Grade 7 Natural Sciences (NS) Term 3 (Via Africa) • Teacha! - Free Printable

Worksheet Analysis and Solution

The provided worksheet contains questions related to energy, matter, and physical processes. Below is a detailed explanation and solution for each question.

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#### Page 75: Formal Assessment Task (FAT) [1]

##### Section 1: Biofuel
1. 1.1 What is 'biofuel'?
- Answer: Biofuel is a type of fuel derived from organic materials, such as plants, algae, or waste products. It is produced through biological processes like photosynthesis and can be used as an alternative to fossil fuels.

2. 1.2 Give an example of biofuel.
- Answer: An example of biofuel is ethanol, which is typically made from corn or sugarcane.

3. 1.3 Explain why biofuel is a renewable source of energy.
- Answer: Biofuel is considered a renewable source of energy because the organic materials used to produce it (e.g., crops, algae) can be replenished naturally over time. Unlike fossil fuels, which take millions of years to form, biofuels can be continuously grown and harvested, making them sustainable in the long term.

##### Section 2: Energy Concepts
4. 2. Explain the difference between position potential energy and kinetic energy.
- Answer:
- Position Potential Energy: This is the energy stored in an object due to its position or state. For example, a book on a high shelf has gravitational potential energy because of its height above the ground. Another example is elastic potential energy, which is stored in a stretched spring.
- Kinetic Energy: This is the energy an object possesses due to its motion. For example, a moving car has kinetic energy because it is in motion. The faster the object moves, the more kinetic energy it has.

##### Section 3: Heat Transfer
5. 3.1 Explain the difference between conduction of heat and convection.
- Answer:
- Conduction: This is the transfer of heat through direct contact between particles in a material. It occurs when particles with higher thermal energy collide with particles with lower thermal energy, transferring energy in the process. Conduction is most effective in solids.
- Convection: This is the transfer of heat through the movement of fluids (liquids or gases). Warmer regions of the fluid become less dense and rise, while cooler, denser regions sink, creating a circular motion called a convection current. Convection is common in liquids and gases.

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#### Page 76: Worksheets

##### Section 3.2: Phase of Matter and Heat Conduction
6. 3.2 In which phase of matter (solid, liquid, or gas) does conduction of heat occur best?
- Answer: Heat conduction occurs best in solids. Solids have tightly packed particles that vibrate but do not move freely, allowing efficient transfer of energy through collisions. Liquids and gases have more space between particles, leading to less efficient conduction compared to solids.

##### Section 3.3: Woolen Jersey and Cold Weather
7. 3.3 Explain how a woollen jersey keeps you warm in cold weather.
- Answer: A woollen jersey keeps you warm in cold weather primarily due to the following reasons:
- Trapping Air: Wool fibers create tiny air pockets that trap air, which is a poor conductor of heat. This trapped air acts as an insulator, preventing body heat from escaping.
- Hydrophobic Properties: Wool fibers are hydrophobic (water-repellent), which means they can absorb moisture without feeling wet. This helps maintain warmth by reducing heat loss due to evaporation.
- Natural Insulation: The natural crimp of wool fibers enhances its ability to trap air and provide insulation.

##### Section 4: Hair Dryer and Energy
8. 4.1 When a hair dryer is used:
- 4.1.1 What is the type of input energy?
- Answer: The input energy for a hair dryer is electrical energy. This is the energy supplied to the hair dryer through an electrical outlet.
- 4.1.2 What are the three types of output energy?
- Answer: The three types of output energy from a hair dryer are:
1. Heat Energy: Generated by the heating element to warm the air.
2. Kinetic Energy: Produced by the motor that drives the fan, causing air to move.
3. Sound Energy: Created by the movement of the fan and other mechanical components.
- 4.1.3 Which two of these are useful forms of energy?
- Answer: The two useful forms of energy are:
1. Heat Energy: Used to dry hair by warming the air.
2. Kinetic Energy: Used to move air, ensuring even drying.

9. 4.2 For every 800 J of input energy, 160 J of input energy is wasted. Calculate the efficiency of this hair dryer.
- Solution:
- Input Energy: 800 J
- Wasted Energy: 160 J
- Useful Energy: Input Energy - Wasted Energy = 800 J - 160 J = 640 J
- Efficiency Formula:
\[
\text{Efficiency} = \left( \frac{\text{Useful Energy}}{\text{Input Energy}} \right) \times 100\%
\]
- Substituting the values:
\[
\text{Efficiency} = \left( \frac{640}{800} \right) \times 100\% = 0.8 \times 100\% = 80\%
\]
- Answer: The efficiency of the hair dryer is 80%.

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Final Answers

1. 1.1: Biofuel is a type of fuel derived from organic materials.
2. 1.2: Example: Ethanol.
3. 1.3: Biofuel is renewable because the organic materials can be replenished naturally.
4. 2: Position potential energy is stored due to position; kinetic energy is due to motion.
5. 3.1: Conduction involves particle collisions in solids, while convection involves fluid movement.
6. 3.2: Heat conduction occurs best in solids.
7. 3.3: Wool traps air, provides insulation, and repels moisture.
8. 4.1.1: Input energy is electrical energy.
- 4.1.2: Output energies are heat, kinetic, and sound.
- 4.1.3: Useful energies are heat and kinetic.
9. 4.2: Efficiency = 80%.

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Boxed Final Answer

\[
\boxed{80\%}
\]