ppt presentation on electric current

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Current Electricity - PowerPoint PPT Presentation

Current Electricity

Chapter current electricity 22 in this chapter you will: explain energy transfer in circuits. solve problems involving current, potential difference, and resistance. – powerpoint ppt presentation.

• Explain energy transfer in circuits.
• Solve problems involving current, potential difference, and resistance.
• Diagram simple electric circuits.
• Describe conditions that create current in an electric circuit.
• Explain Ohms law.
• Design closed circuits.
• Differentiate between power and energy in an electric circuit.
• Flowing water at the top of a waterfall has both potential and kinetic energy.
• However, the large amount of natural potential and kinetic energy available from resources such as Niagara Falls are of little use to people or manufacturers who are 100 km away, unless that energy can be transported efficiently.
• Electric energy provides the means to transfer large quantities of energy over great distances with little loss.
• This transfer usually is done at high potential differences through power lines.
• Once this energy reaches the consumer, it can easily be converted into another form or combination of forms, including sound, light, thermal energy, and motion.
• Because electric energy can so easily be changed into other forms, it has become indispensable in our daily lives.
• When two conducting spheres touch, charges flow from the sphere at a higher potential to the one at a lower potential.
• The flow continues until there is no potential difference between the two spheres.
• A flow of charged particles is an electric current.
• In the figure, two conductors, A and B, are connected by a wire conductor, C.
• Charges flow from the higher potential difference of B to A through C.
• This flow of positive charge is called conventional current.
• The flow stops when the potential difference between A, B, and C is zero.
• You could maintain the electric potential difference between B and A by pumping charged particles from A back to B, as illustrated in the figure.
• Since the pump increases the electric potential energy of the charges, it requires an external energy source to run.
• This energy could come from a variety of sources.
• One familiar source, a voltaic or galvanic cell (a common dry cell), converts chemical energy to electric energy.
• A battery is made up of several galvanic cells connected together.
• A second source of electric energy a photovoltaic cell, or solar cellchanges light energy into electric energy.
• The charges in the figure move around a closed loop, cycling from pump B, through C to A, and back to the pump.
• Any closed loop or conducting path allowing electric charges to flow is called an electric circuit.
• A circuit includes a charge pump, which increases the potential energy of the charges flowing from A to B, and a device that reduces the potential energy of the charges flowing from B to A.
• The potential energy lost by the charges, qV, moving through the device is usually converted into some other form of energy.
• For example, electric energy is converted to kinetic energy by a motor, to light energy by a lamp, and to thermal energy by a heater.
• A charge pump creates the flow of charged particles that make up a current.
• Charges cannot be created or destroyed, but they can be separated.
• Thus, the total amount of chargethe number of negative electrons and positive ionsin the circuit does not change.
• If one coulomb flows through the generator in 1 s, then one coulomb also will flow through the motor in 1 s.
• Thus, charge is a conserved quantity.
• Energy also is conserved.
• The change in electric energy, ?E, equals qV. Because q is conserved, the net change in potential energy of the charges going completely around the circuit must be zero.
• The increase in potential difference produced by the generator equals the decrease in potential difference across the motor.
• Power, which is defined in watts, W, measures the rate at which energy is transferred.
• If a generator transfers 1 J of kinetic energy to electric energy each second, it is transferring energy at the rate of 1 J/s, or 1 W.
• The energy carried by an electric current depends on the charge transferred, q, and the potential difference across which it moves, V. Thus, E qV.
• The unit for the quantity of electric charge is the coulomb.
• The rate of flow of electric charge, q/t, called electric current, is measured in coulombs per second.
• Electric current is represented by I, so I q/t.
• A flow of 1 C/s is called an ampere, A.
• The energy carried by an electric current is related to the voltage, E qV.
• Since current, I q/t, is the rate of charge flow, the power, P E/t, of an electric device can be determined by multiplying voltage and current.
• To derive the familiar form of the equation for the power delivered to an electric device, you can use P E/t and substitute E qV and q It
• Power is equal to the current times the potential difference.
• Suppose two conductors have a potential difference between them.
• If they are connected with a copper rod, a large current is created.
• On the other hand, putting a glass rod between them creates almost no current.
• The property determining how much current will flow is called resistance.
• The table below lists some of the factors that impact resistance.
• Resistance is measured by placing a potential difference across a conductor and dividing the voltage by the current.
• The resistance, R, is defined as the ratio of electric potential difference, V, to the current, I.
• Resistance is equal to voltage divided by current.
• The resistance of the conductor, R, is measured in ohms.
• One ohm (1 O ) is the resistance permitting an electric charge of 1 A to flow when a potential difference of 1 V is applied across the resistance.
• A simple circuit relating resistance, current, and voltage is shown in the figure.
• A 12-V car battery is connected to one of the cars 3-O brake lights.
• The circuit is completed by a connection to an ammeter, which is a device that measures current.
• The current carrying the energy to the lights will measure 4 A.
• The unit for resistance is named for German scientist Georg Simon Ohm, who found that the ratio of potential difference to current is constant for a given conductor.
• The resistance for most conductors does not vary as the magnitude or direction of the potential applied to it changes.
• A device having constant resistance independent of the potential difference obeys Ohms law.
• Most metallic conductors obey Ohms law, at least over a limited range of voltages.
• Many important devices, such as transistors and diodes in radios and pocket calculators, and lightbulbs do not obey Ohms law.
• Wires used to connect electric devices have low resistance.
• A 1-m length of a typical wire used in physics labs has a resistance of about 0.03 O.
• Because wires have so little resistance, there is almost no potential drop across them.
• To produce greater potential drops, a large resistance concentrated into a small volume is necessary.
• A resistor is a device designed to have a specific resistance.
• Resistors may be made of graphite, semiconductors, or wires that are long and thin.
• There are two ways to control the current in a circuit.
• Because I V/R, I can be changed by varying V, R, or both.
• The figure a shows a simple circuit.
• When V is 6 V and R is 30 O, the current is 0.2 A.
• How could the current be reduced to 0.1 A? According to Ohms law, the greater the voltage placed across a resistor, the larger the current passing through it.
• If the current through a resistor is cut in half, the potential difference also is cut in half.
• In the first figure, the voltage applied across the resistor is reduced from 6 V to 3 V to reduce the current to 0.1 A.
• A second way to reduce the current to 0.1 A is to replace the 30-O resistor with a 60-O resistor, as shown in the second figure.
• Resistors often are used to control the current in circuits or parts of circuits.
• Sometimes, a smooth, continuous variation of the current is desired.
• For example, the speed control on some electric motors allows continuous, rather than step-by-step, changes in the rotation of the motor.
• To achieve this kind of control, a variable resistor, called a potentiometer, is used.
• A circuit containing a potentiometer is shown in the figure.
• Some variable resistors consist of a coil of resistance wire and a sliding contact point.
• Moving the contact point to various positions along the coil varies the amount of wire in the circuit.
• As more wire is placed in the circuit, the resistance of the circuit increases thus, the current changes in accordance with the equation I V/R.
• In this way, the speed of a motor can be adjusted from fast, with little wire in the circuit, to slow, with a lot of wire in the circuit.
• Other examples of using variable resistors to adjust the levels of electrical energy can be found on the front of a TV the volume, brightness, contrast, tone, and hue controls are all variable resistors.
• The human body acts as a variable resistor.
• When dry, skins resistance is high enough to keep currents that are produced by small and moderate voltages low.
• If skin becomes wet, however, its resistance is lower, and the electric current can rise to dangerous levels.
• A current as low as 1 mA can be felt as a mild shock, while currents of 15 mA can cause loss of muscle control, and currents of 100 mA can cause death.
• An electric circuit is drawn using standard symbols for the circuit elements.
• Such a diagram is called a circuit schematic. Some of the symbols used in circuit schematics are shown below.
• Are the units correct?
• Current is measured in amperes.
• Is the magnitude realistic?
• There is a fairly large voltage and a small resistance, so a current of 3.00 A is reasonable.
• Step 1 Analyze and Sketch the Problem
• Draw a circuit containing a battery, an ammeter, and a resistor.
• Show the direction of the conventional current.
• Step 2 Solve for the Unknown
• Use I V/R to determine the current.
• Step 3 Evaluate the Answer
• An artists drawing and a schematic of the same circuit are shown below.
• Notice in both the drawing and the schematic that the electric charge is shown flowing out of the positive terminal of the battery.
• An ammeter measures current and a voltmeter measures potential differences.
• Each instrument has two terminals, usually labeled and . A voltmeter measures the potential difference across any component of a circuit.
• When connecting the voltmeter in a circuit, always connect the terminal to the end of the circuit component that is closer to the positive terminal of the battery, and connect the terminal to the other side of the component.
• When a voltmeter is connected across another component, it is called a parallel connection because the circuit component and the voltmeter are aligned parallel to each other in the circuit, as diagrammed in the figure.
• Any time the current has two or more paths to follow, the connection is labeled parallel.
• The potential difference across the voltmeter is equal to the potential difference across the circuit element.
• Always associate the words voltage across with a parallel connection.
• An ammeter measures the current through a circuit component.
• The same current going through the component must go through the ammeter, so there can be only one current path.
• A connection with only one current path is called a series connection.
• To add an ammeter to a circuit, the wire connected to the circuit component must be removed and connected to the ammeter instead.
• Then, another wire is connected from the second terminal of the ammeter to the circuit component.
• In a series connection, there can be only a single path through the connection.
• Always associate the words current through with a series connection.
• Explain how electric energy is converted into thermal energy.
• Explore ways to deliver electric energy to consumers near and far.
• Define kilowatt-hour.
• Energy that is supplied to a circuit can be used in many different ways.
• A motor converts electric energy to mechanical energy, and a lamp changes electric energy into light.
• Unfortunately, not all of the energy delivered to a motor or a lamp ends up in a useful form.
• Some of the electric energy is converted into thermal energy.
• Some devices are designed to convert as much energy as possible into thermal energy.
• Current moving through a resistor causes it to heat up because flowing electrons bump into the atoms in the resistor.
• These collisions increase the atoms kinetic energy and, thus, the temperature of the resistor.
• A space heater, a hot plate, and the heating element in a hair dryer all are designed to convert electric energy into thermal energy.
• These and other household appliances, act like resistors when they are in a circuit.
• When charge, q, moves through a resistor, its potential difference is reduced by an amount, V.
• The energy change is represented by qV.
• In practical use, the rate at which energy is changedthe power, P E/tis more important.
• Current is the rate at which charge flows, I q/t, and that power dissipated in a resistor is represented by P IV.
• For a resistor, V IR.
• Thus, if you know I and R, you can substitute V IR into the equation for electric power to obtain the following.
• Power is equal to current squared times resistance.
• Thus, the power dissipated in a resistor is proportional both to the square of the current passing through it and to the resistance.
• If you know V and R, but not I, you can substitute I V/R into P IV to obtain the following equation.
• Power is equal to the voltage squared divided by the resistance.
• The power is the rate at which energy is converted from one form to another.
• Energy is changed from electric to thermal energy, and the temperature of the resistor rises.
• If the resistor is an immersion heater or burner on an electric stovetop, for example, heat flows into cold water fast enough to bring the water to the boiling point in a few minutes.
• If power continues to be dissipated at a uniform rate, then after time t, the energy converted to thermal energy will be E Pt.
• Because P I2R and P V2/R, the total energy to be converted to thermal energy can be written in the following ways.
• Thermal energy is equal to the power dissipated multiplied by the time. It is also equal to the current squared multiplied by resistance and time as well as the voltage squared divided by resistance multiplied by time.
• Power is measured in watts, and energy is measured in joules.
• For power, 102102101 103, so kilowatts is reasonable. For energy, 103101 104, so an order of magnitude of 10,000 joules is reasonable.
• Sketch the situation.
• Label the known circuit components, which are a 120.0-V potential difference source and a 10.0-O resistor.
• Because R and V are known, use P V2/R.
• Solve for the energy.
• A superconductor is a material with zero resistance.
• There is no restriction of current in superconductors, so there is no potential difference, V, across them.
• Because the power that is dissipated in a conductor is given by the product IV, a superconductor can conduct electricity without loss of energy.
• At present, almost all superconductors must be kept at temperatures below 100 K.
• The practical uses of superconductors include MRI magnets and in synchrotrons, which use huge amounts of current and can be kept at temperatures close to 0 K.
• Hydroelectric facilities are capable of producing a great deal of energy.
• This hydroelectric energy often must be transmitted over long distances to reach homes and industries.
• How can the transmission occur with as little loss to thermal energy as possible?
• Thermal energy is produced at a rate represented by P I2R.
• Electrical engineers call this unwanted thermal energy the joule heating loss, or I2R loss.
• To reduce this loss, either the current, I, or the resistance, R, must be reduced.
• All wires have some resistance, even though their resistance is small.
• The large wire used to carry electric current into a home has a resistance of 0.20 O for 1 km.
• Suppose that a farmhouse were connected directly to a power plant 3.5 km away.
• The resistance in the wires needed to carry a current in a circuit to the home and back to the plant is represented by the following equation R 2(3.5 km)(0.20 O/km) 1.4 O.
• An electric stove might cause a 41-A current through the wires.
• The power dissipated in the wires is represented by the following relationships P I2R (41 A)2 (1.4 O) 2400 W.
• All of this power is converted to thermal energy and, therefore, is wasted.
• This loss could be minimized by reducing the resistance.
• Cables of high conductivity and large diameter (and therefore low resistance) are available, but such cables are expensive and heavy.
• Because the loss of energy is also proportional to the square of the current in the conductors, it is even more important to keep the current in the transmission lines low.
• How can the current in the transmission lines be kept low?
• The electric energy per second (power) transferred over a long-distance transmission line is determined by the relationship P IV.
• The current is reduced without the power being reduced by an increase in the voltage.
• Some long-distance lines use voltages of more than 500,000 V.
• The resulting lower current reduces the I2R loss in the lines by keeping the I2 factor low.
• Long-distance transmission lines always operate at voltages much higher than household voltages in order to reduce I2R loss.
• The output voltage from the generating plant is reduced upon arrival at electric substations to 2400 V, and again to 240 V or 120 V before being used in homes.
• While electric companies often are called power companies, they actually provide energy rather than power.
• Power is the rate at which energy is delivered.
• When consumers pay their home electric bills, they pay for electric energy, not power.
• The amount of electric energy used by a device is its rate of energy consumption, in joules per second (W) times the number of seconds that the device is operated.
• Joules per second times seconds, (J/s)s, equals the total amount of joules of energy.
• The joule, also defined as a watt-second, is a relatively small amount of energy, too small for commercial sales use.
• For this reason, electric companies measure energy sales in a unit of a large number of joules called a kilowatt-hour, kWh.
• A kilowatt-hour is equal to 1000 watts delivered continuously for 3600 s (1 h), or 3.6106 J.
• The electric energy transferred to a light bulb is converted into light energy, but as the bulb glows, it becomes hot, which shows that some part of energy is converted into thermal energy. Why is it so?
• An electric bulb acts like a resistor, and when current is passed through a resistor (light bulb). The current moving through a resistor causes it to heat up because the flowing electrons bump into the atoms in the resistor. These collisions increase the atoms kinetic energy and, thus, the temperature of the resistor (light bulb). This increase in temperature makes the resistor (light bulb) hot and hence some part of electric energy supplied to a light bulb is converted into thermal energy.
• If the current through the motor in the figure is 3.0 A and the potential difference is 120 V, the power in the motor is calculated using the expression P (3.0 C/s)(120 J/C) 360 J/s, which is 360 W.

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