Lesson Notes By Weeks and Term v4 - SHS 3
ATOMIC PHYSICS
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ATOMIC PHYSICS
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Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 18
Grade code: 3.4.1.LI.2
Strand code: 4
Sub-strand code: 1
Content standard code: 3.4.1.CS.1
Indicator code: 3.4.1.LI.2
Theme: ATOMIC AND NUCLEAR PHYSICS
Subtheme: ATOMIC PHYSICS
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Welcome, future scientists and engineers! Today, we are exploring a fascinating and powerful part of the electromagnetic spectrum: X-rays. Many of us have encountered X-rays in our lives, perhaps when visiting a hospital like Korle Bu or Komfo Anokye for a suspected fracture, or seeing security scanners at Kotoka International Airport. But what are they, how are they made, and why are some types different from others? This lesson will demystify X-rays, moving beyond just seeing them as "pictures of bones." We will learn about their production, their fundamental properties, and crucially, how to distinguish between "hard" and "soft" X-rays.
2.1 What are X-rays?
X-rays are high-energy, invisible electromagnetic waves. They have very short wavelengths, typically ranging from 0.01 to 10 nanometres (nm). On the electromagnetic spectrum, they are found between ultraviolet (UV) light and gamma rays. Because of their high energy, they can pass through many materials that are opaque to visible light. 2.2 Production of X-rays: The Coolidge Tube
X-rays are produced when fast-moving electrons are suddenly stopped or decelerated. The device used to do this is called a Coolidge tube or an X-ray tube. It is a vacuum tube containing a cathode and an anode.
Here is the step-by-step process: Thermionic Emission: A low-voltage power supply heats a tungsten filament (the cathode). This heating gives the electrons in the filament enough energy to escape from its surface. This process is called thermionic emission. Acceleration: A very high potential difference (p.d.), often called the accelerating voltage (measured in kilovolts, kV), is applied between the cathode and the anode (the target). Since electrons are negatively charged, they are strongly accelerated from the negative cathode towards the positive anode, gaining a huge amount of kinetic energy. Deceleration/Collision: The high-speed electrons smash into the metal target (anode), which is usually made of a high-melting-point metal like tungsten. Energy Conversion: Upon impact, the electrons decelerate rapidly. Their kinetic energy is converted into other forms. About 99% of the kinetic energy is converted into heat, which is why the anode has cooling fins or a rotating mechanism. About 1% is converted into X-ray photons.