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Electricity and Ohm’s Law

Electricity and Ohm’s Law

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Electricity and Ohm’s Law

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  1. Electricity The flow of electrons is called electric current The unit of electric current is the ampere or amp for short The symbol for amp is A An amp is the measure of how many electrons flow per second Electricity and Ohm’s Law Completing the Circuit

  2. Electricity • For electricity to flow in a circuit it must have: • A complete pathway for the electrons to flow through eg connected wires • An energy supplier eg a battery • An energy user eg a lamp Electricity and Ohm’s Law Completing the Circuit

  3. Draw the 3 circuits Under each one say why you think they will work or won’t work. A B Electricity and Ohm’s Law Completing the Circuit C

  4. Voltage (Potential Difference) e- • A potential difference occurs when there are more electric charges in one location than in another • The unit of potential difference is a volt • The symbol for volt is V • Example: excess electrons will be attracted to the positive plate because of the potential difference + + + + + + + + + + + + + + Electricity and Ohm’s Law Completing the Circuit - - - - - - - - - - - - - - - - - - -

  5. A More Relevant Example • In a household electric outlet, the potential difference (voltage) between the two slots is 240 volts • Because of a power plant many miles away, electrons are under “pressure” to move from one slot to the next

  6. Electrical Circuits • An electrical circuit is formed when electrons are given a path to move across a potential difference • Generally, conducting (metal) wires are used to “complete” the circuit

  7. Electrical Circuits • A circuit like this where the electrons have only one path is called a series circuit.

  8. Bomb Circuit • Pretend a bulb is a bomb and a switch is a key to detonate it. • Draw a circuit where the key will set off the bomb

  9. What type of Circuit is it? A series circuit

  10. Emergency • Now the evil powers of the world rewired the circuit so two keys would detonate the bomb • Draw this circuit

  11. What Type of Circuit is this? A parallel circuit

  12. Electrical Components An electric current is a movement of charged particles. Draw, name and give the main function of the following components + - Name:cellFunction:source of electrical energy Name:LampFunction:turns electrical energy to light Name:resistor Function: to slow current flow Name:fuse Function: to melt when short circuit occurs or too much power is drawn

  13. Copy out and complete the following NameFunction: ammeter A to measure current Ammeters must be placed in circuits in series V NameFunction: voltmeter To measure voltage Voltmeters must be placed in circuits in parallel NameFunction: diode lets current flow in one direction only

  14. Draw the following in symbol form 3 cells joined in series power pack variable power pack open switch

  15. Electrical Components An electric current is a movement of charged particles. Draw, name and give the function of the following components Name:Rheostat Function:to vary resistance Or variable resistor

  16. V A lamp - + A V • Draw a series circuit with a 6volt cell 2 lamps and two ammeters If both lamps are the same comment on their brightness Draw voltmeters to measure the voltage of each lamp If 4 amps was applied to the circuit, what would each ammeter read

  17. Now make the circuit with the gear provided. • Use the multimeters to: V V Measure the voltage across each lamp Lamp 1 = Lamp 2 = Measure the current at 2 different places in the circuit A1 = A2 = A1 - + A2

  18. Voltage • Voltage (also called potential difference) measures: 1. The energy lost across a component 2. Energy supplied by the power supply or battery The unit of voltage, the volt (symbol V) represents the energy change each second for every amp of current If a lamp uses 1.5 volts this means that for every amp of current 1.5 joules of energy is lost every second. Voltage is measured with a voltmeter. Voltmeters must always be connected in parallel to the component for which the voltage is being measured

  19. Current in a Series Circuits • In a series circuit the current flow is along the same path • Current in a series circuit is the same at any point of a series circuit • Current is measured by an ammeter and must always be added to a circuit in series A1 A1 - - + + A2 A2 If 4 amps is supplied to the series circuit A1 = 4 amps A2 = 4 amps

  20. Voltage in a Series Circuit V 6V • If there is only one component in a circuit the voltage across the power supply is equal to the voltage across the components + - V 6V • In a series circuit the voltage across the components add up to the supply voltage

  21. Voltage in a Series Circuits 2V V 4V V + - • In a series circuit the voltage across the components add up to the supply voltage, what is the voltage at V1? • What does the voltage drop over each component depend on? 4V + 2V= 6V V1 The resistance of the component which is related to the energy usage

  22. Voltage in a Series Circuits • What’s an advantage of a series circuit? • What’s a disadvantage of a series circuit? • Where are series circuits used?

  23. Home Wiring: Series Circuits • If lights or appliances are connected in series they will not experience the same voltage • Since most lights and appliances are designed to work at a specific voltage, they cannot be connected in series • The resistance of lights or appliances connected in series is the sum of their resistances Fuse Voltage Lights Switch

  24. Parallel Circuits These are circuits where the lamps are in parallel with each other in separate branches 6 V V All the lamps glow with same brightness. Why? Because the voltage over each lamp is the same as the supply voltage 6 V V - + 6 V

  25. Current in Parallel Circuits Current splits over each branch of a parallel circuit A3 If the current supplied to the parallel circuit at ammeter A1 = 6 amps then: A1 = ____amps A2 = ____ amps A3= ____ amps A4= ____amps A2 A4 6 A1 - + 3 3 6 6 V

  26. Home Wiring: Parallel Circuits • A single switch can be used to turn on multiple lights or appliances if they are connected in parallel • The voltage is the same for each light or appliance, although the current increases with each new addition • A fuse is designed to break if the current gets too large Fuse Voltage Lights Switch

  27. Starter – draw the grid then make a statement Current Voltage Current stays the same at all points in a series circuit Voltage drops over each component in a series circuit Series circuit Parallel circuit Current splits over each branch in a parallel circuit Voltage in a parallel circuit stays the same as the supply voltage

  28. How can two switches work lights in a hallway? switch switch

  29. Conventional Current • Conventional current is the flow of positive charges from the positive terminal of a power supply to the negative terminal of a power supply – • we will use conventional current when looking at the direction of current flow in a circuit Electron flow moves this way through the wire Conventional current (I) This is important when looking at diodes!

  30. Resistance • Resistance is anything that slows the flow of charged particles • Resistance is measured in ohms (symbol )

  31. Ohm’s Law • Voltage, current and resistance in an electric circuit can be related using Ohm’s Law • Ohm’s law states that V = I x R where • V is the voltage (in volts) • I is the current (in amps) • R is the resistance (in ohms)

  32. V I R X Task: Rearrange the formula (V= I x R) using ohms law to find the formula for: V = R = I =

  33. Using Ohms law Example • If a lightbulb in a 240volt circuit is connected and has a resistance of 60 , what current is flowing through the bulb? Answer: • Rearrange V = I x R

  34. Example 1 • If 4 amps of current are flowing through a toaster connected to a 120 V outlet, what is the resistance of the toaster? • Answer: Rearrange V = I x R

  35. Level one Questions – draw the following circuit With both switches closed the current in L1 is 2A 1. Draw the symbol for the component that could measure the voltage across L2 2. What would the device measure? 12 V L1 L2 L3 3. Calculate the resistance of lamp L1 show your working and state the unit 4. Describe the brightness of lamp L2 compared with the brightness of lamp L1 and lamp L3

  36. Investigating voltage – current relationships We will investigate the R = V/I relationship in a circuit to see if resistance changes when voltage and current is changed at constant temperature. Method Set up a series circuit with a heat sync resistor (designed to dissipate heat) a variable voltage supply and an ammeter. Vary the voltage supply, read current and fill in the chart below. Now plot your results on the provided graph paper Voltage on the ‘Y” axis and Current on the “X” axis

  37. Investigating voltage – current relationships Once you have plotted the points draw a straight line graph and find the gradient of the line The gradient of the line gave us the r________ of the circuit which = ____ ( )? Ohmic conductors Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor This is because the temperature of the heat sync resistor remained constant Non Ohmic Conductors (conductors whose temperature increases with voltage and current) Now plot the figures from a circuit with a lamp (a non Ohmic conductor) on page 167 What do you notice?

  38. Investigating voltage – current relationships The gradient of the ohmic voltage vs current graph gave us the ________ of the circuit. Ohmic conductors Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor This is because the temperature of the heat sync resistor remained constant V I

  39. Non Ohmic conductors NonOhmic Conductors (conductors whose temperature and therefore resistance increases with increasing voltage and current) Examples of non ohmic conductors are lamps and resistors V I

  40. Power • In an electrical circuit power (P) is the rate at which electrical energy is : • Used by a component eg a lamp or … • Supplied by a power source eg battery

  41. Power continued • Power is measured in Watts (W) One watt = 1 joule of energy used per second Larger amounts of energy use the unit kilowatts One kilowattt = 1000 watts

  42. Examples A 100 W lamp uses 100 joules of energy per second. A motor which uses 2,500 joules of electrical energy each second has a power rating of 2,500W or 2.5 Kw

  43. Starter – Question Where are parallel circuits used and why?

  44. Calculating Power Power can be calculated using the voltage and current in the circuit. Using the following formula. P = V x I Where: P = power (Watts) V = volts I = current P V I X Can you put these in a triangle to help remember them

  45. Calculate the following power supplied to the lamp P = V I = 12v x 5A = 60 W A 5A V 12V

  46. Use the following circuit to answer the questions V 1. If each of the cells is rated at 2.5 volts what would the voltmeter read? A1 2.5v + 2.5v + 2.5v = 7.5v (A) 2. If each lamp had a resistance of 1.25 ohms calculate the total resistance in the circuit A4 A2 A3 1.25 + 1.25 + 1.25 = 3.75  (A) 3. How much power would the circuit use? (M/E)

  47. 8 V The resistors are identical 4V I= 1.5A 1. What is the voltage across the bulb? 4V 2. What is the current through the bulb? 3 Amps

  48. Draw the circuit 240 V AC Fuse 10A • What is the purpose of a fuse? 0.416A Radio (100 W) To break the circuit if too much current is drawn 0.83A Fridge (200 W) 2.08A b. If all the appliances are turned on at once will the fuse blow? TV (500 W) 4.16A Heater (1000 W) No the fuse would not blow

  49. Read Page 168 Complete questions – 3 , 4 a & 4b on page 169 These are important questions

  50. R= 3  What is the current through the resistor? V 3V A 0.5 A V 3V A car battery supplies a current of 200A to a starter motor at a voltage of 10V. What is the resistance of the starter motor What is the resistance of a bulb if a 240V supply causes a current of 2A to flow through it when it is heated to constant temperature.