

Long Questions

1. What is potential difference, and how does it cause the flow of electrons?

• Potential difference is the difference in electric potential between two points, like the positive and negative terminals of a cell. It causes electrons to flow from the lower potential (negative) to the higher potential (positive) through a conductor.

2. How does the flow of water in a waterfall relate to the flow of electric charge?

• In a waterfall, water flows from a higher level to a lower level due to gravity, just like electric charge flows from a higher potential to a lower potential. This potential difference drives the flow of electrons in a circuit, similar to how gravity drives water.

3. What are free electrons, and how do they help in current flow?

• Free electrons are the outermost electrons in a metal that are weakly attached to the nucleus and can move freely. When a potential difference is applied, these electrons move from the negative to the positive terminal, creating an electric current.

4. How is electric current defined, and what is its unit?

• Electric current is the flow of electrons through a conductor, measured as the charge (Q) passing through a point in unit time (t), so I = Q/t. Its unit is Ampere (A), where 1 A means 1 Coulomb of charge flows per second.

5. What is Ohm’s Law, and how is it used to find resistance?

• Ohm’s Law states that if the physical state of a conductor is constant, the current (I) is directly proportional to the potential difference (V), so V = IR. Resistance (R) can be found using R = V/I, measured in Ohms (Ω).

6. What is the difference between conductors and insulators?

• Conductors, like copper, have low resistance and allow current to flow easily because they have free electrons. Insulators, like rubber, have high resistance and block current flow as they lack free electrons.

7. How does resistivity affect the resistance of a conductor?

• Resistivity (ρ) is a property of a material that affects resistance, given by the formula R = ρL/A, where L is length and A is the area of cross-section. A material with higher resistivity, longer length, or smaller area will have higher resistance.

8. What happens when resistors are connected in series?

• In a series connection, resistors are connected one after another, so the same current flows through each, and the effective resistance is the sum of individual resistances (Rs = R1 + R2 + R3). The total potential difference is divided among the resistors.

9. What happens when resistors are connected in parallel?

• In a parallel connection, resistors are connected side by side, so the potential difference across each is the same, but the total current splits among them. The effective resistance is found using 1/Rp = 1/R1 + 1/R2 + 1/R3, and it is less than the smallest individual resistance.

10. Why are domestic appliances connected in parallel in a house?

• Domestic appliances are connected in parallel so that each gets the same voltage (220V in India) and works independently. If one appliance fails, others still function, and the effective resistance is reduced, allowing sufficient current for each device.