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Chapter 17 Current Electricity

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Conductors Conductors are materials in which the electric charges move freely Copper, aluminum and silver are good conductors In terms of circuits, we will generally be using copper When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

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Electric Current Whenever electric charges of like signs move, an electric current is said to exist The current is the rate at which the charge flows through this surface Current (I) = units of charge (Q) per time I = Q/t The SI unit of current is Ampere (A) 1 A = 1 C/s

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Ex. 1 The amount of charge that passes through the filament of a certain light bulb in 2.00s is 1.67C. A) Determine the current in the light bulb. B) How many electrons passed through the filament per second?

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Ex. 2 A W light bulb draws 0.83A of current. How many electrons pass a given cross-sectional area of the filament in 1 hour?

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Ex. 3 1.5 x 10 7 electrons pass through a given cross section of a wire every 1.0s. A) Find the current in the wire. B) How much charge (in C) passes through the wire per minute?

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Electric Potential Energy of a Charge Wants to move when it has high PE Point b PE = max KE = min Point a PE = min KE = max

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Voltage Voltage (ΔV) – the observed electrical potential difference between two points in a circuit Also the driving force behind the flow of charge Analogous to a height difference with gravitational potential energy or a temperature difference with heat flow.

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Voltage and Current Differences Lower potential difference, but more energy flowing (therefore more current) Higher potential difference, but less energy flowing (therefore less current)

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In this river system, what would the height difference in the waterfall represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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In this river system, what would the flow of water represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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In this river system, what would the water molecules represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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In this river system, what would the riverbed represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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In this river system, what would the water as a whole represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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In this river system, what would the rocks in the river represent? 1. Charge 2. Voltage 3. Current 4. Wire 5. Electrons 6. Resistance

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Analogy to Flow of Water Electric Charge = Water Coulomb = Gallons of Water Electron = Molecule of Water Electric Current = Rate of Water Flow Ampere = Gallons of Water per second Potential Difference or Voltage = Water Pressure (height of waterfall) Wire = Riverbed Resistance = Rocks in the River

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Electric Current, cont When diagramming, conventional current flow is the direction positive charge (+) would flow This is known as conventional current flow In a common conductor, such as copper, the actual current is due to the motion of the negatively charged electrons In a particle accelerator, positively charged protons are set in motion

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Electrical Energy and Power, final The SI unit of power is Watt (W) The unit of energy used by electric companies is the kilowatt-hour (kW-hr) This is defined in terms of the unit of power and the amount of time it is supplied 1 kWh = 3.60 x 10 6 J

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Meters in a Circuit -- Ammeter An ammeter is used to measure current In line with the bulb, all the charge passing through the bulb also must pass through the meter

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Meters in a Circuit -- Voltmeter A voltmeter is used to measure voltage (potential difference) Connects to the two ends of the bulb

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Which will turn the bulb on? 1. A. 2. B. 3. C. 4. D.

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Drift Velocity Drift Velocity is the velocity at which electrons move opposite the electric field (E). Counterintuitively, drift velocity is very small. (eg 2.46 x m/s in Cu wire) So how does the electric light turn on so quickly??? Hmmmmm…

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Charge Carrier Motion in a Conductor The zig-zag black line represents the motion of charge carrier in a conductor The net drift speed is small The sharp changes in direction are due to collisions The net motion of electrons is opposite the direction of the electric field

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Resistance In a conductor, the voltage applied across the ends of the conductor is proportional to the current through the conductor The constant of proportionality is the resistance of the conductor

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Resistance, cont Units of resistance are ohms (Ω) 1 Ω = 1 V / A Resistance in a circuit arises due to collisions between the electrons carrying the current with the fixed atoms inside the conductor (analogous to water colliding with rocks in a river)

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Ohm’s Law In general, resistance remains constant over a wide range of applied voltages or currents This statement has become known as Ohm’s Law ΔV = I R

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Factors affecting resistance Length of a resistor – R increases with length (directly prop.) Cross-sectional area – R increases with smaller cross-sectional area (inv. prop.) Material – different metals have different resistances Temperature – R increases with temperature (dir. prop.)

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Superconductors A class of materials and compounds whose resistances fall to virtually zero below a certain temperature, T C T C is called the critical temperature

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Superconductors, cont Once a current is set up in a superconductor, it persists without any applied voltage Since R = 0

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Superconductor Timeline 1911 Superconductivity discovered by H. Kamerlingh Onnes 1986 High temperature superconductivity discovered by Bednorz and Müller Superconductivity near 30 K 1987 Superconductivity at 96 K and 105 K Current More materials and more applications

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Electrical Energy and Power, cont The rate at which the energy is lost in a circuit is the power From Ohm’s Law, alternate forms of power are

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Electrical Activity in the Heart Every action involving the body’s muscles is initiated by electrical activity Voltage pulses cause the heart to beat These voltage pulses are large enough to be detected by equipment attached to the skin

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Electrocardiogram (EKG) A normal EKG P occurs just before the atria begin to contract The QRS pulse occurs in the ventricles just before they contract The T pulse occurs when the cells in the ventricles begin to recover

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Abnormal EKG, 1 The QRS portion is wider than normal This indicates the possibility of an enlarged heart

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Abnormal EKG, 2 There is no constant relationship between P and QRS pulse This suggests a blockage in the electrical conduction path between the SA and the AV nodes This leads to inefficient heart pumping

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Abnormal EKG, 3 No P pulse and an irregular spacing between the QRS pulses Symptomatic of irregular atrial contraction, called fibrillation The atrial and ventricular contraction are irregular

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Implanted Cardioverter Defibrillator (ICD) Devices that can monitor, record and logically process heart signals Then supply different corrective signals to hearts that are not beating correctly

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