Static ElectricityPositive and negative charges There are two kinds of electric charge. Positive charge (+) and negative charge (-). Like charges (+ and + or – and – ) repel, whereas unlike charges (+ and -) attract. The force between them decreases when the distance increases.
Charges, atoms and electronsSmall central nucleus which containspositively charged particles called protons,are surrounded by an equal number ofnegatively charged called electrons. Aelectrically neutral atom has the samenumber of protons and electrons. Everynucleus, except hydrogen, containsuncharged particles called neutrons.
Electrons, insulators and conductors An electroscope is a device to determine or measure the presence of electrostatic charges. In an insulator all electrons are bound firmly to their atoms and in a conductor some electrons can move freely from atom to atom. Good insulators are plactics such as polyetene, cellulose acetate, Perspex and nylon. All metals and carbon are good conductors. Wood, paper, cotton, the human body and the Earth are poor conductors and insulators.
Electrostatic Induction Electrostatic induction is a quick way of using a charged object to give something a charge, of the opposite sign, without losing any of the original charge. The attraction of an uncharged object by object near it is due to electrostatic induction.
Dangers of static electricity Lightning Refuelling Opering Theatres ComputersUses of static electricity Flue-ash precipitation Photocopiers Inkjet Printers
Van de Graaff generator The basic use of a Van de Graaf generator is to separate electric charges and build them up, frequently for demonstration purposes A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand.
Electric FieldsIt is the region of space where an electric chargeexperiences a force due to other charges.
Electric Current Electric current is a flow of electric charge through a medium An electric current has three effects that reveal its existence : Heatingand lightning Magnetic Chemical
• The ampere and the coulomb ▫ Current is measured in ampere (A). One milliampere (mA) is one-thousandth of an ampere. Ammeter is the measuring instrument of current. ▫ The unit of charge, the coulomb (C), is defined in terms of the ampere. 1C = 1A s.
• Circuit diagrams Current must have a complete path or circuit of conductors if it is to flow. These symbols are the commonly-used ones in a circuit.
Series and parallel circuits Series In a series circuit, the current is the same at all points in a circuit but it has different numbers of potential different. Parallel In a parallel circuit, the potential difference is the same at all points in a circuit but it has different numbers of current.
The battery is said to have a potential difference (p. d.) at its terminals. Potential difference is measured in volts (V) and the term voltage is sometimes used instead of p.d. Its defined by V = I x R
Energy transfers and p.d. Energy in a circuit is supplied from a source such as a battery and is transferred to other forms of energy by devices in the circuit. The volt The p.d. between two points in a circuit is 1 volt of 1 jould of electrical enegry is transferred to other forms of energy when 1 coulomb passes from one point to another.
▫ Cells, batteris and e.m.f. A battery consists of two or more electric cells. Greater voltages are obtained when cellls are joined in series. When no current is drwn from a battery it is said to be an open circuit and its terminal p.d. is a maximum. The maximum voltage is called the electrmotive force (e.m.f)▫ Voltages round a circuit These are how to calculate the total of V in a circuit; Series : V= V1+V2+V3 Parallel : V1=V2
The opposition of a conductor to current is called its resistance. A good conductor has a low resistance, a poor conductor has a high resistance. It increases as its length increases, as its cross-section area decreaes, and it depens on the material. Silver and copper are the best conductors.
The Ohm The resistance is defined by R = V.I and it is measured in Ohms (Ω). The ohm is the resistance of a conductor in which the current is 1 when a voltage of 1 volt is applied to it.
Resistors Conductors intended to have resistance are called resistors. They arre usually made from wires of special alloys or carbon. Rheostat is a variable resistor, used for adjusting the current, can also work as a potential divider to adjust the p.d. in a circuit.
These are how to calculate the total ofresistance in a series; -Series R = R1+R2+R3 -Parallel 1/R = 1/R1+1/R2+1/R3
Capacitors A capacitor stores electric charge and its usefu in many electronic circuits. The insulator called the dielectris is sandwiched between two parallel metal plates.
A capacitor can blok d.c. and passes a.c. Capacitance The more charge a capacitor can store, the greater is its capacitance (C). It is measured in farads but smaller units like microfarad are more convinient.
Types of capacitor Practical Capacitor Plastics are the commonly-used material for this kind of capacitor, with films of metal being deposited on the plastic to act as the plates. Electrolytic capacitor It has a very thin layer of aluminium oxide as the dielectric between two strips of aluminium foul, giving large capacities.
Electric power Energy transfers were measured by the work done and power was defined by the equation; power = work done/time taken = energy taken/time taken Lamp and motor can be used to measure the electric power in a circuit. Instead of using ammeter and a volmeter to measure the electrical energy transferred by an appliance, a joulemeter can be used to obtain it directly in joules.
These are some aspects that are important to be concerned for electricity in the home; circuits in parallel, switches and fuses, staircase circuit, ring main circuit, fused plug, earthing and safety, circuit breakers and double insulation.
• Paying for electricity – A killowat-hour is the electrical energy used by a 1 kW appliance in 1 hour.• Dangers of electricity – Electric shock – Fire risks
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