Mr Cooke s Physics Notes IGCSE Triple Physics 2011 Vers Electricity

Transcription

1 Electricity Introduction... 2 Charge, Current, Voltage and Potential Difference... 2 Charge... 2 Current... 2 Voltage... 3 Mains Electricity... 4 Hazards of Electricity... 5 Safety measures... 5 Heating Effect of Current... 6 Power... 6 Energy... 7 Cost of electricity:... 7 A.C. and D.C... 8 Energy & Potential Difference in Circuits... 9 Current-Voltage Graphs... 9 Ohm s Law Other components Circuit Symbols Electric Charge Charging by Friction Charging by induction: Attraction & Repulsion of Charged Objects Electrostatic Phenomena Uses of Static Electricity Dangers of Static Electricity

2 Introduction Electricity is the topic that has changed our lives most over the last 100 years. Our lives are totally dependent on electricity: without it there would be no Emenem on your ipod, you could not get annoying phone calls or a lovely text on mobile phone, you would not know about facebook as you wouldn t have a computer, no radio would wake you up in the morning, TV would mean nothing else but two letters in the alphabet, and the house would be romantically lit by candles. While electricity is so important in modern Western life, current or voltage cannot be seen. Only the effect of electricity can be experienced. This chapter looks at what electricity is. Special attention is paid to the mains electricity (which we have in the sockets in our homes), to some electronic devices and to static electricity, explaining why you get a shock when you go to Topshop in Ealing. We will start with a look at the basics of electricity. Charge, Current, Voltage and Potential Difference Charge Electrons and Protons have an electric charge. Electrons are negatively charged, protons positively. Ions are atoms with either more or less electrons than protons and can therefore be positively or negatively charged. In this context charge is understood as the number of electrons/protons/ions. A big charge would be e.g. a high number of electrons. Charge is measured in coulombs (C). Nice to know: 1C is the charge of 6.24 x electrons. In metals there are free electrons, sometimes referred to as electron cloud or electron gas which are not fixed to one atom, but shared between the fixed atoms. These free electrons makes metals to such good conductors. Current Current is defined as the rate of flow of charge. In other words, how many coulombs of charge flow through the cross section of a wire per second. Imagine tennis balls (charges) being pushed through a huge pipe. Current would be the number of tennis balls flowing through the cross-section of the pipe per second. Symbol of current: I Unit of current: A (amperes or amps) 2

3 As current is the amount of charges flowing through the cross section of a wire, it can be calculated as Current = charge/time I = Q/t Unit of time in this formula: always s (seconds) This formula is often used in the form: Q = It Examples: 1.) Find the current if a charge of 20C flows through the cross section of a wire within 5s. I = Q/t = 20/5 = 4A 2.) Find the charge transferred if a current of 3A flows over 2min. Q = It= 3 x 120 = 360C Note: conventional current flows from the positive terminal of a battery to the negative. As was discovered later, the electrons in a metal wire flow in the opposite direction. As they are negative they are repelled from the negative terminal and are attracted to the positive. In water or on wet skin positive and negative ions make up the flow of charges. Positive ions (cations) flow to the negative terminal (cathode) and the negative ions (anions) to the positive terminal (anode). Voltage Voltage is the energy transferred per unit charge passed. If the battery provides a voltage of 12V, then 12J will be passed on to every coulomb of charge flowing out of the battery. This energy is then given off to the components. If the voltage in a circuit with a light bulb is increased, the light bulb will become brighter. This is because more energy will be passed on to the bulb. In the tennis-ball model voltage would be how much energy the tennis balls are given by the battery or how much energy they give off to a light bulb. Symbol: V Unit: V (volt) Often the word potential difference (p.d.) is used instead of voltage. Both mean the same. 3

4 Mains Electricity We use mains electricity, supplied by power stations, for all kinds of appliances in our homes. In this chapter you will learn how it is brought to our homes, about its dangers and you will find out about devices that protect users from receiving electric shocks. The electricity you use at home comes from the mains supply. You get charged for the amount of electrical energy you use and it is measured in your home in the electricity meter. On the right you see a diagram of what the older version looks like. The second diagram shows one of the designs you will come across more often in future. The supply cable that connects your house to the National Grid is first connected to the electricity meter. The meter itself is connected to the consumer unit, also known as the fuse box (see left). It is fitted with fuses and circuit breakers, safety devices which will be explained later. All sockets are connected to the ring main circuits. What appears to be a series circuit is actually a parallel circuit. All sockets are provided with the same voltage. The electricity supplied by the mains is different to that of a battery. Current from a battery always flows in the same direction. It is called direct current, usually abbreviated as d.c. The mains current changes direction 50 times per second (in the UK and Europe). This is called alternating current (a.c.). For graphs look at the chapter a.c. and d.c. There are three wires in the mains electricity and therefore three pins on each plug. The wires are: the live wire (brown), which carries a voltage of 230V. the neutral wire (blue), which completes the circuit. the earth wire (green and yellow), which is purely used for safety. The current usually flows through the live and the neutral wire. The diagram on the right is what you would see if you opened a plug. The casing is made of plastic to provide insulation of the metal parts inside. The pins are made of brass, which is a very good conductor, yet harder than copper. For the wires copper is used, which is one of the very best conductors. The cable grip is there so that the cable is held in its place even if you trip over the cable. The fuse is a protection device explained further down. 4

5 Hazards of Electricity Frayed cables (electric shocks) Long cables (when rolled up overheating can potentially cause a fire) Damaged plugs (electric shocks) Water around sockets (electric shocks) Pushing metal objects into sockets (electric shocks) Electric shocks from the mains can potentially be fatal. The likelihood for a shock to be fatal is increased when water is involved as it increases the conductivity of your skin. Safety measures Insulation: conductors of electricity like copper wires in cables are insulated. Therefore users do not get an electric shock when they touch the cable. Double insulation: appliance with a casing made of an insulator (e.g. the plastic casing of a microwave). Should the live wire, which is usually insulated, get loose and touch the casing, a person in contact with the casing would still be safe. A double insulated appliance does not need an earth wire (as you will understand after the next paragraph). Your plastic mobile charger or your hairdryer might therefore have a plastic pin in the place of the earth pin. Sometimes the two layers of insulation on cables (see diagram of plug) are also referred to as double insulation. Earthing: If the casing is made of metal (like this microwave on the left) the user could potentially get a fatal shock should the live wire (due to a serious fault!) touch the casing. The earth wire is connected to the metal casing. The current would flow therefore through the live and the earth wire instead of through the live wire and the person (into the ground, i.e. earth). Earth in electrical terms is like a neutral connector (0V). A metal would usually be buried in the ground and the earth wire connected to it. Excess charge can flow into it or out of it in an emergency. Fuses are made of thin wires which melt and thereby break the circuit when the current flowing through them gets too high. They are placed in the live wire. Fuses are found in plugs and in some appliances. The wire is protected by a little tube around it. Fuses need to be replaced once the metal has melted. The fuses used in plugs in the UK come in three different sizes: 3A, 5A and 13A. These numbers indicate the maximum current that can flow through a fuse before it melts. If a current of 4A flows through an appliance, a 5A fuse needs to be selected as the 3A fuse would melt and the 13A fuse would allow too high a current to flow without breaking the circuit. Other fuse sizes exist, e.g. for car fuses. 5

6 Circuit breakers: the fuse box in the house used to be full of fuses, hence the name. However, there is a disadvantage when a fuse blows and it is Christmas and you spend your evening in the dark because all shops selling fuses are closed. Fuses in the fuse box have therefore mostly been replaced by circuit breakers. This device acts like a fuse and breaks the circuit when the current gets too high. Once it has been tripped it can simply be reset and act like a new fuse. The diagram on the right shows how it works in principle: when the current gets too high, the magnetic pull from the electromagnet becomes strong enough to pull over the Current in live wire soft iron armature. The spring pulls the upper contact away and the circuit is broken. The reset button or switch allows the push the contact back into its place. The picture on the right shows what an actual circuit breaker might look like. If you check your fuse box in your home you will just see the front of it. It usually says in the fuse box which part of the house which circuit breaker is for. Current in live wire Switches for lights or for cookers are always placed in the live wire instead of the neutral wire. The idea is that when you replace a light bulb and you touch the connectors of the bulb, you will not get a shock as long as the switch is off. If the switch was in the neutral wire and you touched the live wire, a current would flow through you into earth, giving you a shock. Heating Effect of Current The heating effect of a current is avoided in cables by making them out of good conductors. However, if you increase the resistance slightly, a wire will heat. This effect is used in toasters, in electric stoves, electric heaters, kettles, etc. Electrical energy is transferred to heat. Power Electrical power can be calculated from current and voltage: Power = current x voltage P (in W) = I (in A) x V (in V) Power is measured in watts (W) or kilowatts (kw). 1000W = 1kW Examples: 1.) A current of 0.26A flows through a light bulb. Find its power rating. The mains voltage is 230V. P = IV = 0.26 x 230 = 60W 6

7 2.) Power rating and fuse sizes are often mixed in questions: a kettle has a power rating of 2.2kW. Find a suitable fuse size for its plug. P = 2.2kW = 2200W I = P/V = 2200/230 = 9.6A The next highest fuse size is therefore 13A. Energy Power is the amount of energy transferred per unit time. A powerful car will gain a high speed (i.e. kinetic energy) in a short time. Mathematically this can be written as: Power = energy / time P = E/t Therefore Energy is : E = P x t As power = current x voltage this formula can be written as: E = I x V x t Electrical energy can be measured in two different units: joules (J) and kilowatthours (kwh). To obtain a result in joules you need current to be in amps (A), voltage in volts (V) and time in seconds (s). To obtain a result in kilowatthours you need to find the power in kilowatts (kw) and the time in hours (h). Conversion of kwh into J: 1Ws = 1J 1kWh = 1000Wh = 1000 x 60min/h x 60 s/min = 3,600,000J Cost of electricity: The cost of electricity can be calculated by multiplying the number of units (kwh) by the cost per unit. Cost for electrical energy = energy (in kwh) x cost per kwh Example: How much does it cost to run a 100W fridge for 24h? Cost per kwh = 9p. Energy for 24h: E= P x t = 0.1kW x 24 = 2.4kWh Cost = 2.4 x 9 = 21.6p 22p 7

8 A.C. and D.C. I So far you have only ever seen current flowing out of a battery. Current from a battery flows in one direction only and is constant. The diagram shows a current that changes in size, but still flows in one direction only. I These two graphs shows an ac current. Important is that the current alternates between positive and negative. This indicates that it changes direction. As mentioned before the voltage of the mains is 230V as opposed to 1.5V of an AA battery. 8

9 Energy & Potential Difference in Circuits Current and voltage are related to each other In most cases the current in a circuit will be higher the higher the applied voltage is. The current also depends on the number of components (e.g. how many light bulbs there are in series) and on the nature of the components. In the following the current-voltage graphs of a few components are shown. Current-Voltage Graphs Filament light bulb The current increases with the voltage. The higher the current the hotter the filament (glowing wire in the bulb) becomes. Its resistance increases with the temperature. The metal atoms vibrate so strongly that it becomes more difficult for the electrons to pass through. The current is no longer proportional to the voltage, the gradient of the graph decreases. The filament in a light bulb is a specific wire that is meant to become very hot. A normal wire has a similar current-voltage graph. It first follows Ohm s law (see below). When the temperature increases due to the high current, the graph curves as its resistance increases. For a resistor, the current-voltage graph is a straight line through the origin. The current through a resistor is directly proportional to the potential difference (voltage) across the resistor. Circuit symbol: 9

10 The diode only allows current to flow in one direction. The diode has a high resistance in the opposite direction. The p.d. at which current starts to flow is around 0.6V. Circuit symbol: Series and Parallel Circuits Series Circuit Parallel Circuit In a series circuit the current is the same throughout the entire loop. In a parallel circuit the current I flowing out of or into the battery splits up into the different branches, here the 6A current splits up into three 2A currents in each branch. Imagine a river running around an island. The total rate of waterflow will add up to the rate of waterflow in the branches. The same principle applies here. 10

11 Series Circuit The voltage is shared between the components. That means the total voltage provided by the battery is the same as the sum of the potential differences (p.d. s) of the individual components. If the components are the same, the shared voltage across them is them is the same, too. The more components are added, the less the p.d. across each component will be. Therefore there would be less energy for each component and in the case of light bulbs they would appear dimmer. If the circuit is broken in any place, the current ceases to flow. If one bulb breaks, all others go out as well. Use: series circuits are sometimes used for Christmas tree lights. In electronic circuits in computers a mixture of series and parallel circuits would be used. Parallel Circuit The potential difference across the different branches is the same as the potential difference of the battery. The p.d. across each component does not change when further branches are added. The energy provided to each branch remains the same. In the case of light bulbs, each bulb maintains the same brightness even if further bulbs are added. If the circuit is interrupted in any of the branches, the current will continue to flow in the other branches. If a light bulb breaks in one of the branches, the others will continue to glow. If the circuit is broken in the main branch near the battery, the current will cease to flow in the entire circuit. Use: As every branch has the same p.d. this setup is best suited to the mains electricity in a house. To every socket, every lightbulb, every appliance the same voltage is supplied. 11

12 Ohm s Law Voltage = current x resistance V = I x R V : the potential difference across a circuit component measured in volts (V) I : the current through the circuit component measured in amperes (A) R : the resistance of the circuit component measured in ohms (Ω) Note: both the symbol for voltage AND the unit are written as capital V. The current through a resistor (at a constant temperature) is directly proportional to the potential difference (voltage) across the resistor. The resistance of a component can be found by measuring the current through, and potential difference across, the component. The current though a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component. Example: An NHEHS girl places a 9 volt battery against her tongue. The shock she feels is from a current of 0.12A. What is the resistance of her tongue? R = V/I = 9/0.12 = 75Ω 12

13 Other components LDRs (light dependent resistors) In a light dependent resistor the resistance decreases with increasing light intensity. LDRs can be used for photographic equipment as well as for switching lights on automatically when it gets dark. Thermistors In a thermistor the resistance decreases with increasing temperature. Thermistors can be used in thermostats (control of temperature). 13

14 LEDs (light emitting diodes) Many devices use little lamps to show that the power has been switched on (e.g. projectors, TVs, etc.). These come in different colours, red and green are the most common. These are called LEDs, a type of diode that emits light. In older circuits filament light bulbs were used for this purpose. Circuit Symbols 14

15 Electric Charge This topic focuses on static electricity. As static electricity is based on the movement of charge (electrons) from one surface to another, the topic is named electric charge. Charging by Friction First of all we will look at materials which can be used to generate static electricity, which are insulators. In insulators electrons are bound to one atom and cannot move around freely. If two insulators are rubbed against each other, they can both become charged. The material that attracts electrons more strongly will remove electrons from the surface of the other material. One material therefore becomes negatively charged, the other one positively charged. Examples of insulators are: acetate, polythene, etc. When rubbed with a cloth, acetate becomes positively charged, i.e. loses electrons from its surface. Polythene becomes negatively charged, i.e. gains electrons. This type of charging materials is called charging by friction. The charge can then be passed on to a conductor, e.g. a metal. (Very good conductors of electricity are: copper, aluminium, gold, silver, etc.) Charging by induction: When a positively charged ruler is brought close to a piece of paper, the electrons on the piece of paper are shifted slightly towards the positive ruler. That side of the paper is then negatively charged, the other side is positively charged. This kind of charging of a neutral object is called charging by induction. The diagram shows a rod being brought to a gold-leaf electroscope (a metal cap on a metal rod with a goldleaf attached to the side). The negatively charged rod induces charges in the previously neutral electroscope. The top is now positive, the bottom negative (the electrons are repelled by the negative rod). 15

16 Attraction & Repulsion of Charged Objects As the charging by induction shows, like charges repel and unlike charges attract. When two charged acetate rods are brought close to each other when one is suspended on a thread, it will turn away. If a charged polythene rod (negative) is brought close to the positively charged acetate rod, the two rods will attract. The diagram shows an example of rods made of different materials, yet showing the same effect. 16

17 Electrostatic Phenomena 1. Attraction and Repulsion of Paper due to Electrostatic Charges The acetate rod gets positively charged when rubbed with a duster. Some electrons from the rod have moved onto the duster. When the rod is brought close to pieces of paper, a charge is induced in the paper. That means: the electrons are attracted by the positive rod and move slightly towards the rod, i.e. this side becomes negative. The opposite side of the paper remains positive as electrons are missing. The overall charge of the paper is still neutral as no charges have left. When the paper touches the ruler, some of the electrons move onto the positive ruler. Therefore the paper becomes positively charged. If the bits of paper have become sufficiently charged and the rod is brought close a second time, they will be repelled this time as like charges repel. If only an insufficient number of electrons has moved from the paper to the rod the first time, the attractive forces will still be bigger than the repulsive forces and the paper will be pulled towards the rod. 17

18 2. As a thin line of water runs out of the tap a charged ruler is brought close to the water. As the water molecules are dipoles (nice to know: oxygen is negative, hydrogen is positive), the water bends towards the ruler. 3. Lightning The lower side of clouds get negatively charged, the top positively charged (most of the time). Due to the excess electrons at the bottom of the cloud, electrons in the ground are repelled. The ground becomes positively charged. Lightning is the discharge of the charged cloud into the ground ( earth ). Van der Graaf Generator Friction occurs between the belt and the plastic roller. The charges from the negatively charged belt are transferred by a metal comb (B) onto the dome. Over time charges built up on the dome. If a person touches the dome, the dome discharges through the body of the person into the ground (earth). If a person touches the dome continuously (preferably at the beginning when the dome is not charged yet), then the charges spread on their body as well. As like charges (here excess electrons) repel, the hair will stand up. 18

19 Uses of Static Electricity Photocopiers The following diagram shows how photocopiers work: Inkjet Printers The inkdroplets are charged as they leave the nozzle. Due to the charged plates the stream of droplets can be directed to the right position on the paper. The diagram shows negatively charged droplets as they are repelled from the negative plate. Spray painting By connecting the spray nozzle to a negative electrode, it is possible to charge each droplet of paint. If the car part is then given the opposite charge, the paint droplets will be attracted to the car body part. This has several advantages: More paint goes to the charged body part and not to the electrically neutral floor. Less wastage most of the paint ends up on the body part, very little is wasted. 19

20 Neater job - The paint is distributed evenly across the surface as droplets repel each other and therefore spread out more. Smoke Precipitator Smoke precipitators are important to reduce pollution. While carbon dioxide or sulfur dioxide cannot be removed from fumes in a chimney, smoke particles can be. The diagram shows a chimney. The smoke moves upwards, the particles get negatively charged ( gain electrons ) as they pass through the negatively charged metal grid. They then get attracted to the positively charged plates on the side. From time to time these plates are struck with a hammer and the collected dust falls down and can be disposed. Dangers of Static Electricity When fuelling an aircraft, the friction between hose and kerosene leads to a build-up of charge. To prevent sparks/fires the aircraft needs to be earthed (connected to the ground). The same applies to tankers, they can be charged due to the fuel moving inside the tank when the tanker drives along the road. A chain touching the ground can be used to discharge the tanker. Similar effects can occur in flour storage when flour is blown through pipes. Sparks (discharge of static electricity) can occur and cause fire/explosions. 20