S2 Sc Practical Electricity Notes PDF

Learning @ The Pinnacle The electrical energy, E, of the circuit is defined as: S2 Sc – Practical Electricity E = Pt Name...

Learning @ The Pinnacle The electrical energy, E, of the circuit is defined as: S2 Sc – Practical Electricity E = Pt Name Date : ____________ where P is the power output of the circuit and t is the amount of time the A conductor heats up after some time when electricity or electric current is electrical energy has been used. passed through the conductor. The larger the resistance of the conductor, the more electrical energy can be converted to heat energy. This result is used by Since P = VI, we have the following: engineers to build electrical devices as electric kettles, ovens and heater for the convenience of mankind. E = VIt In an electric kettle, a conductor is placed at its bottom and a current is passed through it. This process converts electrical energy to heat energy The cost of energy is specified in terms of kWh. where water in the kettle is heated up.The heating element is placed at the For example, a cost of 20 cents per kWh means that it costs $0.20 to use bottom because if it is placed near the top, the hot water will always stay on 1000 W of power in an hour. top. The mechanism of heat transfer is mainly convection. Example A kettle is plugged into the mains which is 240 V. The kettle is switched on for 20 minutes and the current that flows in the kettle is 40 A. Calculate the cost of energy if electricity costs 20 cents per kWh. cable wire nichrome wires An electric oven and a heater make use of the same concept as an electric kettle. The heating elements are normally placed on the top, bottom and/or sides of the oven. The heated air heats up food via convection and conduction. The heating elements used in electric kettles, ovens and heaters are usually made of nichrome wire. Nichrome wire is used because it has high resistivity and is able to withstand high temperatures. Electrical Energy and Power The electric power is the amount of electrical energy required by the electrical appliance to convert into other forms of energy in one second. The power output of a circuit or a particular component in the circuit is defined as P = IV where I is the current in the circuit or across the component and V is the voltage of the source or the potential difference across the component. Danger of Electricity Damp conditions It is hazardous to operate electrical appliances or use electricity under the Damp conditions are extremely dangerous circumstances for operating following conditions. electrical appliances. This is because when our skin is damp or wet, the  Damaged insulation resistance at the contact between skin and appliance is reduced. This can  Overheating of cables cause a short circuit.  Damp conditions Damaged insulation touching a power point The insulation casing around the exterior of a wire can be damaged either by with a wet hand is overuse, misuse or normal wear and tear. When this happens, it becomes a dangerous hazard and it is dangerous to continue using the damaged appliance. When a short circuit happens, current may flow through our body, causing electric shocks, burns and even death. Therefore, to avoid such accidents from occurring, we must not use any electrical appliances in wet or damp places such as toilets and swimming pools. Safe Use of Electricity At Home exposed wire Fuse A fuse is simply a very thin wire. It is fitted onto a live wire that melts and Bare or exposed, live wire can endanger living things, especially humans. It breaks the circuit when the current exceeds a safe level to prevent the causes burns, electric shocks, and in the worst case, death. It can even result wire from overheating. in fire and damage to properties, causing further risks to nearby humans and properties. Therefore, damaged insulation should be reported immediately to trained personnel such as the technicians. short thin wire Overheating of cables For a rating fuse of 13A, the fuse will allow currents less than 13 A to pass Overheating of cables is a common reason for most hazards, which include through it. If the current exceeds this, the fuse will heat up. Due to the low destruction of properties and loss of lives. It is due mainly to overuse of the melting point of the fuse material, it will melt, causing the circuit to break. electrical supply. Overheating causes the melting of the wire, which can When this happens, the circuit is opened and the current stops flowing. cause a short circuit and fire. To prevent overheating, the supply must not be Therefore, overheating is prevented. overloaded with electrical appliances. Fuses are rated by current or voltage and are designed to operate on A.C. or D.C. circuits. The current rating of a fuse is the current that it can carry for indefinite time periods without breaking the circuit. A higher current rating an overloaded power outlet fuse allows more current to pass through without breaking the circuit compared to a low rated fuse. Fuses are rated according to how much current they can carry before melting. In plugs fuses are usually 3 A (red), 5 A (black) or 13 A (brown). The correct fuse for an appliance is one that is just above the normal working current for To prevent such accidents from occurring, we should install and check that that appliance. However, since fuses are unlikely to act quickly enough to safety measures such as circuit breakers and fuse ratings are working prevent human electrocution, their main purpose is to prevent fires due to properly. large currents. Circuit breaker Earthing A circuit breaker is an electrical switch designed to protect an electrical Metal objects such as metal cases, are highly susceptible to charge build ups. circuit from damage caused by an overload or a short circuit. It operates Static charges can build up when there is contact with some materials. automatically. Unlike a fuse, which needs to be replaced once it is used, a Charge can also build up when electricity, in the form of lightning, strikes the circuit breaker can be reset (either manually or automatically to resume metal case. When charge builds up, a potential difference exists between the normal operation. Circuit breakers are made in varying sizes depending on ground we are stepping on and the metal case. the scale of the operation. Furthermore, home appliances with metal casing will be charged if a live wire Two types of circuit breakers are normally found near the front door of a becomes loose and touches the case. Under these circumstances, if we house: touch the case, current flows through our body either to or from the metal  The Miniature Circuit Breaker (MCB) cuts off the electric current case due to a difference in potential. This can cause burns, electrical shock or supply to one particular electric circuit to prevent excessive current even death. flow due to overloading or short circuit.  The Residual Current Circuit Breaker (RCCB) that monitors the Earthing ensures that any static charge that builds up is immediately released amount of current flowing from the live wire and cuts off the electrical by flowing back to the ground. This is done by connecting a wire or a metal supply in the house when there is an earth current leakage in an chain from the metal case to the ground. electric circuit. The setting of a circuit breaker determines the maximum current that can Double insulation pass through before the circuit trips. For some appliances such as vacuum cleaners or electric drills, there are no earthing systems present. These appliances are equipped with double Fuses have the advantages of being less costly and simpler than circuit insulation. It ensures that users will not be exposed to the current when there breakers of Similar ratings. The blown fuse must be replaced with a new is an electricity leakage and comes in contact with the casing of the vacuum device which is less convenient than simply resetting a breaker and therefore, cleaner or electric drill. This casing is made of reinforced insulating plastics likely to discourage people from ignoring faults. On the other hand, replacing and the wire is encased with double layers of insulating materials. However, a a fuse without isolating the circuit first can be dangerous in itself, particularly if wet double insulated appliance is exceptionally dangerous. This is because the fault is a short circuit. water is a good conductor of electricity and will easily reach the live electrical components within the case. Anyone who touches the casing will then receive Circuit breakers which have interrupted a severe fault should be removed an electric shock. It is important to note that wet hands allow a greater current from service, inspected and replaced if damaged. They must be maintained to flow through the casualty. For this reason, it is advisable not to operate a on an annual basis to ensure their mechanical operation will not impede their mains radio, hairdryer or double insulated appliance in any wet area, performance during an interruption, especially a bathroom. For example, your household circuit breakers should be switched to the “off” position and back to the “on” position at least once per year, to exercise * Do not touch a person who has been electrocuted until you are sure that the circuit breakers. Failure to do so could result in the circuit breaker’s failure the electricity supply has been shut off. to trip when an overcurrent is present. This is not the case with fuses, in which no mechanical operation is required for the fuse to operate under faulty conditions. The three-pin plug Most appliances such as kettles, toasters, dishwashers and washing A three-pin plug is used to connect an electric appliance to the main electric machines have metal cases. If there is a leakage of electricity in the supply via the power socket. The diagram below shows the parts of a three- appliance and you happen to touch its metal case, the current will flow pin plug. through you to the earth. This causes an electric shock. The earth wire is just live wire a wire connected to the metal case of the appliance. It goes down the flex into the socket and travels down to the earth through the earthing system. earth wire The earth pin of a three-pin plug is longer than the live and neutral pins. This fuse ensures that the earth pin always connects with the socket first. neutral wire cable grip earth pin outlet insulation neutral pin live pin The fuse in the plug protects the appliance when there is an electric fault. When excessive current flows in the appliance, the fuse breaks the circuit and isolates the faulty appliance so that other appliances plugged in the same Procedures for wiring in the mains plug: circuit can still function. The cable from the appliance consists of three wires,  The earth wire should be wired first. This is the reason why it is made namely the live wire, the neutral wire and the earth wire. Each of these wires the longest. Doing so provides a protection against any electricity is made of copper and is covered by a plastic sheath. leakage.  The neutral wire is then connected. The table below summarises the information about the three wires.  Finally, the live wire is connected. This is because in case the wire Live wire Neutral wire Earth wire becomes loose, the current can flow through the earth wire and will not cause any harm to the user. Its plastic sheath is Its plastic sheath is blue Its plastic sheath is brown in colour. in colour. green and yellow in colour. Connection of safety devices in a circuit It carries current from It forms a closed path It conducts current to the Safety devices such as fuses, switches and circuit breakers must be the main supply to the with the live pin when an ground in case of connected to the live wire in the circuit. appliance. electric appliance is electrical fault.  Switches connected to the live wire allow users to turn on or off the connected to the socket. current that flows in an appliance safely and effectively.  Fuses and circuit breakers must be wired to the live wire because Its high voltage is Its voltage does not vary. It is a safety wire and when the current exceeds the safe limit, the fuse or circuit breaker will alternating between It stays close to 0 V voltage is not involved. trip and break the circuit. +240 V and -240 V. The neutral and earth wires do not normally have a current flowing through them, unless the live wire has tripped. Hence, it is ideal for the circuit to break first rather than to have the live wire trip and then break.