Electron
Atomic structure: Atoms are tiny particles that make up everything around us. Each atom is made of three main parts: protons, neutrons, and electrons. These parts are called subatomic particles because they are smaller than atoms.
Electron characteristics: Among these subatomic particles, electrons are the lightest. That means they have the smallest mass compared to protons and neutrons.
Charge of electrons: Electrons carry a type of electric charge called a negative charge. This means they are attracted to positively charged particles, like protons.
Electron mass: The mass of one electron is extremely small—it is 9.1 × 10⁻³¹ kilograms. This is about 1/1836 the mass of a proton, making electrons almost 2,000 times lighter.
Free Electrons on Metal Surfaces
Electron movement on metals: In metals, some electrons are not tightly held by atoms and are free to move around. These free electrons move in random directions across the surface of the metal.
Electron retention: Even though electrons are free to move, they don’t leave the metal because they are still attracted to the positive charges in the metal’s atoms. This attraction comes from the nuclei inside the metal atoms.
Thermionic Emission
Definition: Thermionic emission is a process that happens when a metal gets hot enough that its electrons gain enough energy to break free from the surface of the metal. These electrons then escape into the surrounding space. This is important in many electrical devices like vacuum tubes and cathode ray tubes.
Heating effect: When a metal is heated, the electrons inside it start moving faster because heat gives them energy. If the electrons gain enough energy from the heat, they can overcome the attractive forces holding them inside the metal and escape from the surface.
Surface area effect: A larger surface area on the metal gives more room for electrons to escape. So, the bigger the area, the more electrons can be released at the same time, which increases the current of escaping electrons.
Temperature effect: As the metal gets hotter, the electrons gain even more energy. This means more electrons have enough energy to leave the metal. So, higher temperatures result in more thermionic emission.
Metal type effect: Some metals hold onto their electrons more tightly than others. These metals need more heat to release electrons. Other metals release electrons more easily because of their special material properties, meaning they require less energy to do so.
Examples of easy emitters: Metals like barium oxide and strontium oxide are especially good at letting go of their electrons. These materials can emit electrons even when the temperature is not very high, which makes them useful in many devices.
Surface properties effect: The condition of the metal’s surface also affects how easily electrons can escape. If the surface is clean and smooth, electrons can escape more easily. But if it’s rough, dirty, or covered with oxides, it can be harder for the electrons to break free.
Cathode Rays
Definition: Cathode rays are streams of fast-moving electrons that travel from a heated metal part called the cathode to another part called the anode inside a vacuum tube. These rays were important in discovering the electron.
Electron source: The electrons in cathode rays come from a heated wire or filament. When the filament gets hot, thermionic emission happens, and electrons are released.
Acceleration: A high voltage is applied between the cathode and the anode. This voltage creates an electric field that pulls or pushes the electrons toward the anode, making them speed up.
Vacuum travel: The tube is a vacuum, which means it has no air or gas inside. This allows the electrons to move freely without hitting air molecules, so they can travel easily from the cathode to the anode.
Straight-line motion: Because there is nothing in the way inside the vacuum, the electrons can travel in straight lines. This makes it easier to predict and study their motion.
Kinetic energy: Cathode rays carry kinetic energy, meaning they are moving and can push things, like a tiny paddle wheel. They also have momentum, which means they can keep moving unless something stops them.
Negative charge: Since cathode rays are made up of electrons, and electrons are negatively charged, the rays themselves carry a negative charge.
Field deflection: If you put the cathode rays into an electric or magnetic field, the rays will bend. This bending shows that they are charged particles because only charged particles react to electric or magnetic fields.
Fluorescence: When cathode rays hit certain materials, they make them glow. This glowing effect is called fluorescence, and it helps scientists see where the rays are hitting.
Stopping cathode rays: If you place a thin metal sheet in the path of the cathode rays, it can stop them. This proves that cathode rays are made of tiny particles with mass, not just waves or energy.
Cathode Ray Tube (CRT)
CRT function: A cathode ray tube is a sealed glass tube with nearly all the air removed (a vacuum). It uses a hot cathode to release electrons and create a focused beam of cathode rays.
Beam control: Inside the tube, electric or magnetic fields are used to control where the electron beam goes. These fields help steer the beam to different spots on the screen.
Voltage use: A high voltage is used to make the electrons move very quickly from the cathode to the anode. This high-speed beam of electrons is what makes the images on old TV and computer screens.
Deflection Tube
Purpose: A deflection tube is a special kind of glass tube that has had almost all the air removed so it becomes a vacuum. It is used in science experiments to show how electrons—tiny negatively charged particles—can have their path bent or changed when they go through an electric field. This helps us learn how charged particles behave.
Plate setup: Inside the tube, there are two flat metal plates placed side by side with a small space between them. They face each other and are kept parallel. When we connect these plates to a battery or power supply and give them opposite charges, one becomes positive and the other becomes negative. This creates an invisible electric field between the plates.
Deflection behavior: When electrons enter the space between the plates, they feel a push and a pull from the electric field. Since electrons are negatively charged, they are pulled toward the positive plate and pushed away from the negative plate. This changes their path and makes them curve in the direction of the positive plate.
Parabolic path: While the electrons are being pushed sideways by the electric field, they are still moving forward at the same time. This causes their path to become curved like the shape of a U or a smile, which is called a parabola. This shows that electric fields can change the direction of moving charges.
Straight line path: But if we turn off the voltage, the plates don’t have any charge and no electric field is present. That means the electrons don’t get pushed or pulled and just keep going in a straight line. This shows that it is the electric field that causes the bending.
Maltese Cross Tube
Straight-line proof: The Maltese cross tube is another kind of vacuum tube that helps us prove something important about cathode rays (which are just streams of electrons). It is designed to show that these rays move in straight lines when there is nothing in their way.
Shadow formation: Inside the tube, there is a metal shape that looks like a cross. When the cathode rays travel toward a glowing screen, they hit the cross and leave a clear shadow behind it. This is just like when light shines on a solid object and forms a shadow. This proves that cathode rays move in straight lines and are blocked by solid objects.
Obstacle interaction: The cross stops the cathode rays from reaching the screen behind it. This means that the rays behave like little particles instead of waves—they can’t go around or through solid objects. This supports the idea that cathode rays are made of particles.
Magnetic deflection: If we bring a magnet near the tube, the shadow on the screen starts to shift or move. This is because the magnetic field changes the path of the cathode rays, making them bend. It shows that cathode rays respond to magnetic forces.
Fleming’s rule: The direction in which the cathode rays bend follows a rule called Fleming’s left-hand rule. This rule helps us figure out which direction a moving charged particle will go when it moves through a magnetic field.
Fluorescent screen: At the end of the tube is a screen that glows (or fluoresces) when it is hit by cathode rays. This glowing screen makes it easy to see exactly where the beam of electrons is going and how it moves.
Maximum Electron Velocity
Energy principle: The fastest speed (velocity) that an electron can reach inside a vacuum tube depends on how much electrical energy it is given. When we give the electron energy using electricity, that energy turns into motion, which we call kinetic energy.
Potential energy formula: The energy that the electron gets from the electric voltage is called electrical potential energy, and it is calculated using the formula eV. In this formula, e stands for the charge of the electron and V is the voltage (electrical push) that we apply.
Constants: The charge of one electron is always the same: 1.6 × 10⁻¹⁹ coulombs (C). Its mass is also constant: 9.1 × 10⁻³¹ kilograms (kg). These are special numbers scientists use in physics equations.
Kinetic energy formula: The energy an object has because of its movement is called kinetic energy. For an electron, we find this using the formula (1/2)mv², where m is the mass of the electron and v is its speed or velocity.
Velocity equation: If we say that all of the electrical energy (eV) is changed into kinetic energy ((1/2)mv²), we can set the two equations equal to each other. Solving for v, we get the formula vmax = √(2eV/m). This tells us how fast the electron can go when it is given energy by a voltage.
Dependencies: The speed of the electron depends on three main things: the voltage (V) we apply, the charge of the electron (e), and its mass (m). If we increase the voltage, the electron gets more energy and moves faster.