Soil

24.1 How soil is formed
Soil is a mixture of decayed plants and animals, and small pieces of broken rocks such as sand, silt and clay. The rocks are broken as a result of a process called weathering.

Weathering
Soil is largely made up of weathered rocks. Weathering is the process that loosens and breaks down rocks at or near the surface of the Earth. It eventually converts the broken rocks into sand, clay, silt and other dissolved materials.

Weathering is brought about by various agents in our environment, such as:

• wind
• waves
• water
• heat from the sun
• plants and animal
• human activities
• frozen water

24.2 Soil erosion
The removal of soil by wind and rain is called soil erosion. soil erosion occurs when soil gets blown by the wind  or washed away by water.
Factors that affect soil erosion:

• speed and volume of running water
• amount of vegetation
• slope of  land
• dryness and size of soil particles

Effects of soil erosion:

• Shape of land
• condition of soil
• condition of water

How to prevent soil erosion

• Stop cutting down trees unnecessarily as this exposes the soil to the Sun, rain and wind.
• Grow plants and crops along the slopes of hills and mountains.
• build retaining walls to hold back slopes of hill and mountains.

Sound

23.1 Sound travels away from the source
Sound also travels in a similar way. When an object produces a sound, the sound travels away from the object in all directions. The object producing the sound is called the source of the sound.
23.2 Sound requires a medium to travel
Sound requires a medium to travel. When an alarm clock rings, we can hear the sound because the sound travels through the air and reaches our ears, In this case, air is the medium through which sound travels.

Electric Circuits

22.1 Electric Circuits
An electric circuit is made up of different components.
Which is:

• Energy source
• Wires
• Output device
• Switch

22.2 Circuit Diagram and Symbols
A circuit diagram is a drawing that uses different symbols to represent an actual electric circuit.
Symbols in an electric circuits

circuit diagram

22.3 Series and Parallel Circuits
We can classify electric circuits based on the arrangement of the components in the circuit. Electric circuits are classified into :

• Series Circuit
• Parallel Circuit

Series Circuit
A series circuit is a circuit that has components which are connected such that there is only one path for the electric current to flow through.




Parallel Circuits
A parallel circuit is a circuit that has parts which are connected such that there are more than one path for the electric current to flow through.




22.4 Brightness of a bulb
The brightness of the bulb in the circuit depends on the amount of electric current flowing through it. We can increase or decrease the amount of electric current flowing through the circuit by changing the:

• number of batteries
• number of bulbs
• arrangement of bulbs

Changing the number of batteries
connecting more batteries to the series circuit makes the bulbs light up brighter.
Changing the number of bulbs
connecting another bulb to the series circuit, without changing the number of batteries, can make the bulbs dimmer.
changing the arrangement of bulbs
connecting more batteries to the series circuit makes the bulb light up lighter.