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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.

This YouTube video investigates the structure and composition of the sun.

This online resource investigates the internal structure, outer layers and surface features of the sun.

These four short online lessons cover the following:

• What is the sun?
• Interior of the sun
• Outer layers of the sun
• Surface features of the sun

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

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 resource explores the terrestrial, or inner, planets - the planets closest to the sun: Mercury, Venus, Earth and Mars.

Short tutorial on trigonometry

• 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.