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5.2 heating effect of currents

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5.2 heating effect of currents

  1. 1. Electricity andElectricity and MagnetismMagnetism Topic 5.2 Heating effect ofTopic 5.2 Heating effect of currentscurrents
  2. 2. ResistanceResistance A tungsten filament lamp has a highA tungsten filament lamp has a high resistance, but connecting wires have a lowresistance, but connecting wires have a low resistance.resistance. What does this mean?What does this mean? The greater the resistance of a component,The greater the resistance of a component, the more difficult it is for charge to flowthe more difficult it is for charge to flow through it.through it.
  3. 3. The electrons make many collisions with theThe electrons make many collisions with the tungsten ions as they move through thetungsten ions as they move through the filament.filament. But the electrons move moreBut the electrons move more easily through the coppereasily through the copper connecting wires becauseconnecting wires because they make fewer collisionsthey make fewer collisions with the copper ions.with the copper ions.
  4. 4. Resistance is measured in ohms (Resistance is measured in ohms (ΩΩ) and is defined) and is defined in the following way:in the following way: The resistance of a conductor is the ratio of theThe resistance of a conductor is the ratio of the p.d.p.d. applied across it, to theapplied across it, to the currentcurrent passingpassing through it.through it. In fact:In fact:
  5. 5. ResistorsResistors Resistors are components that are made toResistors are components that are made to have a certain resistance. They can behave a certain resistance. They can be made of a length of nichrome wire ormade of a length of nichrome wire or chiseled ceramic and carbon.chiseled ceramic and carbon.
  6. 6. Circuit DiagramsCircuit Diagrams You need to be able to recognize and useYou need to be able to recognize and use the accepted circuit symbols included in thethe accepted circuit symbols included in the Physics Data BookletPhysics Data Booklet
  7. 7. Ohm’s LawOhm’s Law The current through a metal wire isThe current through a metal wire is directly proportional to the p.d. across itdirectly proportional to the p.d. across it (providing the temperature remains(providing the temperature remains constant).constant). This is Ohm's law.This is Ohm's law. Materials that obey Ohm's law are calledMaterials that obey Ohm's law are called ohmic conductors.ohmic conductors.
  8. 8. When X is aWhen X is a metal resistance wiremetal resistance wire the graph is a straight line passingthe graph is a straight line passing through the origin: (if thethrough the origin: (if the temperature is constant)temperature is constant) This shows that:This shows that: II is directlyis directly proportional to V.proportional to V. If you double the voltage, the currentIf you double the voltage, the current is doubled and so the value of V/I isis doubled and so the value of V/I is always the same.always the same.
  9. 9. Since resistance R =V/I, the wire hasSince resistance R =V/I, the wire has a constant resistance.a constant resistance. The gradient is the resistance on a VThe gradient is the resistance on a V against I graph, and 1/resistance in aagainst I graph, and 1/resistance in a I against V graph.I against V graph.
  10. 10. For a filament lamp, doubling theFor a filament lamp, doubling the voltage produces less than double thevoltage produces less than double the current.current. This means that the value of V/I risesThis means that the value of V/I rises as the current increases.as the current increases. As the current increases, the metalAs the current increases, the metal filament gets hotter and the resistancefilament gets hotter and the resistance of the lamp rises.of the lamp rises.
  11. 11. Resistance of wireResistance of wire Resistance through a solid depends onResistance through a solid depends on 1)1)LengthLength 2)2)Cross-sectional AreaCross-sectional Area 3)3)Material (ie free electrons and atomicMaterial (ie free electrons and atomic diameter)diameter) R = ρ l/A
  12. 12. ResistivityResistivity Data table from physnet.co.ukData table from physnet.co.uk Calculate resistance ofCalculate resistance of a)a)2m of copper of 0.5mm2m of copper of 0.5mm diameterdiameter b)b)50cm of graphite, 25050cm of graphite, 250μμm inm in diameterdiameter c)c)1.7mm of aluminium, 201.7mm of aluminium, 20μμmm in diameterin diameter
  13. 13. Power DissipationPower Dissipation
  14. 14. Power DissipationPower Dissipation
  15. 15. Resistance CombinationsResistance Combinations
  16. 16. Resistors in seriesResistors in series
  17. 17. The diagram shows three resistors connected inThe diagram shows three resistors connected in series.series. There are 3 facts that you should know forThere are 3 facts that you should know for a series circuit:a series circuit: 1.1. the current through each resistor in series isthe current through each resistor in series is the samethe same 2.2. the total p.d., V across the resistors is thethe total p.d., V across the resistors is the sum of the p.d.s across the separatesum of the p.d.s across the separate resistors, so: V = Vresistors, so: V = Vll + V+ V22 + V+ V33 3.3. the combined resistance R in the circuit isthe combined resistance R in the circuit is the sum of the separate resistors,the sum of the separate resistors, R = RR = Rll + R+ R22 + R+ R33
  18. 18. Suppose we replace the 3 resistors withSuppose we replace the 3 resistors with one resistor R that will take the sameone resistor R that will take the same current I when the same p.d. V is placedcurrent I when the same p.d. V is placed across it.across it.
  19. 19. This is shown in the diagram. Let's calculateThis is shown in the diagram. Let's calculate R. We know that for the resistors in series:R. We know that for the resistors in series: V = VV = Vll + V+ V22 + V+ V33 But for any resistor: p.d. = current xBut for any resistor: p.d. = current x resistance (V = I R).resistance (V = I R). If we apply this to each of our resistors, andIf we apply this to each of our resistors, and remember that the current through eachremember that the current through each resistor is the same and equal to I, we get:resistor is the same and equal to I, we get: IR = IRIR = IRll+IR+IR22+IR+IR33 If we now divide each term in the equationIf we now divide each term in the equation by I, we get:by I, we get: R = RR = R11 + R+ R22 + R+ R33
  20. 20. Resistors in parallelResistors in parallel
  21. 21. We now have three resistors connected inWe now have three resistors connected in parallel.parallel. There are 3 facts that you should knowThere are 3 facts that you should know for a parallel circuit:for a parallel circuit: 1.1. the p.d. across each resistor in parallel isthe p.d. across each resistor in parallel is the samethe same 2.2. the current in the main circuit is the sum ofthe current in the main circuit is the sum of the currents in each of the parallelthe currents in each of the parallel branches, so:branches, so: I = II = I11 + I+ I22 + I+ I33 3. the combine3. the combined resistanced resistance RR is calculated from theis calculated from the equation:equation:
  22. 22. Suppose we replace the 3 resistors with oneSuppose we replace the 3 resistors with one resistorresistor RR that takes the same total current Ithat takes the same total current I when the same p.d. V is placed across it.when the same p.d. V is placed across it.
  23. 23. This is shown in the diagram. Now let'sThis is shown in the diagram. Now let's calculatecalculate R.R. We know that for the resistors inWe know that for the resistors in parallel:parallel: I = II = I11+I+I22+I+I33 But for any resistor, current = p.d. =But for any resistor, current = p.d. = resistanceresistance (I = V/R ).(I = V/R ). If we apply this toIf we apply this to each of our resistors, and remembereach of our resistors, and remember that the p.d. across each resistor is thethat the p.d. across each resistor is the same and equal to V,same and equal to V,
  24. 24. we get:we get: V/V/R=V/RR=V/R11 + V/R+ V/R22 + V/R+ V/R33 Now we divide each term by V, to get:Now we divide each term by V, to get: 1/1/R=1/RR=1/R11 + 1/R+ 1/R22 + 1/R+ 1/R33 You will find that the total resistanceYou will find that the total resistance R isR is alwaysalways less than the smallest resistance in the parallelless than the smallest resistance in the parallel combination.combination.
  25. 25. Ammeters and VoltmetersAmmeters and Voltmeters In order to measure the current, an ammeter isIn order to measure the current, an ammeter is placedplaced in series,in series, inin the circuit.the circuit. What effect might this have on the size of theWhat effect might this have on the size of the current?current? TheThe idealideal ammeter hasammeter has zerozero resistance, so thatresistance, so that placing it in the circuit does not make the currentplacing it in the circuit does not make the current smaller.smaller. Real ammeters do have very small resistances ‑Real ammeters do have very small resistances ‑ around 0.01around 0.01 ΩΩ..
  26. 26. A voltmeter is connectedA voltmeter is connected in parallelin parallel with awith a component, in order to measure the p.d. acrosscomponent, in order to measure the p.d. across it. Why can this increase the current in theit. Why can this increase the current in the circuit?circuit? Since the voltmeter is in parallel with theSince the voltmeter is in parallel with the component, theircomponent, their combinedcombined resistance is lessresistance is less than the component's resistance.than the component's resistance. TheThe idealideal voltmeter hasvoltmeter has infiniteinfinite resistance andresistance and takes no current.takes no current. Digital voltmeters have very high resistances,Digital voltmeters have very high resistances, around 10 Maround 10 MΩΩ, and so they have little effect on, and so they have little effect on the circuit they are placed in.the circuit they are placed in.
  27. 27. Potential dividersPotential dividers A potential divider is a device or a circuitA potential divider is a device or a circuit that uses two (or more) resistors or athat uses two (or more) resistors or a variable resistor (potentiometer) to provide avariable resistor (potentiometer) to provide a fraction of the available voltage (p.d.) fromfraction of the available voltage (p.d.) from the supply.the supply.
  28. 28. The p.d. from the supply is divided acrossThe p.d. from the supply is divided across the resistors in direct proportion to theirthe resistors in direct proportion to their individual resistances.individual resistances.
  29. 29. Take the fixed resistance circuit - this is aTake the fixed resistance circuit - this is a seriesseries circuit therefore the current in thecircuit therefore the current in the same at all points.same at all points. IIsupplysupply = I= I11 = I= I22 Where IWhere I11 = current through R= current through R11 II22 = current through R= current through R22
  30. 30. Using Ohm’s LawUsing Ohm’s Law
  31. 31. ExampleExample
  32. 32. With sensorsWith sensors A thermistor is a device which will usuallyA thermistor is a device which will usually decrease in resistance with increasingdecrease in resistance with increasing temperature.temperature. A light dependent resistor, LDR, willA light dependent resistor, LDR, will decrease in resistance with increasing lightdecrease in resistance with increasing light intensity. (intensity. (LLightight DDecreases itsecreases its RResistance).esistance).
  33. 33. ExampleExample Calculate the readings on the meters shownCalculate the readings on the meters shown below when the thermistor has a resistancebelow when the thermistor has a resistance ofof a) 1 ka) 1 kW (W (warm conditions) and b) 16 kwarm conditions) and b) 16 kW.W. ((cold conditionscold conditions))
  34. 34. Kirchoff’s lawKirchoff’s law Solving electric circuit problems involvesSolving electric circuit problems involves finding current and voltage values for everyfinding current and voltage values for every component in the circuit. There are rulescomponent in the circuit. There are rules that can assist you in these calculations.that can assist you in these calculations. AA junctionjunction is a point where 3 or more wiresis a point where 3 or more wires connect, aconnect, a looploop is all the wire andis all the wire and componenets in a circle and acomponenets in a circle and a branchbranch is allis all the wires and components connecting onethe wires and components connecting one junction to another.junction to another.
  35. 35. 1.1. Assign current labels to each branch. AAssign current labels to each branch. A current entering a junction is positive, onecurrent entering a junction is positive, one leaving is negative. From theleaving is negative. From the conservation of charge you get theconservation of charge you get the sumsum of currents to a junction is zero.of currents to a junction is zero. ApplyApply this rule to each junction to obtain a set ofthis rule to each junction to obtain a set of equations.equations.
  36. 36. 2. Give each resistor a voltage and each cell2. Give each resistor a voltage and each cell an emf. Sum of all the energy in each loopan emf. Sum of all the energy in each loop should be zero from the conservation ofshould be zero from the conservation of energy, so theenergy, so the sum of pd = zerosum of pd = zero..

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