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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Slide 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 tvs), its dangerous if you are this path! animation ???? Slide 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 Slide 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! Slide 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. Slide 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 ( ) Slide 7 OHMS 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 Slide 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 Slide 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. Slide 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. Slide 11 Factors affecting resistance The units of resistance are volts per ampere (VA -1 ). However, a separate SI unit called the ohm 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. Slide 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 Slide 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. Slide 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 dont need to add additional energy in oder to keep them going. The dream: current without cost!!!!!!!!! Both in money and damage to environment!!!!!!!! Slide 15 Resistance of a wire when the temperature is kept constant is: The resistivity, (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 m. 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. Slide 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 Slide 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. Slide 18