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Electricity

Electricity. Show video clip on introduction to electricity Write down four things you learnt from the video clip about electricity. Questions on Video clip. What is electricity? What does a battery do? What is current and what is it measured in? What is meant by short circuit.

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Electricity

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  1. Electricity Show video clip on introduction to electricity Write down four things you learnt from the video clip about electricity

  2. Questions on Video clip What is electricity? What does a battery do? What is current and what is it measured in? What is meant by short circuit. What is a circuit? What happens the electic energy when a light bulb is in the circuit? What happens the current when the light bulb is in the circuit? Are electrons big or small?

  3. Keywords

  4. Lesson objectives To be able to clearly define the meaning of the terms “current”, and “voltage / potential difference” To be able to correctly use a voltmeter and an ammeter and an ohmmeter. Identify basic circuit symbols. To understand what resistors and variable resistors are and how they work. To understand the relationship between potential difference and current. To preform calculations using ohm's law.

  5. Lesson objectives To measure resistance of various electronic components. To distinguish between series and parallel circuits and their effects ón light bulbs. To know the three effects of electric current To distinguish between a.c and d.c. Lch. 2.

  6. INTRODUCTION TO ELECTRICITY This is a brief introduction to the main facts of electricity, potential difference, current, switches, parallel and series circuits and resistance. In the diagram are of the common symbols for drawing electric circuits.

  7. ELECTRIC CIRCUITS • An electric current is a flow of negatively charged electrons. • A circuit is a complete path along which a current may flow. • A circuit consists of materials which are conductors (usually metals), which have “free” electrons, loosely attached to atoms, which as they move from atom to atom, carry an electric charge. This flow of charge is the current. • The electrons flow when there is a difference in potential (voltage) across the wire.

  8. Remember to show virtual physics lab. Demo.

  9. In fact because electrons have a negative charge, current actually flows from – to +, but it is the agreed convention that currents shown in circuit diagrams flow from + to -. We call this conventional current Insulators are materials which will not carry currents; they have no free electrons How could you show if a material conducts electricity or not? What apparatus would you need? How would you connect the test substance into the ciruit.

  10. Circuit to test if a substance is a conductor or an insulator.

  11. Conductors and insulators

  12. Conclusions • All metals are conductor as because the electrons are free to move. • In non metals the electrons are held tightly by the nucleus of the atom so cannot flow we call these materials insulators. • Graphite is one of the few non-metals which can conduct electricity.

  13. Voltage Voltage is a measure of the energy carried by the current. Voltage is causes the current to flow. (Technically it is a measure of the difference in energy between two points – hence the name potential difference). Voltage is a measure of the energy carried by the charge. Voltage is supplied by the battery (or power supply). We say voltage across a component. Voltage is measured in volts, V. Voltage is measured with a voltmeter, connected in parallel. The symbol V is used for voltage in equations.

  14. Voltage / potential difference(p.d.) Voltage is measured with a voltmeter. Voltmeters are always placed in parallel in a circuit. Voltage is the driving force that pushes the current like an electric pump. Voltmeter in parallel

  15. CURRENT • Current is the rate of flow of charge. • Current is not used up, only energy • Current is measured in amps (amperes), A. • Current is measured with an ammeter, connected in series. • To connect in series you must break the circuit and put the ammeter acoss the gap, as shown in the diagram. • The symbol I is used for current in equations. • Why is the letter I used for current? ... please see FAQ. Ammeter in series

  16. A Measuring current flow • The brightness of a lamp is an indicator of the amount of current flowing through it. The brighter the lamp the more current flowing. • A better, and more accurate way of measuring current is to use an AMMETER Unit = AMPS (A) symbol A current measuring circuit:

  17. Resistance • Resistance is anything in the circuit which slows the current flow down. • Resistors use up the energy supplied by the battery and turn it into other forms of energy for example • Light bulb – light and heat energy • Electric motor – kinetic and heat energy

  18. VARIABLE RESISTOR This is a component which can vary (change) the resistance of a circuit. This relies on the fact that longer wires have more resistance, by changing the length you can control the current in a circuit Remember to show circuit board with variable resistor as a dimmer switch.

  19. Measuring RESISTANCE Aim: To measure the resistance of various components. A multimeter set to measure Resistance in ohms (Ω) is used. Simply connect the resistor accross] The terminals and read the resistance.

  20. Learning Check • What is electric current? • Explain potential difference? • What is resistance? • What are conductors and insulators? • How could you show if a substance is a conductor or an insulator? • What is a circuit? • Why must electric circuits be complete? • With what is (a) potential difference, (b) current (c) resistance measured with? • What are the units of (a) potential difference, (b) current (c) resistance

  21. The ammeter and the voltmeter have to be connected with their + terminals towards the positive side of the battery. Ohms Law The potential difference is directly proportional to the current flowing in a conductor for a constant temperature. V = constant (called the resistance R) or V = I x R I

  22. Let's see how Ohm's law can be used to calculate the current, voltage or resistance in a circuit. If we know the values of any two of the three quantities (voltage, current, and resistance) in this circuit, we can use Ohm's Law to determine the third.

  23. calculate the amount of resistance (R) in a circuit, given values of voltage (E) and current (I):

  24. calculate the amount of voltage supplied by a battery, given values of current (I) and resistance (R):

  25. Showing the Relationship between current and voltage – across a fixed resistor Proving Ohm's Law Set up this circuit and measure the current and voltage across the resistor at different values – measure at least 5 values Voltage (V) Current (A) A V Use 470 resistors

  26. Relationship between current and voltage – across a fixed resistor Now plot a graph of voltage against current for the fixed resistor circuit. Given what we observed using the virtual physics laboratory when we changed the voltage or current what would you expect the graph to look like? Voltage, V Current, A

  27. Series and Parallel Circuits A. IN SERIES The current flowing through a lamp makes it light. The more current the brighter the lamp.

  28. Series and Parallel Circuits A. IN PARALLEL All lamps are normalbrightness All lamps have the same amount of current flowing through them.

  29. SWITCHES In a series circuit –one switch at either X or Y would control the lamps. In a parallel circuit- Each lamp is controlled independently by each switch.

  30. Effects of Electric Current. Three effects of an electric current. Remembering what has been done in the lesson to date and work we did in first year what effects of electric current have you noticed? • Heating effect. How would you show this? • Magnetic effect. How would you show this? • Chemical effect. How would you show this?

  31. Heating effect of an Electric Current. What would expect to happen the temperature? Why? What does it demonstrate? Uses of the heating effect. Kettle, oven, electric fire, fuse, immersion heater.

  32. Magnetic effect of an electric current • What happens the compass when the switch is closed? • Why? • Can you think of any other way of demonstrating this effect of electricity? Uses of the Magnetic effect • Electric motors, transformers, loudspeakers, computers.

  33. Chemical effect of an electric current Here electricity is being passed through water. A chemical reaction takes place and splits water into hydrogen and oxygen. This type of reaction is called Electrolysis and is an example of the chemical effect of electricity. Uses of the chemical effect Electrolysis -extracting and purifying metals. Electroplating e.g silver plating

  34. Keywords

  35. Electricity in the Home Mains supply two cables • Live (brown) at ±230 V • Neutral (blue) at 0 V • Main fuse on live coming into house. • Both wires go through electricity meter recording the number of units used. • Both wires go through a distribution box with circuit breakers.

  36. Circuit Breakers and Fuses • Circuit breakers and fuses cut off the current if it gets to high for safety, to prevent a fire etc. • Fuses melt if the current is too high (heating effect of an electric current) • Fuses come in various current ratings e.g 1A, 5A, 13A. Must be replaced if they blow. • Circuit breakers trip off if the current gets too high (magnetic effect of an electric current), can be switched back on when the fault is corrected. • Circuit breakers and fuses are always placed on the live wire (why?).

  37. Plugs Most appliances are connected to the electricity supply by pushing a plug into a wall socket. Three wires • Live (brown) with fuse connected in it. • Neutral (blue) • Earth (green and yellow) • What are the safety features of a plug? (3)

  38. Earthing The wire in your house is connected to a metal plate buried in the ground. It is a safety device. If the live wire get damaged and results in the metal body of an appliance such as an electric kettle becoming live then because it is earthed if you touch it the current flows to earth instead of through you.

  39. Learning Check • What is fuse? What effect of electricity does it show? • How does a fuse work? • What is circuit breaker? What effect of electricity does this show? • Why do we have an earth wire? How does it work? • What are the colours for live, neutral and earth wires? • What two advantages doe circuit breakers have over fuses?

  40. The Cost of Electricity Power = work done ÷ time measured in Watts (W) The higher the power rating of an appliance the more energy it uses per second. Rem. 1000W = 1 kW. Examples. 1500 W heater uses 1500 J of energy per second . 2kW cooker uses 2000 J of energy per second. How much energy does a 8.5 kW electric shower use? 8500 J /s

  41. The Cost of Electricity Unit of electricity sold by ESB = the kilowatt-hour kW h (the joule is too small) A kilowatt-hour is the electrical energy converted in other forms in 1 hour by a 1 kW appliance. The number of kWh (units) = no. of kW x no. of hours How many kWh are used by the following: • 8 kW shower for 20 minutes. • 1500 W iron for 1.5 hours. • 100 W bulb for 20 hrs. • 25 W i-pod for 40 hrs.

  42. The Cost of Electricity ANSWERS The number of kWh (units) = no. of kW x no. of hours • Number of units = 8 kW x 0.33 hrs = 2.66 • Number of units = 1.5 kW (1500W) x 1.5 hrs. = 2.25 • Number of units = 0.1 kW (100W) x 20 hrs. = 2.0 • Number of units = 0.025 kW (25 W) x 40 hrs. = 1

  43. The Cost of Electricity The cost of the electricity Cost = number of kWh (units) x price per unit (kWh) Example What is the cost of running a 3kW electric fire for 5 hours if the price per unit is 18 cent.? ANSWER Number of kWh = number of kW x number of hrs = 3 x 5=15 Cost = number of units x cost per unit = 15 x 18 cent = 270 cent or €2.70

  44. The Cost of Electricity Example 2 Find the cost of using the following appliances if a unit of electricity costs 18 cent. • 2000 W heater for 3 hrs. • 1000 W iron for 10 mins. • 150 W freezer for 24 hrs. • 25 W light bulb for 40 hrs. *** you must convert the W to kW first (how?)*** ***then you must find the number of units (kWh)(how?)***

  45. The Cost of Electricity Answer The number of kWh (units) = no. of kW x no. of hours Number of units = 2 kW (2000 W) x 3 hrs. = 6 Number of units = 1 kW (1000W) x 0.167 hrs. = 0.167 Number of units = 0.15 kW (150W) x 24 hrs. = 3.6 Number of units = 0.025 kW (25W) x 40 hrs. = 1 Total number of units = 10.767 Cost = number of kWh (units) x price per unit (kWh) Cost = 10.767 x 18 cent =194 cent or €1.94

  46. The Cost of Electricity Working out the cost of your electricity from the bill. Example The final reading electricity bill ón a 2 months ago is 21185 and the new reading is now 21935. the price per unit is 18 cent. The standing charge is €16.89. V.A.T is charged @ 13.5%. What is the cost of the electricity bill for 2 months? Number of units used = new reading – previous reading = 21935 – 21185 = 750

  47. The Cost of Electricity ANSWER (cont.) Cost = number of kWh (units) x price per unit = 750 x 18 cent = €135.00 standing charge = €16.89 (now add the standing charge) Total before VAT = €151.89 now calculate the V.A.T. VAT @ 13.5% = €20.51 Total (including VAT)= €172.40

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