Many Circuits use a change in the potential difference to operate certain chips and functions. Potential difference is achieved by connecting two resistors in series. The difference is then worked out by the formula (R2/(R1+R2))*V with R1 being the top resistor and R2 the bottom one. ==Food Technology == File:Headline textItalic text
Resistant Materials involves working with woods, metals and plastics. In GCSE Resistant Materials CAD and CAM is used. Useful revision website: http://www.technologystudent.com/
Woods can be classified into two types- softwoods and hardwoods. The use of the words 'soft' and 'hard' are not necessarily an accurate description of the wood's hardness- balsa is a prime example; it is a hardwood and yet lighter in weight than many softwoods.
Hardwoods come from deciduous trees, and tend to take around 100 years to grow. On the other hand, softwoods, which come from coniferous trees, only take around 30 years to grow. This makes softwoods cheaper and more commercially available than hardwoods.
Knowledge of some of the properties of certain softwoods and hardwoods is essential for the exam.
Beech: Hardwood, hard, tough, prone to warping
Oak: Hardwood, hard, tough, durable, heavy and contains an acid which can cause steel to corrode
Ash: Hardwood, tough, flexible
Pine: Softwood, knotty, prone to warping
Generally, softwoods are less dense than hardwoods. Hardwoods contain more fibrous material than softwoods.
Metals can be classified into 3 areas: ferrous, non-ferrous and alloys. Ferrous means the metal consists of mainly ferrite or iron. Ferrous metals are magnetic. Non-ferrous metals are not magnetic.
Popular metals include iron, mild steel, stainless steel, aluminium, copper, lead and tin.
Plastics are split into 2 types: thermoset and thermoplastics. Thermoplastics can be re-moulded over and over again because they have very few cross links. Thermoset plastics, on the other hand, have molecules lined up and connected by cross links. This means they can be shaped for the 1st time, but after that, they form a rigid and permanent shape.
Thermoplastics: Acrylic, ABS and polyester
Thermoset Plastics: Epoxy Resin and Urea Formaldehye
Adhesives are used for the process of fabrication-joining one material to another. Joints which are made using adhesives are permanent. The most common school workshop adhesives are:
Polyvinyl Acetate: PVA, joining wood to wood, most types not waterproof
Contact Adhesive: Gluing large sheet materials together, takes 15-20 mins to dry, used for dissimilar materials
Epoxy Resin: Expensive, joining any materials, full strength 2-3 days after application Example: Araldite
Tensol: Gluing acrylic to acrylic, clear liquid
Safety when using adhesives: Harmful fumes may be given off, so application of adhesives should only take place in well-ventilated areas. Contact with skin should be avoided and an apron should be worn.
There are many types of wood joints used in manufacturing, from very simple ones to very complex ones. Here are the ones which are required for the exam;
Butt Joint: The simplest and weakest joint.
Lap Joint: Also known as a 'rebate' joint.
Mitre Joint: 2 pieces of wood at a right angle, cut at 45 degrees. Often used in picture frames and skirting boards.
Dowel Joints: A butt joint with dowels- acting as reinforcement.
Mortice & Tenon Joint: Very strong joint.
Other joining methods: Threading, rivets, brazing, soldering and welding/
Useful Textiles revision website http://textiles4u.wikispaces.com/
Pens&Markers for Graphic Design
The Lead on Pencils are actually GRAPHITE composites, a mix of clay and graphite. Pencils are graded with (H) or (B) according to the hardness and blackness of this composite.
Soft Pencils are ones with less clay but more graphite, meaning that the composite is darker and richer, but will wear down easily. They are graded form 9B to HB.
Hard Pencils are ones with more clay but less graphite. The point of the pencil won't wear down easily. They are graded from H to 9H.
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