37 Static Electricity Atoms are made up of electrons , protons and neutrons . These particles have different properties. Electrons are tiny, very light particles that have a negative electrical charge (-). Protons are much larger and heavier than electrons and have the opposite charge, protons have a positive charge. Neutrons are large and heavy like protons, however neutrons have no electrical charge. Each atom is made up of a combination of these particles. Objects may be charged by rubbing. eg: Rub a piece of ebonite (very hard, black rubber) across a piece of animal fur. • The fur does not hold on to its electrons as strongly as the ebonite. At least some of the • electrons will be ripped off of the fur and stay on the ebonite. Now the fur has a slightly • positive charge (it lost some electrons) and the ebonite is slightly negative (it gained some • electrons).The net charge is still zero between the two… remember the conservation of charge. • No charges have been created or destroyed, just moved around.
Conductors & InsulatorsConductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule. An object made of a conducting material will permit charge to be transferred across the entire surface of the object. eg. WireIn contrast to conductors, insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. eg. RubberElectrostatic Induction: redistribution of electrical charge in an object, caused by the influence of nearby charges.
Dangers & Uses of static electricity Lightning Flue-ash precipitation Refuelling Photocopiers Operating theaters Inkjet printers ComputersElectric fieldElectric field is the region of space where an electric chargeexperiences a force due to other charges and force can be felt.
38 Electric CurrentElectrical current is a measure of the amount of electrical charge transferred per unit time. It represents the flow of electrons through a conductive material. Current is a scalar quantity (though in circuit analysis, the direction of current is relevant). The SI unit of electrical current is the ampere, defined as 1 coulomb/second. Q= coulombs I= amperes/current t= time in seconds Q= I.t Circuit symbols
CircuitsSeries : The current is the same at all points in a series circuit.Parallel : The sum of the current in the branches of a parallel circuitequals the current entering or leaving the parallel section.
Direct & Alternating current In a direct current (d.c.) the electrons flow in one direction only. Steady d.c. Varying d.c.In an alternating current (a.c.) the direction of flow reverses regularly.
39 Potential differencePotential difference is measured in volts (V) and the term voltage is sometimes used instead of p.d. In a circuit, note that electrical energy, not change or current, is “used up”. V= volts V= W / Q or W = Q . V W= energy in joules If Q is steady = W = I . t . V Q= coulombs Voltage in series : V1+V2+V3...=Vtotal Voltage in parallel : V1=V2
40 Resistance The opposition of a conductor to current is called its resistance. A good conductor has low resistance and a poor conductor has a high resistance. The resistance of a wire of a certain material 1) increases as its length increases 2) increases as its cross- section area decreaces 3) depends on the material. R=V/I R= resistance in ohm V= voltage in volts V=I.R I= current in amperes I=V/R Resistors in series = R1+R2+R3...=Rtotal Resistors in parallel = 1/R1+ 1/R2+1/R3...=3/Rtotal Rtotal/3= answer
ResistivityThe resistivity of a material is numerically equal to the resistance of a 1m length of it of cross-section area 1m2. Potential divider V =I.R
41 Capacitors Practical capacitors are often classified according to the material used as the dielectric, with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films. Charging a capacitor: by connecting a battery across it. Discharging a capacitor: when a conductor is connected across a charged capacitor, there is a brief flow of electrons from the negatively charged plate to a positively charged one.
Effect of capacitor in d.c. & a.c. circuitsIn a d.c. Circuit the lamp does not light because a capacitor blocks d.c.In an a.c. Circuit the lamp lights, suggesting that a capacitor passes a.c. In fact, no current actually passes through the capacitor since its plates are seperated by an insulator. But the a.c. reverses direction. d.c.
42 Electric power Power = work done/time taken = energy transfer/time taken P=W/t There are experiments to measure electric power. eg. Lamp, motor Lamp: connect a circuit, note the ammeter and voltmeter readings and work out the electric power supplied to the bulb in watts. Motor: replace a bulb by a small electric motor. Attach a known mass in kg to the axle of the motor with a length of thin string and find the time t in seconds required to raise the mass through a known height in m at a steady speed. Then the power output Po in W of the motor is given by : Po=work done/ time taken
Electricity LightingFilament lamps: a small coil of tungsten wire, which becomes white hot when current flows through it. The higher the temperature of the filament the greater is the proportion of electrical energy transferred to light and for this reason it is made of tungsren, a metal with a high melting point.Fluorescent strips: five times as efficient and may last 3000 hours compared with the 1000 hour life of filament lamps.Compact fluorescent lamps: energy saving fluorescent lamps fit straight into normal light sockers, either bayonet or screw in. They last up to eight times longer and use about five times less energy than filament lamps for the same light output.
Electric heatingHeating elements: domestic appliances are made from nichrome wire. This is an alloy of nickel and chromium which does not oxidize when the current makes it red hot. The elements in radiant electric fires are at red heat and the radiation they emit is directed into the room by polished reflectors.Three heat switch: this is sometimes used to control heating appliances . It has three setting and uses two identical elements. On “HIGH” the elements are in parallel across the supply voltage. On “Medium” current only passes through one and on “LOW” they are in series.Fuses: a fuse should ensure that the current carrying capacity of the wiring is not exceeded.
JoulemeterInstead of using an ammeter and a voltmeter to measure the electrical energy transferred by an appliance, a joulemeter can be used to obtain directly in joules.
43 Electricity in the HOME Every circuit is connected in parallel with the supply. Switches and fuses are always in the live wire and the lights are controlled from two places by the two two-way switches. A house may have ring circuits each serving a different area tp prevent wires to over heat. Fuses are plug to cut short circuit. Earthing is important to discharge. The residual circuit breaker also called the residual current device is an adapted circuit breaker which is used when the resistanve of the earth path between the consumer and the sub-station is not small enough for a fault current to blow the fuse or trip the circuit breaker. Double insulation is used to not make a direct connection with the internal electrical parts.
Paying for electricityA kilowatt-hour is the electrical energy used by a 1 kW appliance in 1 hour. eg. 1 kWh=1000J/s x 3600s =3600000J =3.6 MJ