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Transcript
WHAT IS ELECTRIC CURRENT? An electric current is a flow of microscopic particles called ELECTRONS flowing through wires and electronic components. It can be compared to the flow of water through pipes and radiators etc. As water is pushed through pipes by a pump; electric current is pushed through wires by a battery. Electrons have a negative charge. Electric current will flow from the positive terminal of a battery, through the lamp, to the negative terminal. This is called electron current flow. The current flows round the circuit. This is the conventional current flow, and it is so because as electrons are negative charged, when they leave the electrode, it remains positive. RESISTORS Resistors are electronic components which oppose the flow of electronic current. The higher the value of resistance (measured in ohm =) the lower the current will be. This was discovered by Mr Ohm. The ohm is a small unit, so sometimes it is necessary to use some multiples. The most important are: k= 1000 = 1x103 M = 1000000 = 1x106 = 1000 k = 1x103k In electronic textbooks sometimes we can find the resistors values written as 3k7 that means 3,7k and 4M6 that means 4,6M. THE RESISTOR COLOUR CODE Most resistors have three coloured bands close together at one end and one single band at the other. Each band means a digit according to its position in the resistor, so the three adjacent bands give the resistor value. First you have to place the separated band to the right position and then: The first band on the left, gives the value of the first digit. e.g Brown = 1. The second band gives the value of the second digit e.g. Red = 2 The third band gives the number of zeros which follows the two digits e.g. orange = 3 zeros = 000. It is called the Multiplier band. Therefore a resistor with brown, red, orange bands would have a value of 12000 ohms. TO KNOW HOW TO USE THE RESISTORS COLOUR CODE, FOLLOW THIS LINK: http://ealnet.com/m-eal/resistor/resistor.htm The fourth band indicates the tolerance. e.g. brown indicates plus or minus 1%. a 100 ohm 1% resistor can be in value between 99 ohms and 101 ohms. Tolerance band: brown 1%, red 2%, gold 5%, silver 10%, none 20% If there is a fifth pink band this indicates a high stability resistor. STANDARD PREFIXES FOR INTERNATINAL SYSTEM OF UNITS NAME MULTIPLES SUBDIVISIONS SYMBOL deca- hecto- da kilo- h k MegaM giga- tera- G T FACTOR 101 102 103 106 109 1012 NAME deci- centi- mili- micro- nano- pico- SYMBOL FACTOR d 10-1 c 10-2 -1- m 10-3 10-6 n 10-9 p 10-12 TASKS – PART I 1. COMPLETE THE FOLLOWING TABLE: COLOURS VALUES ( in Ohms - ) Red-red-brown—gold Brown-brown-brown—silver Blue-yellow-green –-red Green-yellow-orange—brown Red-green-brown—none Orange-red-blue—red Brown-black-black—gold White-grey-orange—none Violet-blue-orange—silver 33000±10% 27000000±5% 4K7±2% 9800±1% 100±2% 5M8±20% 6900 ±10% 17K±5% 2. Convert the following values: a)- 1M2 into k b)- 1K8 into c)- 5M6 into K d)- 4K7 into e)- 3M9 into k 3. What is a resistor? 4. Which is the unit used to measure resistance? 5. Name the most common multiples of Ohm and explain their equivalences. -2- OHM’S LAW Different conductors resists current flow by different amounts under different conditions, this property is called resistance R. Ohm’s law states that the current through a conductor is directly proportional to applied p.d. (v), and inversely proportional to the intensity, (I) ,of the current applied. The unit of resistance is Ohm (), the unit of potential difference (p.d.) is V (Volt) and the unit of intensity, I, is A (Ampere). TYPES OF CIRCUITS There are two main types of circuits SERIES and PARALLEL. Some circuits have some components connected in series and other in parallel. They are called MIXED CIRCUITS. SERIES CIRCUIT The p.d. across the cell (V) is equal to the sum of the p.d. across the two components (V1 and V2). V= V1+V2 The current (I) is the same through all parts of the circuit. I = I1 = I2 Total resistance is equal to the sum of the values of all the resistor in the circuit: RT = R1+R2 PARALLEL CIRCUIT The current from the source (I) is equal to the sum of the currents in the two branches (I2 and I2) IT = I1+I2 The potential difference (p.d.) across the two components is equal to the value of Volts from the source (V). V= V1=V2 The formula used to calculate total resistance is: 1 R T ELECTRIC POWER (potencia) P 1 1 R R 1 ... 2 1 R N Electric power is a magnitude that measures how fast is electric energy used in the circuit. The SI unit for power is watt (W). The relationship with other electrical magnitudes are P=I·V and as V=I·R (according to Ohm´s Law) so P=I2·R Units: 1 kW = 1000W = 103 W ELECTRIC ENERGY: P·t // Units : kW·h -3- CIRCUIT ELEMENTS AND SYMBOLS WIRES & CONNECTIONS Component Circuit Symbol Function of the component Wire To pass current very easily from one part of a circuit to another. Wires joined A 'blob' should be drawn where wires are connected (joined), but it is sometimes omitted. Wires not joined In complex diagrams it is often necessary to draw wires crossing even though they are not connected. Power Supplies Cell Supplies electrical energy. A single cell is often wrongly called a battery, but strictly a battery is two or more cells joined together. Battery Supplies electrical energy. A battery is more than one cell. DC supply Supplies electrical energy. DC = Direct Current, always flowing in one direction. Electrons leave the +ive pole and goes to the –ive pole. Output Devices Lamp A transducer which converts electrical energy to light. Motor A transducer which converts electrical energy into kinetic energy (motion). Bell / buzzer A transducer which converts electrical energy to sound. -4- SWITCHES This switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell. Push Switch (push-to-make) This type of push switch is normally closed (on), it is open (off) only when the button is pressed. Push to break switch On off switch An on-off switch allows current to flow only when it is in the closed (on) position. A 2-way switch directs the flow of current to one of two routes according to its position. Two way switch A relay is an electrical switch that opens and closes under the control of another electrical circuit Relay switch HOW DOES A RELAY WORK? The switch is operated by an electromagnet to open or close one or many sets of contacts. The control circuit of the relay is connected to a coil with an iron nucleus that becomes a magnet when the current flows through it. So, the contacts of the controlled circuit will move and open or close the circuit. Label the parts of the following relays. TO LEARN MORE ABOUT RELAYS, FOLLOW THESE LINKS: http://www.youtube.com/watch?v=zY7XXSG6nbo http://www.youtube.com/watch?v=E8Os4WFUALA&feature=related -5- RESISTORS Resistor It restricts the flow of current, for example to limit the current passing through an LED. Variable resistor (potentiometer) It has got 3 contacts. It is usually used to control voltage. Examples include: adjusting lamp brightness, adjusting motor speed… METERS Voltmeter A device used to measure potential difference (voltage). It must be always connected in parallel to the points between the p.d. is being measured. Ammeter A device used to measure current intensity. It must be always connected in series in the branch where intensity is being measured. Ohmmeter A device used to measure resistance. TASKS 1. Calculate the intensity that flows through a 100W lamp connected to 220V. 2. Your music equipment electric power is 40W. If you listen to music for 5 hours, how many kW.h did you use? How much must you pay to the electric company if 1 kW.h costs 0,08€? 3. The electric power used by an iron is 2400 W. If you are ironing for 2 hours, how many kW.h did you use? How much must you pay to the electric company if 1 kW·h costs 0,08 €? 4. Represent, using the adequate symbols, a series circuit with 4 different elements and indicate the position of a voltmeter and an ammeter in it, 5. Represent, using the adequate symbols, a parallel circuit with 4 different elements and indicate the position of a voltmeter and an ammeter in it, 6. Investigate the electric values for 5 electric or electronic devices you use at home. Use a table to record your investigation. -6-