Physics Notes Part 3

  v Some materials become charged when they are rubbed. Their charge is sometimes called electrostatic charge or static electricity. Like charges repel; unlike charges attract. Closer charges, greater force between them.

v Conductors are materials that let electrons pass through them. Metals are the best conductors(electrical). Some of their electrons are so loosely held to atoms that they can pass freely between them. These free electrons make metals good thermal conductors. Most non-metals conduct charge poorly or not at all, although carbon is an exception. (silver, carbon, aluminium and copper.)

v Semiconductors are in between materials. They are poor conductors when cold, better in warm. (silicon, germanium)

v Insulators are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move, although they can be transferred by rubbing. Insulators are easy to change by rubbing because any electrons that get transferred tend to stay where they are. (water, human body, earth, PVC, plastic, Perspex, polythene, glass, rubber, dry air)

v If enough charge builds up on something, electrons may be pulled through the air and cause sparks, which can be dangerous. To prevent charge building up, objects can be earthed: they can be connected to the ground by conducting material so that the unwanted charge flows away. Charges that appear on an uncharged object because of a charged object nearby are called induced charges. A metal sphere is being charged by induction. The sphere ends up with an opposite charge to that on the rod, which never actually touches the sphere.

v SI unit of p.d.(Potential difference) is volt. SI unit of charge is coulomb. (6 billion billion electrons) If electric charges feel a force, then scientifically speaking, they are in an electric field. Ions are electrically charged atoms. Atoms become ions if they lose, or gain electrons. An electric cell (commonly called a battery) can make electrons move, but if there’s a conductor connecting two terminals. Chemical reactions inside the cell push electrons from the negative terminal round to the positive terminal.

v The conducting path through the lamp, wires, switch and battery is called a circuit. A flow of charge is called an electric current. SI unit of current is ampere. Currents of about an ampere can be measured using an ammeter, or milliammeter. Some circuit diagrams have arrowheads marked on them. It shows the conventional current direction: positive to negative in the circuit. The higher the voltage, the more work is done in pushing out each coulomb. The scientific name for voltage is potential difference(pd). It can be measured using a voltmeter across terminals. The p.d. across a cell is highest when it isn’t connected in a circuit. This maximum p.d. is electromotive force (emf). Moving around a circuit, from one battery terminal to the other, the sum of pds across the components is equal to the pd across the battery.

v SI unit of resistance is ohm (Ω). Resistance is the potential difference across the conductor divided by current through the conductor. Factors affecting resistance: length, cross-section area, material, temperature. There’s a heating effect whenever a current flows in resistance. Resistors are made to provide resistance. In a simple circuit, it reduces current. Variable resistors (rheostats) are used for varying currents. In hifi equipment, rotary (circular) variable resistors are used as volume controls. Thermistors have high resistance in cold. They contain semiconductor materials. Some electrical thermometers use a thermistor to detect temperature change. Light-dependent resistors (LDRs) have high resistance in the dark. It is used in electronic circuits which switch off and on lights automatically. Diodes have high resistance and allow current to flow in 1 direction only.

v Ohm’s law: Current is proportional to p.d.

v Series Lamps: If one is removed, the other goes out because the circuit is broken. Lamps share p.d. from the battery, so each glows dimly. Resistors: The current in each component is the same. The total voltage across all components is the sum of p.d.s across each of them.

v Parallel Lamps: Each gets full p.d. from the battery as each is connected directly, so glows brightly. If one is removed, the other keeps working as it is still part of an unbroken circuit. Resistors: The voltage across each component is the same. The total current in the main circuit is the sum of the current in the branches.

v Rectification is the process of changing AC to DC. It is done using diodes, which doing this job are known as rectifiers.

v LED stands for Light-Emitting Diode. It’s a diode that gives off light when current is passed through it. Arrays of white LEDs are a source of light in some low-energy bulbs and torches. (red, green, blue)

v Reed switch is operated by a magnetic field. Contacts lose if the magnet is brought closer. Burglar alarm circuits use a reed switch. With a coil around it, a reed switch becomes a reed relay. The current in 1 circuit through coil switches on another circuit through the contacts. Appliances have power ratings marked in kilowatts.

v Power is the rate at which energy is transferred from one store to another. SI unit of power is watt.

v The main current is alternating current (AC), it flows forwards and backwards. The mains frequency is 50Hz.

v Live wire (line): This goes alternately negative and positively making current flow backwards and forwards in the circuit.

v Neutral wire (cold): This completes the circuit. In many systems, it is kept at zero voltage by the electricity supply company.

v Switch: It’s fitted in live wire. This is dangerous if the cable is cut accidentally.

v Fuse: A thin piece of wire which overheats and melts if current is too high. It is fitted in live wire as a cartridge. Many circuits use a circuit breaker (trip switch) instead of a fuse.

v Earth wire (grounded): It’s a safety wire. It connects the metal body of the kettle to the earth and stops it from becoming live. This means the kettle is safe to touch.

v Double insulation: Some appliances don’t have earth wire, as outer case is made up of plastic and not metal. The plastic acts as an extra layer of insulation around the wires.

v Residual current device (RCD): It compares current in live and neutral wires. The RCD senses the difference and switches off current before any harm can be done.

v If an accident happens, and someone is electrocuted, you must switch off the socket and pull out the plug before giving any help.