9 current, potential difference & resistance hbl (edited )


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9 current, potential difference & resistance hbl (edited )

  1. 1. Current, Potential Differenceand Resistance Jurongville Secondary School Secondary 3A Science
  2. 2. Topic Overview1. Introduction2. Electric Current3. Potential Difference4. Resistance5. Factors Affecting Resistance
  4. 4. Who Discovered Electricity? The history of electricity goes back more than two thousand years, to the time the Ancient Greeks discovered that rubbing fur on amber caused an attraction between the two. By the 17th century, many electricity-related discoveries had been made, such as the invention of an early electrostatic generator, the differentiation between positive and negative currents, and the classification of materials as conductors or insulators. In the year 1600, English physician William Gilbert conned the term electric, from the Greek elektron, to identify the force that certain substances exert when rubbed against each other. While many believe Benjamin Franklin to be the father of electricity, current findings seem to show otherwise. In 1752, Franklin is said to have performed the famous experiment of flying a kite during a thunderstorm, which led to the discovery that lightning and electricity were somehow related. Modern scientists know this to be something of a tall tale, since being hit by lightning would have been fatal. Its likely that Franklin was actually insulated, away from the path of lightning.
  5. 5. Who Discovered Electricity? The kite experiment helped Franklin establish a relationshipbetween lightning and electricity, which led to the invention of the lightningrod. Benjamin Franklin went on to observe other phenomena related toelectricity, but many believe that he didnt actually discover its true nature. In 1800, Italian-born physicist Alessandro Volta constructed thevoltaic pile, later known as the electric battery, the first device to produce asteady electric current. It was Volta, not Franklin, who discovered thatcertain chemical reactions could produce electricity. Volta also created thefirst transmission of electricity by linking positively-charged and negatively-charged connectors and driving an electrical charge, or voltage, throughthem. It wasnt until 1831 that electricity became viable for use intechnology. English scientist Michael Faraday created the electric dynamo,a crude precursor of modern power generators. This invention opened thedoor to the new era of electricity. A few decades later, in 1879, Thomas AlvaEdison invented the light bulb.
  6. 6. Famous Scientists- Click on the links below to read more about them… Coulomb Volta Ampere Charles Augustin de Coulomb 70yrs Alessandro Giuseppe Antonio Anastasio Volta 82yrs André Marie Ampère 61yrs
  7. 7. 1. Electric Current
  8. 8. Electric Current• Electric current, I is defined as the rate of flow of charge.• The SI unit is the ampere (A)• Mathematically, Amount of charge Q flowing past a point Current I t Time taken for charges to flow
  9. 9. CCharges flowing slowly – small currentCharges flowing quickly – large current
  10. 10. Video on Electric Current• Click here to view a video on current.
  11. 11. Electric Current 1000 1000000• 1 A = ___________ mA = ____________ μA.• Conventional current describes the flow of electric charges from the positive terminal to the negative terminal of a battery or power supply.
  12. 12. Electric Current + - cellConventional Flow of current electrons switch bulb
  13. 13. Electric Current Conventional current e- Flow of electrons
  14. 14. Measuring Electric Current
  15. 15. Measuring Electric CurrentWe use an ammeter to measure the size of an electriccurrent. The ammeter must be connected in series tothe circuit as shown below. A
  16. 16. Measuring Electric CurrentNegative Positiveterminal terminal
  17. 17. Measuring Electric CurrentThe positive terminal of ammeter must be connectedto the positive terminal of the battery.
  18. 18. Measuring Electric Current “+” to “+” “-” to “-”
  19. 19. Example 1What is wrong with the electrical circuit shownbelow? – +The ammeter is connected to the batterywrongly. i.e. positive terminal of the battery isconnected to negative terminal of the ammeter.
  20. 20. Example 2What is the reading on the ammeter shownbelow?10 divisions  0.2 A1 division  0.2 / 10 = 0.02 AAmmeter reading = 14 divisions = 0.02 x 14 = 0.28 A
  21. 21. 2. POTENTIAL DIFFERENCE (p.d.)
  22. 22. 2. Potential Difference (p.d.)In an electric circuit, electric charges require energyto move around the circuit.The energy is given by a electrical energy source suchas a battery. battery bulb
  23. 23. Potential Difference (p.d.)The electric cell has positions of high potential andlow potential.The difference between the high and low potentialsprovides the energy that moves the charges out ofthe battery through the wires in a closed circuit. High potential Low potential
  24. 24. Potential Difference (p.d.)• When there is a potential difference across a conductor, a current will flow through the conductor.• The SI unit is the volt (V)
  25. 25. Measuring Potential difference.• We use a voltmeter to measure the – voltage of a electrical source or – potential difference across an electric component.
  26. 26. Measuring Potential DifferenceNegative Positiveterminal terminal
  27. 27. Measuring Potential Difference• The voltmeter must be connect in parallel.• The positive terminal of voltmeter must be connected to the positive terminal of the battery.
  28. 28. Example 3What is wrong with the electrical circuit shownbelow? voltmeter – +The voltmeter is connected wrongly. i.e. itshould be connected in parallel with the bulb.
  29. 29. Example 4What is the reading on the voltmeter shownbelow?10 divisions  1 V1 division  1 / 10 = 0.1 VVoltmeter reading = 12 divisions = 0.1 x 12 = 1.2 V
  30. 30. 3. RESISTANCE
  31. 31. What is resistance?• Resistance is a measure of how difficult it is for an electric current to pass through a material.• It is a property of the material that restricts the movement of charges in the material.
  32. 32. Optional: Video on ResistanceClick on links to view videos onresistance.• Video 1• Video 2
  33. 33. ResistanceResistance is defined as the ratio of thepotential difference across the resistor to thecurrent flowing through it. Volt, V V R I Ampere, AThe SI Unit of resistance is ohm (Ω).
  34. 34. Example 5A coil of wire is connected across a 6 V batter. If thecurrent in the wire is 0.3 A, what is the resistance ofthe wire? V 6 R 20 I 0.3The resistance of the wire is 20 Ω