* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Current Electricity - HSphysics
Survey
Document related concepts
Nanogenerator wikipedia , lookup
Giant magnetoresistance wikipedia , lookup
Surge protector wikipedia , lookup
Superconductivity wikipedia , lookup
Thermal runaway wikipedia , lookup
Power MOSFET wikipedia , lookup
Rectiverter wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Lumped element model wikipedia , lookup
Opto-isolator wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Negative resistance wikipedia , lookup
Current source wikipedia , lookup
Transcript
CURRENT ELECTRICITY Name: ________________ Class: _________________ Index: ________________ Objectives --state that a current is a rate of flow of charge measured in amperes --distinguish between conventional current and electron flow --recall and apply the relationship charge = current x time to new situations or to solve related problems -- define electromotive force (e.m.f.) as the work done by a source in driving a unit charge around a complete circuit -- calculate the total e.m.f. where several sources are arranged in series --state that the e.m.f. of a source and the potential difference across a circuit component is measured in volts --define the p.d. across a component in a circuit as the work done to drive a unit charge through the component --state the definition that resistance = p.d./ current -- apply the relationship R= V/I to new situations or to solve related problems --describe an experiment to determine resistance using a voltmeter and an ammeter and make the necessary calculations -- recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems --recall and apply the relationship of the proportionality between resistance and length and the cross-sectional area of a wire to new situations or to solve related problems --state Ohm’s law --describe the effect of temperature increase on the resistance of a metallic conductor --sketch and interpret the V-I characteristic graph for metallic conductor at constant temperature, a filament lamp and for a semiconductor diode -- show an understanding of the use of a diode as a rectifier Electric Current • An electric current I is a measure of the rate of flow of electric charge Q through a given cross section of a conductor. • Symbol of Electric Current = I • SI Unit of Electric Current = ampere (A) I = Q/t where I = current in ampere (A) Q = amount of charges in coulombs (C) t = time in seconds (s) 4 Conventional Current and Electron Flow Conventional current flows from the positive to the negative ends Electric charges flow from the negative to the positive ends 5 Conventional Current and Electron Flow Measuring current • An ammeter is an instrument used for measuring electric current. • Ammeters must be connected in series in a circuit A ammeter symbol Positive (negative) side of ammeter is connected to the positive (negative) terminal of the cell / battery. 6 Conventional Current and Electron Flow Measuring current Since the circuit consists of only one loop, the same current flows through the circuit; does not matter where the ammeter is placed on the circuit A1 - + A6 cell A2 A5 resistor A3 A4 Conventional Current and Electron Flow Measuring current The digital multimeter (DMM) is starting to replace the ammeter. has a wide range of between a few hundred A to several A can be used for direct current (D.C.) and alternating current (A.C.) able to read voltage and resistance too Electromotive Force (e.m.f) electric current is produced when there is a flow of charges a source of energy (provided by a cell, group of cells or generator) is needed to enable charges to be pumped or forced around a circuit electromotive force is the electric force that provides the pumping action for electric current to flow from the positive terminal to the negative terminal of the + battery cell I lamp Electromotive Force (e.m.f) Electromotive Force (e.m.f) Definition • The electromotive force (e.m.f.) of an electrical source is the work done by the source in driving a unit charge round a complete circuit. – is the potential difference between the two terminals of the cell or battery. (From higher p.d. to lower p.d) – A point of high potential is a region where there is a large number of positive charges whereas a point of low potential has lesser positive charges (more negative charges) 10 Electromotive Force (e.m.f) Electromotive Force (e.m.f) • Symbol of Electromotive Force = • SI Unit of Electromotive Force = volts (V) or joules per coulomb (JC-1) = W/Q where = e.m.f. (V) W = Energy converted from non–electrical forms to electrical form (J) [work done] Q = amount of charge in coulombs (C) 11 Potential Difference Potential Difference (p.d.) • The Potential Difference (p.d.) between two points in an electric circuit is defined as the amount of electrical energy converted to other forms of energy when one coulomb of positive charge passes between the two points • Symbol of Potential Difference (p.d.) = V • SI Unit of Potential Difference (p.d.) = volts (V) V = W/Q where V = Potential difference (V) W = Energy converted from electrical form to other forms (J) Q = amount of charge in coulombs (C) 12 Potential Difference Measuring p.d./e.m.f. • An voltmeter is an instrument used for measuring potential difference or electromotive force. • As charges flow round a circuit, they lose their P.E., transforming P.E. into other forms of energy. • It is connected in parallel to the circuit. • The SI unit for p.d. / e.m.f. is volt (V) V voltmeter symbol Voltmeters will measure the potential difference across 2 points of the circuit, so we connect it in parallel with respect to those 2 points 13 Potential Difference Potential difference around a simple circuit sum of all the e.m.f.’s of the cells must be equal to the sum of potential differences across all the components in the circuit V + - 1 2 V V V1 V2 1 + 2 = V 1 + V2 + V 3 V3 Resistance In a circuit, the size of the current depends on the resistance in the circuit. Any component of a circuit resisting the flow of electricity is called a resistor The greater the resistance in a circuit, the lower the current. different types of resistors Resistance Definition: • Resistance R of a component is the ratio of the potential difference V across it to the current I flowing through it. • Symbol of Resistance = R R I • SI Unit of Resistance = ohms () V Where R = resistance in ohms () V = p.d. across the component in volts (V) I = current in ampere (A) 16 Ohm’s Law Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them. where I is the current through the resistance in units of amperes, V is the potential difference measured across the resistance in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current. Resistance If a cell is connected to a resistance, the current gets smaller as the resistance increases. Resistance Uses of high and low resistances materials. All metals have finite resistance. Materials Low resistance High resistance Uses copper, gold, silver, aluminium connecting wires, conductors or connectors tungsten used in light bulbs nichrome (an alloy of nickel and chromium) heaters, such as coils of electric kettles carbon resistors for radio and television sets Resistance Resistors • Is a conductor that has a known value of resistance • Primary purpose is to control the size of the current flowing in the circuit. • Two types: fixed resistors & variable resistors (or rheostats) • Variable resistor (or rheostat) allows resistances to be changed easily fixed resistor symbol variable resistor symbol 20 Resistance Rheostats are variable resistors used for controlling the size of the current in a circuit are used as brightness controls for lights, volume controls on radio and television sets Resistance Measuring Resistance • To determine the resistance of a metallic conductor, we use the following circuit: • We can find the current flowing through R from the ammeter reading. • We can find the potential difference across R from the voltmeter reading • R can be calculated from the equation: R=V/I 22 Resistance Experiment to Determine Resistance of a resistor 1. Set-up the apparatus as shown in the diagram. 2. As a safety precaution, adjust the rheostat to the maximum resistance so that a small current battery rheostat flows in the circuit initially. 3. Record the ammeter reading (I) & voltmeter R reading (V). ammeter 4. Adjust the rheostat to allow a larger current to flow in the circuit. Again record the values of I and V. 5. Repeat Step 4 for at least 5 sets of I and V readings. 6. Plot the graph of V(V) against I (A). Determine the gradient of the graph. voltmeter Note that: Always connect: Voltmeter in Parallel Ammeter in Series 23 Resistance Experiment to Determine Resistance of a resistor Result: The gradient of the graph gives the resistance of the load, R V/V Gradient = V / I = resistance 0 I /A 24 Resistance Factors Affecting Resistance There are several factors that affect the resistance of an object such as a wire: 1. Cross-sectional area of wire / thickness of wire thicker wire smaller resistance (R 1/A) 25 Resistance Factors Affecting Resistance 2. Length of wire longer wire larger resistance (R l) 26 Resistance Factors Affecting Resistance 3. Type of material Wires of the same length and thickness but made of different materials will have a different resistances. This is because they have different resistivities. (Units: Ωm) 27 Resistance • These factors can be placed together to find resistance R = l /A Where R = resistance in ohms () = resistivity in ohm meter (m) l = length of wire (m) A = cross-sectional area in meter square (m2) 28 Resistance Example • The diameter of the copper wire used in a circuit is 2.0 mm. If the resistively for copper is 1.7 x 10-8 m, what is the resistance for 50 cm of the wire? Solution L = 50 cm = 0.5 m diameter = 2.0 mm = 0.002 m A = (d/2)2 = (0.002/2)2 = (0.001)2 m2 R = (1.7 x 10-8)(0.5) / (0.001)2 = 0.0027 29 Resistance resistors in series since resistors are in series, current I passing through each resistor is the same effective resistance I R1 R2 R3 V1 V2 V3 is equivalent to I Rseries = R1 + R2 + R3 Rt V Resistance resistors in parallel since resistors are in parallel, potential difference across each resistor is the same I1 I2 R1 R2 I I3 effective resistance R3 is equivalent to I R V V Temperature Dependence Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material using the following formula: R = Ro[α(T-To)+1] -- Formula not in syllabus where T is its temperature, To is a reference temperature (usually room temperature), R0 is the resistance at T0, and α is the percentage change in resistivity per unit temperature. The constant α depends only on the material being considered. V Ohmic Conductors The uniform gradient shows uniform resistance I O (a) Pure metal Pure metal, V carbon and copper sulphate O I (b) Copper sulphate solution Non-Ohmic Conductors At low temperature, the tungsten wire obey Ohm’s Law but at higher temperature it is not obeyed the Law. V Constant resistance Higher resistance due to higher temperature I O filament bulb Non-Ohmic Conductors Semiconductor diode A diode allows an electric current to pass in one direction (called the diode's forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a valve. Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph). Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown. Bridge Rectifiers Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. References http://www.cartft.com/image_db/1n4001.jpg http://image.wistatutor.com/content/current-electricity/vacuum-diodegraph.gif http://cyberchalky.files.wordpress.com/2010/03/web_ohms_law_triangle.gif http://www.kpsec.freeuk.com/components/diode.htm