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Chemistry - Historical models of the atom As scientists discovered new evidence, the scientific model of the atom was updated or replaced. 1)An early model Atoms were originally thought to be tiny spheres that could not be made smaller. 2 Plum pudding model Electrons were discovered. negative electron The atom is a positively charged sphere containing electrons. The alpha particle-scattering experiment showed that most positive alpha particles fired at an atom went through the atom. 3) Nuclear model Atom’s mass is mainly a positive central nucleus 4) Niel Bohr’s model Bohr's calculations agreed with experiments showing the electron orbits. small, positively charged sphere experiment showed that the positive nucleus was made of small, positively charged particles, which were given the name 'protons'. 5) Atomic model with protons 6) Atomic model with neutrons James Chadwick's experiments showed, about 20 years after the nucleus had been accepted, that the nucleus also contained particles with no charge; these were given the name neutron 
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Chemistry -Distillation Distillation is used to separate liquid from a mixture. Simple distillation The condenser has two tubes, one inside the other. Cold water runs through the space between the two tubes, keeping the condenser cold. The cooling water does not mix with the substance being separated. In a mixture of two liquids, the liquid with the lower boiling point is collected as the distillate. In a solution of a solid in water, the water is collected as the distillate.  Fractional Distillation Fractional distillation is used to separate a mixture of more than two liquids using the laboratory apparatus shown. The column is packed with glass beads. The different liquids leave the top of the column in order of increasing boiling point.  Filtration, crystallisation and chromatography Mixtures are separated by physical processes. Separation of mixtures does not require chemical reaccions and no new substances are made. Filtration filter paper evaporating basin filtrate The mixture is poured into the tunnel. 1. Insoluble solids remain in the filter paper. 2. Solutions and liquids go through into the flask.  Filtration Crystallisation The solution is heated to concentrate the solution, and crystals may start to form. It is then left in a warm place for the remaining water to evaporate Crystals form.  Paper chromatography To set up paper chromatography: draw a pencil line near the bottom of the paper add spots of colourings to the line put the paper into a chromatography tank with a solvent at the bottom and below the line. The soluble dyes are carried up the chromatography paper. Different dyes travel upwards at different rates.  Chemistry - elements, mixtures and compounds Substances can be (1) pure elements, (2) pure compounds or (3) mixtures of elements or compounds. Atoms All substances are made of atoms. An individual atom is too small to see, so everything around you contains very many atoms. If an element is divided into smaller and smaller pieces, an atom is the smallest part that can exist. Elements An element is a substance that is made of only one sort of atom. Oxygen is an element because it contains only oxygen atoms. There are about 100 different elements. Atoms of each element are given a chemical symbol. Every symbol starts with a capital letter, often followed by a lower case letter. For example, N represents a nitrogen atom, but Na represents a sodium atom. •The elements are shown in the periodic table. Compounds Compounds form when atoms of different elements are combined during a chemical reaction. Compounds can separate back into elements only through chemical reactions. The different elements combined in a compound are in fixed proportions, shown in the chemical formula. Example H20 The formula of every water molecule is H20 because in each molecule two hydrogen atoms are chemically combined with one oxygen atom. Mixtures A mixture consists of two or more elements and/or compounds. These components are not chemically bonded. The chemical properties of each component in the mixture remain unchanged. Worked example Some iron powder and some sulfur powder are mixed, and tested with a magnet. The mixture is heated and cooled. The product is then tested with the magnet. Explain why the magnet attracts some of the mixture before heating, but none of the product after heating. (2 marks) The magnet attracts the iron, which is magnetic. When the mixture is heated iron sulfide is formed, which is non-magnetic. In the mixture, iron retains its magnetic property,  Secondary Storage Computer software and data are permanently stored in memory, which is often referred to as secondary storage or auxiliary storage. Since this kind of memory can keep its contents even when the power is switched off, it is referred to as non-volatile memory. Non-volatile memory is employed for backup and long-term storage. A secondary storage device, such a hard disk drive, is where you store files, applications, and operating systems that you install. There are three categories of secondary storage devices: solid state, optical, and magnetic. Hard disk drives and other magnetic storage systems store data by magnetizing certain areas of the disk with the help of a magnetic field. These gadgets are typically inexpensive and have enormous capacities. Flash memory is used by solid state devices to store data. These gadgets are usually quick, costly, and have little capacities. Optical devices read data from CDs, DVDs, and Blu-ray disks using a laser. Additionally, writing data to a disk is possible, but it is typically permanent and cannot be removed. SATA cables are typically used to connect secondary storage devices to the motherboard. The hard drive is located on the left in this image, and the SATA cable is connected to a SATA port on the motherboard.  Computer Structure – Primary Storage Devices Primary Storage Devices Primary storage, also known as main memory, or internal memory, is memory that is accessed directly by the CPU. Random Access Memory (RAM) Computer memory is made up of silicon chips and is the computer’s working area. This is where software instructions and data are stored. This type of memory is known as volatile memory as it requires a power source to maintain the stored data. Once the power is cut, the data is lost.  For example if you are typing a document in Microsoft Word, both Microsoft Word and your document are loaded into and stored in the computer’s memory while you are working on it. This is not to be confused with the Hard Disk. DRAM The first type of RAM we’ll take a look at is DRAM. DRAM stands for Dynamic Random Access Memory and is the memory used to store data in personal computers. DRAM stores each bit of information in a cell composed of a capacitor and a transistor. The transistor admits current to the capacitor during writes, and discharges the capacitor during reads.  If there is a charge in the capacitor, it is read as a 1, if there is no charge, it is read as a 0. The capacitor in a DRAM cell can hold a charge for only a few milliseconds, and is said to be ‘dynamic’, because the cell must be constantly recharged (or refreshed) in order to retain its data. SDRAM The next type of RAM is called SDRAM (or synchronous DRAM). This type of DRAM is called synchronous because its operation is synchronized with an external clock signal (such as the computer’s internal clock). SDRAM uses only the rising edge of the clock signal to transfer data  DDR SDRAM DDR stands for Double Data Rate and is a memory technology that works by allowing operations to occur on both the rising and falling edge of the clock cycle, thereby effectively doubling the data rate without increasing the clock frequency.  The data is stored in memory cells arranged in a grid like pattern called a memory array. The rows are called wordlines and the columns are called bitlines. Each memory array can contain thousands of cells or even millions, but to simplify the design so we can understand what’s going on, we’ll use an 8x8 memory array. During a read cycle: 1. During a read cycle: 1. The row address is placed on the address pins via the address bus. 2. The RAS pin is activated. This places the row address into the Row Address Buffer. 3. The Row Decoder selects the row which is sent to the Sense Amp. 4. The Write Enable pin is deactivated. 5. The column address is placed on the address pins via the address bus. 6. The CAS pin is activated. This places the column address into the Column Address Buffer. 1. The Column Decoder selects the data from the selected column in the Sense Amp, and places it on the Data Out pin, which is connected to the data bus. 2. Refresh row with data from Sense Amp. 3. The RAS and CAS pins are both deactivated and the cycle begins again. During a write cycle: 1. The row address is placed on the address pins via the address bus. 2. The RAS pin is activated. This places the row address into the Row Address Buffer. 3. The Row Decoder selects the row which is sent to the Sense Amp. 4. The input value placed on the Data In pin, then the Write Enable pin is activated. 5. The column address is placed on the address pins via the address bus 6. The CAS pin is activated. This places the column address into the Column Address Buffer. 7. The value from the Data In pin is loaded into the Sense Amp according to 8. Refresh row (with the modified data from Sense Amp). 9. The RAS and CAS pins are both deactivated and the cycle begins again. A single memory array would just store and retrieve one bit of data. To store a byte, we would need an 8 of these arrays  A set of memory arrays is called a bank. On a stick of RAM, you’ll usually find 8 banks on a side. This is called a rank. A stick of RAM is called a DIM (Dual Inline Memory Module). DDR SDRAM comes in different forms such as: DDR, DDR2, DDR3 and DDR4. Laptops have their own type of memory. It’s more or less the same except for the physical size. These memory modules are called SO-DIMMs  SRAM SRAM or static RAM is a lot faster than DRAM because it doesn’t need to be refreshed and is reserved for cache memory. Cache Memory The cache is usually an extremely fast memory chip that stores data so that subsequent requests for that data can be served faster. Data read from a hard disk drive can be stored in a cache, so when a program requests that data again, it can be read from the cache instead of from the hard disk drive.  Read Only Memory (ROM) Read Only Memory is non-volatile memory or storage containing data that cannot be changed. Read Only Memory is useful for storing a program that very rarely change. An example is the BIOS program needed to start a PC, sometimes known as firmware.  Computer Structure - Hardware Parts Hardware components such as a central processor unit (CPU), random-access memory (RAM), hard disk drive (HDD), and others make up computer systems. Usually connecting to a motherboard, this is referred to as internal hardware. Peripherals are external devices such as printers, scanners, keyboards, mouse, cameras, and so on. Additionally, there is removable storage that is meant to be portable, including memory cards, USB sticks, and external hard drives. Hardware Types The casing of a desktop computer contains all of the internal components. External hardware is what's located outside the case and is referred to as peripherals. Internal Hardware Hardware found inside the computer casing is referred to as internal hardware. Examples include the hard drive, graphics card, RAM, CPU, or processor. External hardware / Perioherals The most common peripherals are Monitors Printers Keyboards Mice Scanners Cameras These all sit outside the case and connect to your computer via USB cable or similar. Some familiar ones are pictured below. Here, you have a computer monitor and printer. These are sometimes called output devices.  Computer Structure - Utilities Utilities are small programs that are designed to configure, analyse, optimise or maintain a computer, such as antivirus, scandisk or defrag. Windows comes pre-installed with Windows Defender which is an antivirus program.  Computer Structure - Applications & Apps Applications are huge software packages that were initially intended to operate on desktop computers. A keyboard and mouse are used to interact with the application. Applications such as the Adobe Creative Suite, which includes Photoshop, Premiere, and Word, Excel, and PowerPoint, and the Microsoft Office Suite are examples. The program and its UI are made with the understanding that you use a keyboard and mouse to interact with the system. An illustration of a desktop program is this one. A laptop could possibly be running this application. The App is a more modern version of software. Apps are typically made with a touch screen in mind and have a smaller file size. You use your finger to directly manipulate the screen with various finger motions, such as tapping, dragging, and pointing. This is taken into consideration when designing the interface, as larger icons allow you to tap on them with your finger.  Computer Structure - Operating Systems Software is required for the computer to function in addition to all the hardware and accessories. An operating system is the first component of required software. This might be: On a PC, Windows 10 or 11 If using a Mac computer, use MacOS. If you use a ChromeBook, use ChromeOS; if you use a tablet, use Android and iPadOS. These appear to be comparable to the ones shown below. Both MacOS and Windows 11 are visible here. We can see iOS running on an iPhone in the image below. Instead of use a mouse, the iPhone's operating system enables users to interact with the system by tapping on icons. As you can see, the operating system offers a user interface through which you can carry out tasks using a variety of instruments known as programs or software. One software program suite that enables you to produce documents, spreadsheets, presentations, and other types of files is Microsoft Office. Photoshop is an image editing application.  |
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