1 CURRENT ELECTRICITY Name: ________________ Class: _________________ Index: ________________
2 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
3 -- 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
4 Electric Current I = Q/t 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 = ISI Unit of Electric Current = ampere (A)I = Q/twhereI = current in ampere (A)Q = amount of charges in coulombs (C)t = time in seconds (s)
5 Conventional Current and Electron Flow Conventional current flows from the positive to the negative endsElectric charges flow from the negative to the positive ends
6 Conventional Current and Electron Flow Measuring currentAn ammeter is an instrument used for measuring electric current.Ammeters must be connected in series in a circuitPositive (negative) side of ammeter is connected to the positive (negative) terminal of the cell / battery.Aammeter symbol
7 Conventional Current and Electron Flow Measuring currentSince the circuit consists of only one loop, the same current flows through the circuit; does not matter where the ammeter is placed on the circuitA4resistor+-A3A2A5A6A1cell
8 Conventional Current and Electron Flow Measuring currentThe digital multimeter (DMM) is starting to replace the ammeter.has a wide range of between a few hundred A to several Acan be used for direct current (D.C.) and alternating current (A.C.)able to read voltage and resistance too
9 Electromotive Force (e.m.f) electric current is produced when there is a flow of chargesa source of energy (provided by a cell, group of cells or generator) is needed to enable charges to be pumped or forced around a circuitelectromotive 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 batteryI+-lampcell
10 Electromotive Force (e.m.f) DefinitionThe 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)
11 Electromotive Force (e.m.f) Symbol of Electromotive Force = SI Unit of Electromotive Force = volts (V) or joules per coulomb (JC-1) = W/Qwhere = e.m.f. (V)W = Energy converted from non–electrical forms toelectrical form (J) [work done]Q = amount of charge in coulombs (C)
12 Potential Difference V = W/Q 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 pointsSymbol of Potential Difference (p.d.) = VSI Unit of Potential Difference (p.d.) = volts (V)V = W/QwhereV = Potential difference (V)W = Energy converted from electricalform to other forms (J)Q = amount of charge in coulombs (C)
13 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)Vvoltmeter symbolVoltmeters will measure the potential difference across 2 points of the circuit, so we connect it in parallel with respect to those 2 points
14 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 circuitV+V31V1V2-21 + 2 = V1 + V2 + V3
15 different types of resistors ResistanceIn 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 resistorThe greater the resistance in a circuit, the lower the current.different types of resistors
16 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 = RSI Unit of Resistance = ohms ()VRIWhereR = resistance in ohms ()V = p.d. across the component in volts (V)I = current in ampere (A)
17 Ohm’s LawOhm'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.
18 ResistanceIf a cell is connected to a resistance, the current gets smaller as the resistance increases.
19 Resistance Uses of high and low resistances materials. All metals have finite resistance.MaterialsUsesLow resistancecopper, gold, silver, aluminiumconnecting wires, conductors or connectorsHigh resistancetungstenused in light bulbsnichrome(an alloy of nickel and chromium)heaters, such as coils of electric kettlescarbonresistors for radio and television sets
20 variable resistor symbol ResistanceResistorsIs a conductor that has a known value of resistancePrimary 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 easilyvariable resistor symbolfixed resistor symbol
21 ResistanceRheostatsare variable resistors used for controlling the size of the current in a circuitare used as brightness controls for lights, volume controls on radio and television sets
22 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 readingR can be calculated from the equation:R = V / I
23 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 flows in the circuit initially.3. Record the ammeter reading (I) & voltmeter reading (V).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.batteryrheostatammetervoltmeterRNote that:Always connect:Voltmeter in ParallelAmmeter in Series
24 Resistance Experiment to Determine Resistance of a resistor Result: The gradient of the graph gives the resistance of the load, RV / VI / AGradient = V / I= resistance
25 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 wirethicker wiresmaller resistance(R 1/A)
27 Factors Affecting Resistance 3. Type of materialWires 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)
28 ResistanceThese factors can be placed together to find resistanceR = l /AWhere R = resistance in ohms () = resistivity in ohm meter (m)l = length of wire (m)A = cross-sectional area in meter square (m2)
29 ResistanceExampleThe 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?SolutionL = 50 cm = 0.5 mdiameter = 2.0 mm = mA = (d/2)2 = (0.002/2)2 = (0.001)2 m2R = (1.7 x 10-8)(0.5) / (0.001)2=
30 Resistance resistors in series Rseries = R1 + R2 + R3 since resistors are in series, current I passing through each resistor is the sameeffective resistanceR1R2R3RtIis equivalent toIVV1V2V3Rseries = R1 + R2 + R3
31 Resistance resistors in parallel since resistors are in parallel, potential difference across each resistor is the sameI1R1effective resistanceR2I2RIis equivalent toIR3I3VV
32 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 syllabuswhere 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.
33 Ohmic Conductors Pure metal, VThe uniform gradient shows uniform resistanceOhmic ConductorsIO(a) Pure metalPure metal,carbon and coppersulphateVIO(b) Copper sulphate solution
34 Non-Ohmic Conductors V At low temperature, the tungsten wire obey Ohm’s Law but at higher temperature it is not obeyed the Law.Higher resistance due to higher temperatureConstant resistanceIOfilament bulb
35 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.
36 Forward Voltage DropElectricity 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 VoltageWhen 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.
37 Bridge RectifiersRectifier 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.