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Electrical Energy Storage ◊ We can store electric energy in a capacitor : ◊ Found in nearly all electronic circuits eg. in photo-flash units. ◊ Simplest.

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Presentation on theme: "Electrical Energy Storage ◊ We can store electric energy in a capacitor : ◊ Found in nearly all electronic circuits eg. in photo-flash units. ◊ Simplest."— Presentation transcript:

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2 Electrical Energy Storage ◊ We can store electric energy in a capacitor : ◊ Found in nearly all electronic circuits eg. in photo-flash units. ◊ Simplest is: two close but separated parallel plates. When connected to a battery electrons get transferred from When connected to a battery electrons get transferred from one plate to the other until the potential difference between one plate to the other until the potential difference between them = voltage of battery. them = voltage of battery. ◊ How? Positive battery terminal attracts electrons on LH plate; these are then pumped through battery, through the terminal to the opposite plate. Process continues until no more potential difference btn plate and connected terminal. - - - - - - - - - - - - + + + + + + + + + + + + ◊ ◊ Discharging: when conducting path links the two charged plates. ◊ ◊ Discharging is what creates the flash in a camera. ♦If very high voltages (eg caps in tv’s), its dangerous if you are this path! animation ????

3 Potential difference or Voltage (symbol V) When the ends of an electric conductor are at different electric potential, charge flows from one end to the other. Voltage is what causes charge to move in a conductor. Charge moves toward lower potential energy the same way as you would fall from a tree. Voltage plays a role similar to pressure in a pipe; to get water to flow there must be a pressure difference between the ends, this pressure difference is produced by a pump A battery is like a pump for charge, it provides the energy for pushing the charges around a circuit

4 Voltage and current are not the same thing You can have voltage, but without a path (connection) there is no current.You can have voltage, but without a path (connection) there is no current. voltage Anelectricaloutlet Current– flow of electric charge If I connect a battery to the ends of the copper bar the electrons in the copper will be pulled toward the positive side of the battery and will flow around and around.  this is called current – flow of charge copper Duracell + An electric circuit!

5 Electric current (symbol I) DEF: the rate at which charge flows past a given cross-section.DEF: the rate at which charge flows past a given cross-section. measured in amperes (A) q ◊ the flow of electric charge q that can occur in solids, liquids and gases. Solids – electrons in metals and graphite, and holes in semiconductors Liquids – positive and negative ions in molten and aqueous electrolytes Gases – electrons and positive ions stripped from gaseous molecules by large potential differences.

6 Electrical resistance (symbol R) Why is it necessary to keep pushing the charges to make them move?Why is it necessary to keep pushing the charges to make them move? The electrons do not move unimpeded through a conductor. As they move they keep bumping into the ions of crystal lattice which either slows them down or bring them to rest.The electrons do not move unimpeded through a conductor. As they move they keep bumping into the ions of crystal lattice which either slows them down or bring them to rest.. atoms free electron (actually positive ions) path The resistance (R) is a measure of the degree to which the conductor impedes the flow of current. Resistance is measured in Ohms (  )

7 OHM’S LAW - Current, Voltage and Resistance DEF: Current through resistor (conductor) is proportional to potential difference on the resistor if the temperature of a resistor is constant (the resistance of a conductor is constant). ◊ math def: if resistance R is constant/ temperature is constant if resistance R is constant/ temperature is constant I – current V – potential difference across R I – current V – potential difference across R

8 Examples If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw?If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw? I = V / R = 3 V / 9  = 1/3 Amps I = V / R = 3 V / 9  = 1/3 Amps If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb?If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb? R = V / I = 120 V / 2 A = 60  R = V / I = 120 V / 2 A = 60 

9 Effects of electric current on the BODY- electric shock Current (A) Effect 0.001 can be felt 0.005 painful 0.010 involuntary muscle contractions (spasms) 0.015 loss of muscle control 0.070 if through the heart, serious disruption; probably fatal if current lasts for more than 1 second questionable circuits: live (hot) wire ? how to avoid being electrified? questionable circuits: live (hot) wire ? how to avoid being electrified? 1.keep one hand behind the body (no hand to hand current through the body) 2. touch the wire with the back of the hand. Shock causing muscular contraction will not cause their hands to grip the wire.

10 human body resistance varies: 100 ohms if soaked with salt water; moist skin - 1000 ohms; normal dry skin – 100 000 ohms, extra dry skin – 500 000 ohms. What would be the current in your body if you touch the terminals of a 12-V battery with dry hands? I = V/R = 12 V/100 000  = 0.000 12 A quite harmless I = V/R = 12 V/100 000  = 0.000 12 A quite harmless But if your hands are moist (fear of AP test?) and you touch 24 V battery, how much current would you draw? I = V/R = 24 V/1000  = 0.024 A I = V/R = 24 V/1000  = 0.024 A a dangerous amount of current. a dangerous amount of current.

11 Factors affecting resistance The units of resistance are volts per ampere (VA -1 ). However, a separate SI unit called the ohm Ω is defined as the resistance through which a current of 1 A flows when a potential difference of 1 V is applied. Conductors, semiconductors and insulators differ in their resistance to current flow. DEF: The electrical resistance of a piece of material is defined by the ratio of the potential difference across the material to the current that flows through it.

12 Wires, wires, wires As you are going to see, the resistance of a wire can be completely ignored – if it is a thin wire connecting two, three or more resistors, or becoming very important if it is a long, long wire as in the case of iron, washing machine, toaster ….., where it becomes resistor itself. The resistance of a conducting wire depends on four main factors: length cross-sectional area resistivity temperature

13 Cross Sectional Area (A) The cross-sectional area of a conductor (thickness) is similar to the cross section of a hallway. If the hall is very wide, it will allow a high current through it, while a narrow hall would be difficult to get through. Notice that the electrons seem to be moving at the same speed in each one but there are many more electrons in the larger wire. This results in a larger current which leads us to say that the resistance is less in a wire with a larger cross sectional area. Length of the Conductor (L) The length of a conductor is similar to the length of a hallway. The length of a conductor is similar to the length of a hallway. A shorter hallway will result in less collisions than a longer one. A shorter hallway will result in less collisions than a longer one. Temperature To understand the effect of temperature you must picture what happens in a conductor as it is heated. Heat on the atomic or molecular scale is a direct representation of the vibration of the atoms or molecules. Higher temperature means more vibrations. In a cold wire ions in crystal lattice are not vibrating much so the electrons can run between them fairly rapidly. As the conductor heats up, the ions start vibrating. As their motion becomes more erratic they are more likely to get in the way and disrupt the flow of the electrons. As a result, the higher the temperature, the higher the resistance. To understand the effect of temperature you must picture what happens in a conductor as it is heated. Heat on the atomic or molecular scale is a direct representation of the vibration of the atoms or molecules. Higher temperature means more vibrations. In a cold wire ions in crystal lattice are not vibrating much so the electrons can run between them fairly rapidly. As the conductor heats up, the ions start vibrating. As their motion becomes more erratic they are more likely to get in the way and disrupt the flow of the electrons. As a result, the higher the temperature, the higher the resistance. At extremely low temperatures, some materials, known as superconductors, have no measurable resistance. This is called superconductivity. Gradually, we are creating materials that become superconductors at higher temperatures and the race is on to find or create materials that superconduct at room temperature. We are painfully far away from the finish line. At extremely low temperatures, some materials, known as superconductors, have no measurable resistance. This is called superconductivity. Gradually, we are creating materials that become superconductors at higher temperatures and the race is on to find or create materials that superconduct at room temperature. We are painfully far away from the finish line.

14 Resistance also depends on temperature, usually increasing as the temperature increases. At low temperatures some materials, known as superconductors, have no resistance at all. Resistance in wires produces a loss of energy (usually in the form of heat), so materials with no resistance produce no energy loss when currents pass through them. And that means, once set up in motion (current) you don’t need to add additional energy in oder to keep them going. The dream: current without cost!!!!!!!!! Both in money and damage to environment!!!!!!!!

15 Resistance of a wire when the temperature is kept constant is: The resistivity, ρ (the Greek letter rho), is a value that only depends on the material being used. It is tabulated and you can find it in the books. For example, gold would have a lower value than lead or zinc, because it is a better conductor than they are. The unit is Ωm. Of course, resistance depends on the material being used. In conclusion, we could say that a short fat cold wire makes the best conductor. If you double the length of a wire, you will double the resistance of the wire. If you double the cross sectional area of a wire you will cut its resistance in half.

16 Example A copper wire has a length of 160 m and a diameter of 1.00 mm. If the wire is connected to a 1.5-volt battery, how much current flows through the wire? The current can be found from Ohm's Law, V = IR. The V is the battery voltage, so if R can be determined then the current can be calculated. The first step, then, is to find the resistance of the wire: L = 1.60 m. r = 1.00 mm  = 1.72x10 -8  m, copper - books The current can now be found from Ohm's Law: The resistance of the wire is then: R =  L/A = (1.72x10 -8  m)(1.67)/(7.85x10 -7 m 2 ) = 3.50  I = V / R = 1.5 / 3.5 = 0.428 A

17 Ohmic and Non-Ohmic conductors How does the current varies with potential difference for some typical devices? current potentialdifference devices are non-ohmic if resistance changes current potentialdifference current potentialdifference metal at const. temp. filament lamp diode Devices for which current through them is directly proportional to the potential difference across device are said to be ‘ohmic devices’ or ‘ohmic conductors’ or simply resistors. There are very few devices that are trully ohmic. However, many useful devices obey the law at least over a reasonable range.

18 Do Now Answer the following 1)What is power? What is unit for power? 2)Can you figure out 2 reasons for this year Nobel prize in physics? “for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources" It is an invention of greatest benefit to mankind

19 The main advantage is efficiency. In conventional tungsten/wolfram bulbs, the light-production process involves generating a lot of heat (the filament must be warmed). This is completely wasted energy, unless you're using the lamp as a heater, because a huge portion of the available electricity isn't going toward producing visible light. LEDs generate very little heat, relatively speaking. A much higher percentage of the electrical power is going directly to generating light, which cuts down on the electricity demands considerably. https://www.youtube.com/watch?v=oCEKMEeZXug

20 By the End of Today’s Class You Should Be Able To… Define and calculate electric power Describe the difference between current and electron flow Contrast DC and AC current Differentiate between real and drift speed of electrons Calculate how much your parents pay for devices you use

21 Agenda… Do Now 10 minutes INM: Using Electrical Energy 20 minutes Guided Practice Problems 20 minutes Independent Practice 30 minutes Closing, Exit Ticket 10 minutes

22 Electrical Energy, Resistance, and Power Energy supplied to a circuit can be transformed in many useful ways. Remember that current moving through a resistor increases its thermal energy because flowing electrons collide with lattice ions of the resistor. As a result the amplitude of vibrations of the ions increases and therefore the temperature of the device increases. We say device dissipates energy in it.As a result the amplitude of vibrations of the ions increases and therefore the temperature of the device increases. We say device dissipates energy in it. However not all of the electrical energy delivered ends up in useful form. The light bulb to the left transforms some of the electrical energy from the battery into thermal energy Where is that energy coming from? This energy is equal to the potential energy lost by the charge as it moves through the potential difference that exists between the terminals of the device.

23 DEF: Power is the rate at which electric energy is converted into another form such as mechanical energy, heat, or light. into another form such as mechanical energy, heat, or light. Power dissipation in resistors If a vacuum cleaner has a power rating of 500 W, it means it is converting electrical energy to mechanical, sound and heat energy at the rate of 500 J s -1. A 60 W light globe converts electrical energy to light and heat energy at the rate of 60 J s -1. Power is measured in J s -1 called watts W. Appliance Power rating Blow heater 2 kW Kettle 1.5 kW Toaster 1.2 kW Iron 850 W Vacuum cleaner 1.2 kW Television 250 W

24 Basic definition of power: Deriving expressions for determining power Remember: W = qV → and I = q/t, so P = I V P = IV = V 2 /R = I 2 R

25 Comparison of US and other countries that use voltage of 240 V. CFU The power of appliances must be roughly the sameThe power of appliances must be roughly the same Voltage in USA is 120V and in other countries 120 V.Voltage in USA is 120V and in other countries 120 V. Which appliances have to draw a greater current?Which appliances have to draw a greater current? A) in USA, A) in USA, B) other countries B) other countries Which appliances have less resistance?Which appliances have less resistance? A) in USA, A) in USA, B) other countries B) other countries Which countries use thicker wire (both used for connecting and in appliances)?Which countries use thicker wire (both used for connecting and in appliances)? A) in USA, A) in USA, B) other countries B) other countries So, 220V was an economically more meaningful choice. There is really not too much safety difference between getting zapped by 110V or 220V !!!

26 example How much current is drawn by a 60 Watt light bulb connected to a 120 V power line?How much current is drawn by a 60 Watt light bulb connected to a 120 V power line? P = 60 W = I V = I x 120 P = 60 W = I V = I x 120 so I = 0.5 A What is the resistance of the bulb?What is the resistance of the bulb? I = V/R R = V/I = 120 V/0.5 A I = V/R R = V/I = 120 V/0.5 A R = 240  R = 240  CFU

27 Direct Current (DC) electric circuits a circuit containing a battery is a DC circuita circuit containing a battery is a DC circuit in a DC circuit the current always flows in the same direction.in a DC circuit the current always flows in the same direction. Duracell + a circuit must provide a closed path for the current to circulate arounda circuit must provide a closed path for the current to circulate around when the electrons pass through the light bulb they loose energy  the bulb gives light and heats upwhen the electrons pass through the light bulb they loose energy  the bulb gives light and heats up the battery is like a pump that re-energizes them each time they pass through itthe battery is like a pump that re-energizes them each time they pass through it

28 Direction of Current Current is defined as the direction positive charges would flow From + side of battery to – side of batteryFrom + side of battery to – side of battery Fun Fact Benjamin Franklin defined current in this manner long before we knew much about charges. Now, we know that positive charges stay put and negative charges flow. So, electrons actually flow opposite current. hystoric explanation click me

29 When a battery is connected across the ends of a metal wire, an electric field is produced in the wire.When a battery is connected across the ends of a metal wire, an electric field is produced in the wire. All free electrons in the circuit start moving at the same time.All free electrons in the circuit start moving at the same time. ree electrons are accelerated reaching enormous speeds of about 10 6 ms -1. They collide with positive ions of crystal lattice generating heat that causes the temperature of the metal to increase.Free electrons are accelerated reaching enormous speeds of about 10 6 ms -1. They collide with positive ions of crystal lattice generating heat that causes the temperature of the metal to increase. After that, they are again accelerated because of the electric field,After that, they are again accelerated because of the electric field, until the next collision occurs. until the next collision occurs. Due to the collisions with positive ions of crystal lattice, hence changing direction, it is estimated that the drift velocity is only a small fraction of a metre each second (about 0.1mm/s).Due to the collisions with positive ions of crystal lattice, hence changing direction, it is estimated that the drift velocity is only a small fraction of a metre each second (about 0.1mm/s). Drift speed example: it takes ~ 3 hour for an electron to travel through 1m in an electric circuit of a car. it’s not even a snail’s pace!!!!!

30 the electricity that you get from the power company is not DC it is AC (alternating) created by an AC electric generator.the electricity that you get from the power company is not DC it is AC (alternating) created by an AC electric generator. In an AC circuit the current reverses direction periodicallyIn an AC circuit the current reverses direction periodically_ + AC current alternates in direction. The back-and-forth motion occurs at frequency of 50 or 60 Hz, depending on the electrical system of the country. AC movement of electrons in a wire !!!!!!! the source of electrons is wire itself – free electrons in it !!!!!! If you are jolted by electric shock, electrons making up the current in your body originate in your body. They do NOT come from the wire through your body into the ground. Alternating electric field causes electrons to vibrate. Small vibrations – tingle; large vibrations can be fatal.

31 current time AC current time DC How does the voltage and current change in time? DC does not change direction over time; the actual voltage in a 120-V AC circuit varies between +170V and -170V peaks.

32 AC vs. DC current for heaters, hair dryers, irons, toasters, waffle makers, the fact that the current reverses makes no difference. They can be used with either AC or DC electricity.for heaters, hair dryers, irons, toasters, waffle makers, the fact that the current reverses makes no difference. They can be used with either AC or DC electricity. battery chargers (e.g., for cell phones) convert the AC to DCbattery chargers (e.g., for cell phones) convert the AC to DC Why do we use AC ?? DC seems simpler?Why do we use AC ?? DC seems simpler? late 1800’s  the war of the currentslate 1800’s  the war of the currents Edison (DC) vs Tesla (Westinghouse) (AC)Edison (DC) vs Tesla (Westinghouse) (AC) Edison opened the first commercial power plane for producing DC in NY in 1892Edison opened the first commercial power plane for producing DC in NY in 1892 Tesla who was hired by George Westinghouse believed that AC was superiorTesla who was hired by George Westinghouse believed that AC was superior Tesla was right, but Edison never gave up!Tesla was right, but Edison never gave up!

33 Why AC is better than DC DC power is provided at one voltage only There is energy lost in a power line due to dissipation of energy to heat throughout the length of the cable. So DC power plants must be close to users The major advantage of AC: AC voltages can be transformed to higher or lower voltages (can be stepped up or down to provide any voltage required) by the use of the transformers. This means that low voltage produced in a generator can be stepped up to higher voltage. High voltages in power lines are actually needed for sending power from one place to another over great distances. Power is equal to the current times the voltage. That means that if you want to send a lot of power you can use: a) high current b) high voltage if you use high current, resistance should be small. To reduce the resistance of power lines becomes very expensive since, in order to do this, bigger cables must be used (heavier cables aren’t very safe either). To avoid a lot of the power to be lost to the resistance in the wires it is much better to use high voltage since the currents are smaller then!

34 Electrical energy sent at high voltage over great distances from the power station can be eventually reduced to a safer voltage for use in the house. Some long distance power lines use voltages of more than 500,000 V AC plants can be far outside cities by 1895 DC was out and AC was in

35 Paying for electricity You pay for the total amount of electrical energy (not power) that is used each monthYou pay for the total amount of electrical energy (not power) that is used each month In Irving the cost of electric energy used is 12 ¢ per kilowatt-hour.In Irving the cost of electric energy used is 12 ¢ per kilowatt-hour. How do we get kilowatt-hour and what is that?How do we get kilowatt-hour and what is that? Power = energy/timePower = energy/time Physicists measure energy in joules, but utility companies customarily charge energy in units of kilowatt-hours (kW h), where : Kilowatt-hour (kWh) = 10 3 W x 3600 s 1 kWh = 3.6 x 10 6 J 1W x 1s = 1J

36 amount of power in kW used by the clock over 30 days E = Pt ; where time is in hours and P is in kW cost = E($.12) Cost = $0.10

37 Agenda… Do Now10 minutes INM: Using Electrical Energy20 minutes Guided Practice Problems20 minutes Independent Practice30 minutes Closing, Exit Ticket10 minutes As you work with a partner, take 10 minutes to answer the problems on the handout. we will finish together as a whole group and review our progress.

38 Answer the following problems 1) A 15 ohm electric heater operates on a 120 V outlet. a) What is the current running through the heater? I = 8.0 A b) How much energy is used by the heater in 30.0 s? E = 2.9x10 4 J c) How much thermal energy is liberated in this time? E = 2.9x10 4 J 2) An electric space heater draws 15.0 A from a 120 V source. It is operated, on the average, for 5.0 h each day. a) How much power does the heater use? P = 1.8 kW b) How much energy in kWh does it consume in 30 days? E = 270 kWh c) At $0.12 per kWh, how much does it cost to operate the heater for 30 days? $32.40 Guided Practice

39 Agenda… Do Now10 minutes INM: Using Electrical Energy20 minutes Guided Practice Problems20 minutes Independent Practice30 minutes Closing, Exit Ticket10 minutes Work in silence please. Raise your hand if you have a question, I will help you but I will not just give you the answer to any problem during this time.

40 Agenda… Do Now10 minutes INM: Using Electrical Energy20 minutes Guided Practice Problems20 minutes Independent Practice30 minutes Closing, Exit Ticket10 minutes

41 We have defined potential difference as the amount of work that has to be done to move a unit positive charge from one point to the other in an electric field. Electromotive force (emf – ε or E) DEF: emf (ε) of the source is the potential difference between the terminals when NO current flows to an external circuit.DEF: emf (ε) of the source is the potential difference between the terminals when NO current flows to an external circuit. (IT IS A VOLTAGE NOT A FORCE). (IT IS A VOLTAGE NOT A FORCE). A battery or an electric generator that transforms one type of energy into electric energy is called source of electromotive force In the true sense, electromotive force (emf) is the work (energy) per unit charge made available by an electrical source.

42 D.C. circuit analysis An electric circuit has three essential components 1. A source of emf. 2. A conducting pathway obtained by conducting wires or some alternative. wires or some alternative. 3. A load to consume energy such as a filament globe, other resistors and electronic components. globe, other resistors and electronic components. Electric Circuits: Any path along which electrons can flow is a circuit. For a continuous flow of electrons, there must be a complete circuit with no gaps. A gap is usually provided by an electric switch that can be opened or closed to either cut off or allow electron flow. When the switch is closed, a current exists almost immediately in all circuit. The current does not “pile up” anywhere but flows through the whole circuit. Electrons in all circuit begin to move at once. Eventually the electrons move all the way around the circuit. A break anywhere in the path results in an open circuit, and the flow of electrons ceases.

43 Terminal voltage, emf and internal resistance In the circuit the total energy supplied is determined by the value of the emf. When electrons flow around a circuit, they gain potential energy in the cell and then lose the energy in the resistors. In a closed circuits charge must flow between the electrodes of the battery and there is always some hindrance to completely free flow. So when the current I is drawn from the battery there is some resistance called INTERNAL RESISTANCE (r ) of the battery causing the voltage between terminals to drop below the maximum value specified by the battery’s emf. Thus the TERMINAL VOLTAGE (the actual voltage delivered) is: V =  - Ir

44 In the mid-nineteenth century, G.R. Kirchoff (1824-1887) stated two simple rules using the laws of conservation of energy and charge to help in the analysis of direct current circuits. These rules are called Kirchoff’s rules.

45 ‘The sum of the currents flowing into a point in a circuit equals the sum of the currents flowing out at that point’. 1. Junction rule – conservation of charge. I 1 + I 2 = I 3 + I 4 + I 5 I 1 + I 2 = I 3 + I 4 + I 5 2. loop rule – conservation of energy principle: Energy supplied equals the energy released in this closed path ‘In a closed loop, the sum of the emfs equals the sum of the potential drops’. V = V 1 + V 2 + V 3

46 Burning out of one of the lamp filaments or simply Burning out of one of the lamp filaments or simply opening the switch could cause such a break. opening the switch could cause such a break. Resistors in Series connected in such a way that all components connected in such a way that all components have the same current through them. have the same current through them. logic: the total or effective resistance would have length L 1 + L 2 + L 3 and resistance is proportional to the length Equivalent or total or effective or resistance is the one that could replace all resistors resulting in the same current.

47 Resistors in Parallel Electric devices connected in parallel are connected to the same two points of an electric circuit, so all components have the same potential difference across them. Electric devices connected in parallel are connected to the same two points of an electric circuit, so all components have the same potential difference across them. The current flowing into the point of splitting is equal to the sum of the currents flowing out at that point: I = I 1 + I 2 + I 3. The current flowing into the point of splitting is equal to the sum of the currents flowing out at that point: I = I 1 + I 2 + I 3. A break in any one path does not interrupt the flow of charge in the other paths. Each device operates independently of the other devices. The greater resistance, the smaller curent. A break in any one path does not interrupt the flow of charge in the other paths. Each device operates independently of the other devices. The greater resistance, the smaller curent. equivelent resistance is smaller than the smallest resistance.

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49 RESISTORS IN COMPOUND CIRCUITS Now you can calculate current, potential drop and power dissipated through each resistor

50 example: Find power of the source, current in each resistor, terminal potential, potential drop across each resistor and power dissipated in each resistor. R eq = 120  I =  ε  R eq = 0.3 A terminal potential: V = ε – Ir = 36 – 0.3x6.7 = 34 V current through resistors 100Ω and 50Ω : I = I 1 + I 2 I 1 R 1 = I 2 R 2 0.3 = I 1 + I 2 100 I 1 = 50 I 2 → I 1 = 0.1 A I 2 = 0.2 A potential drops V = IR power dissipated P = IV 80 Ω 0.3x80 = 24 V0.3x24 = 7.2 W 100 Ω0.1x100 = 10 V0.1x10 = 1 W 50 Ω0.2x50 = 10 V0.2x10 = 2 W 6.7 Ω0.3x6.7 = 2 V0.3x2 = 0.6 W ε = Σ all potential drops 36 V = 2 V + 24 V + 10 V power dissipated in the circuit = power of the source 0.6 + 2 + 1 + 7.2 = 0.3x36

51 In practical use, we need to be able to measure currents through components and voltages across various components in electrical circuits. To do this, we use AMMETERS and VOLTMETERS. Ammeters and voltmeters

52 An ammeter – measures current passing through it is always connected in series with a component we want to measure in order that whatever current passes through the component also passes the ammeter. is always connected in series with a component we want to measure in order that whatever current passes through the component also passes the ammeter. has a very low resistance compared with the has a very low resistance compared with the resistance of the circuit so that it will not alter the current the current being measured. would ideally have no resistance with no potential would ideally have no resistance with no potential difference across it so no energy would be dissipated in it.

53 A voltmeter – measures voltage drop between two points is always connected across a device (in parallel). is always connected across a device (in parallel). has a very high resistance so that it takes very little has a very high resistance so that it takes very little current from the device whose potential difference is being measured. an ideal voltmeter would have infinite resistance an ideal voltmeter would have infinite resistance with no current passing through it and no energy would be dissipated in it.

54 A potential divider In electronic systems, it is often necessary to obtain smaller voltages from larger voltages for the various electronic circuits. A potential divider is a device that produces the required voltage for a component from a larger voltage. It consists of a series of resistors or a rheostat (variable resistor) connected in series in a circuit. Potential divider equation

55 In the potential divider shown, calculate: (a) the total current in the circuit (b) the potential difference across each resistor (c) the voltmeter reading if it was connected between terminals 2 and 6. example: (a)The total resistance R = 12 Ω. I = V / R = 12 V / 12 Ω = 1 A I = V / R = 12 V / 12 Ω = 1 A (b) 6 x V = 12 V → V = 2 V ( 12 V is equally shared by each 2 Ω resistor. or or V = IR = 1x2 = 2 V V = IR = 1x2 = 2 V (c) R = 4 x 2 = 8 Ω (Between terminals 2 and 6 there are 4 resistors) potential difference between the terminals is potential difference between the terminals is V = IR = 1 x 8 = 8 V V = IR = 1 x 8 = 8 V

56 Because resistance is directly proportional to the length of a resistor, a variable resistor also known as a potentiometer or as a “pot” can also be used to control the potential difference across some device. Potentiometer Pots have a rotating wheel mounted in plastic and they are commonly used as volume and tone controls in sound systems. They can be made from wire, metal oxides or carbon compounds. Sliding contact A can connect anywhere from one end to the other of the resistor chain. This way it can control voltage across a device and therefore the current through it, from maximm down to zero. 1. step is to do a circuit without device and then adjust point A in such a way that there is no current passing through potentiometer. Potential difference across potentiometer is 6 V. For some other battery point A would be somewhere else. If you include a lamp into circuit and the pointer is at A, potential difference across the lamp is zero. However, if the pointer is moved up to two-thirds the length of the potentiometer as in the figure, then the output voltage across the filament lamp would be ⅔ × 6V = 4V.

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