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Electric charge comes in two varieties, which we call positive and negative. Like charges repel each other, and unlike charges attract each other. Thus, two positive charges repel each other, as do two negative charges. A positive charge and a negative charge attract each other.

How do we know there are two types of electric charge? When various materials are rubbed together in controlled ways, certain combinations of materials always result in a net charge of one type on one material and a net charge of the opposite type on the other material. By convention, we call one type of charge positive and the other type negative.

In this lesson you will learn about the following forms of renewable energy sources:

• Solar energy
• Water energy
• Wind energy
• Sea wave energy
• Geothermal energy

This YouTube documentary highlights the ecological and socio-economic importance of the Tana River Basin in Kenya.

In this unit you will: 

• Understand that temperature is a measure of average kinetic energy.
• Learn about the various temperature scales that are commonly used in science.
• Learn to read temperature scales.
• Differentiate between various thermometers (alcohol or mercury-based).
• Understand that physical properties change when there is a change of temperature.

This online tutorial will provide you with some guidelines on how to interpret both topographic and geologic maps.

• Describe a vector in your own words
• Explain a method to add vectors
• Compare and contrast the component styles
• Decompose a vector into components
• Describe what happens to a vector when it is multiplied by a scalar
• Arrange vectors graphically to represent vector addition or subtraction

Viewing g as the value of Earth's gravitational field near the surface rather than the acceleration due to gravity near Earth's surface for an object in freefall.

When a circuit is connected and complete, charge can move through the circuit. Charge will not move unless there is a reason, a force to drive it around the circuit. Think of it as though charge is at rest and some- thing has to push it along. This means that work needs to be done to make charge move. A force acts on the charges, doing work, to make them move. The force is provided by the battery in the circuit.

This 32 minute long YouTube video investigates the water contamination problems in Tanzania, with specific focus on Dar Es Salaam. 

This 11 minute long YouTube video describes the various causes and types of water pollutants.