Presentation on theme: "In unit 2 we will learn about"— Presentation transcript:
1In unit 2 we will learn about the physics of electricity and electronics.This includes circuits, Ohm’s law, resistance, electrical energy and power, electromagnetism and electronic components.Print for lab books
3Key words: electrons, conductors, insulators, charge, currentBy the end of this lesson you will be ableto:State that electrons are free to move in aconductorDescribe the electrical current in terms ofmovement of charges around a circuitDistinguish between conductors and insulatorsand give examples of each.Carry out calculations involving Q = It
4What is inside an atom? Rutherford Bohr model Thomson’s Plum pudding modelWhat is inside an atom?Quantum model of the nucleusCharge cloud modelRutherford model
5The atom An atom is a fundamental unit of matter made up of protons (with a positive charge)neutrons (neutral – no charge)electrons (with a negative charge)
6What is electricity?Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS.Proton (+)Electron (-)Neutron
7An atom will usually have the same number of positives and negatives This makes the atom NEUTRAL.Proton (+)Electron (-)Neutron
8Q Electrical Charge Electrons are the charge carriers Electric charge is given the symbolQElectrons are the charge carriersthat flow in an electrical circuit –from the negative to positiveterminals.
9Coulombs C Electrical Charge which is given the symbol Charge is measured inCoulombswhich is given the symbolC
10Electrical ChargeThe charge on a proton is1.6 x 10-19Cwhich is the same size as the charge on anelectron.
11What is electricity? Electrons have a negative charge (Q) measured in coulombs (C).Electrons move round a circuit fromnegative to positive (remember likecharges repel, opposites attract)giving rise to an electric current.
17I Charge, Current & Time Electric current is the movement of Electric current is given the symbolIElectric current is the movement ofnegative charges (electrons) in acircuit
18Amps (A) Charge, Current & Time Current is the amount of charge flowingper second and is given the unitAmps (A)
19Charge, Current & Time If current is charge flowing per second then so a current of 1 A is 1 C of charge transferred in 1 s.Charge transferred in coulombs (C)Current in Amps (A)time in seconds (s)
20Charge, Current & TimeThis can be rearranged asor
21Key words: series, current, ammeter, voltmeter, battery, resistor, variable resistor, fuse, switch, lamp,voltageBy the end of this lesson you will be ableto:Draw circuit diagrams to show the correct positions ofan ammeter in a series circuit.Draw and identify the circuit symbols for anammeter, voltmeter, battery, resistor, variableresistor, fuse, switch and lamp.State that in a series circuit, the current is the same atall positions.
22Different types of circuit There are different ways in which you canconnect cells and components (such aslamps) to create a circuit:seriesparallela mixture of both
23Series Circuit A series circuit has only one electrical path. You can trace from one side of the battery to the other, through each component, without lifting your finger from the page.
24Different types of circuit There are different ways in which you canconnect cells and components (such aslamps) to create a circuit:seriesparallela mixture of both
25Series Circuit A series circuit has only one electrical path. You can trace from one side of the battery to the other, through each component, without lifting your finger from the page.Physics Animations – Series Circuits
26Name that component On the back of p2 carefully draw each symbol and ResistorFuseAmmeterBatteryOn the back of p2 carefully draw each symbol andlabel – in pencil!VoltmeterSwitchLampCellVariable resistor
27Build a series circuit On the worksheet you will find four building circuit activities.Follow the instructions carefully!Answer each question as you go.Make careful observations.Lesson 2 build a series circuit.pub
28Build a series circuit Build a series circuit which contains a 6V battery pack, three 3.5 V lamps inlamp holders, and a meter used formeasuring current.What is the meter called?Where is it positioned in the circuit?
31Activity 3 - Change your circuit… Move your ammeter to different positionsin the series circuit.Make a note of the positions each time,and of the current at each position.What can you say about the current in aseries circuit?
32Successful Circuit Diagrams On your worksheet you have drawn a circuitdiagram.To be successful at circuit diagrams:use a ruler and pencildraw components carefullydraw wires as straight lines (with corners asright angles!)make sure all components are correctly drawand joined in the circuit.
34Notice in this circuit, current is the same at all points
35Notice in this circuit, current is the same at all points
36Series Circuits and Current We are measuring the current I in a series circuit.What have we observed?We find that the current is the same atall points.How can this be written mathematically?I1 = I2 = I3 = I4 and so onVirtual Int 2 Physics – Electricity & Electronics – Circuits – Series Circuits
37Think… How could you make use of a series circuit to investigate which materials areconductors and which materials areinsulators?Which components would you need?What would you observe?
38…and learn components and names formulae and symbols what is a series circuit?current in series circuitdrawing a series circuit diagram
40Key words: series, parallel, ammeter, current, By the end of this lesson you will be ableto:Draw circuit diagrams to show the correct positions of anammeter in a parallel circuit.Draw and identify the circuit symbols for an ammeter, andlamp.State that in a series circuit, the current is the same atall positions.State that in a parallel circuit, the sum of the current inthe branches adds up to the current drawn from thesupply.
41Quick Quiz What is a series circuit? What is the symbol for current? What are the units of current?What is the relationship between current andtime?What do we know about the current in a seriescircuit?How do we measure current?Draw the symbol for this.Describe how to measure current in a seriescircuit.
42Build another circuit Build a series circuit which includes a 6V battery, a 6V lamp and an ammeter.Draw the circuit diagram for your circuit:
43Build another circuit We will now take one of your series circuits, and “add it” to someone else’s.Another ammeter has been added.What do you notice about the readings onthe ammeter?
44Build another circuit We will now “add” another series circuit. What do you notice about the readings onthe ammeter?
45What sort of circuit is this? We have constructed a parallel circuit.What does the circuit diagram look like?Try drawing it on Crocodile Physics.
47Parallel Circuit can divide and take different branches. We have constructed a parallel circuit.This is a circuit with different branches.When it reaches a junction, the currentcan divide and take different branches.
48Parallel Circuits and Current We are measuring the current I in aparallel circuit.What have we observed?We find that the current in each of thebranches adds up to the total current.How can this be written mathematically?IT = I1 + I2 + I3 and so on
49How many ways can you make two light bulbs work? Electric CircuitsHow many ways can you make two light bulbs work?
50A SIMPLE CIRCUIT Close the switch, what happens? SWITCH CELL LIGHT BULB
56Key words: voltage, potential difference, voltmeter, series, parallelBy the end of this lesson you will be ableto:Draw and identify the circuit symbols for avoltmeter, battery, and lampState that the voltage of a supply is a measureof the energy given to the charges in a circuit.Draw circuit diagrams to show the correct positions of avoltmeter in a circuit.State that the sum of potential differences across thecomponents in series is equal to the voltage of thesupply.State that the potential difference across componentsin parallel is the same for each component.
57What is electricity? What is a voltage? What is a volt? DiscussionDemonstrationVoltage in series and parallel
58What is the energy change which takes place in a battery? Chemical to Electrical
59When a battery is in a circuit… The electrical energy is carried by theelectrons that move round the circuit.It is converted into others forms ofenergy.
60If there is a bulb in the circuit, it is converted from to
61The amount of electrical energy the electrons have at any point in a circuit isknown as their “potential”.As they move the electrons transfer energyinto other forms.This means at any two points the electron hasdifferent amounts of energy.
62Electrons start with (for example) 6J of energy. They have “potential”. As they pass through the bulb, some of the energy is converted to light.Electrons which have passed through the bulb have less energy. Or less “potential”.There is a “potential” difference in the circuit
63What has “potential difference” got to do with voltage? It is the same thing!The potential difference (p.d.), or voltage,of a battery is a measure of the electricalenergy given to one coulomb of chargepassing through the battery.
64Potential Difference or Voltage (V) A 9 V battery will give how much energyto each coulomb of charge passingthrough the battery?9 J
65Potential Difference or Voltage (V) A 1.5 V battery will give how much energyto each coulomb of charge passingthrough the battery?1.5 J
66Potential Difference or Voltage (V) A battery with a p.d. of 6V will give howmuch energy to each coulomb of chargepassing through the battery?6 J
67volts V Voltage or p.d. and is given the symbol Voltage (or p.d.) is measured involtsand is given the symbolV
68Summary of Units Quantity Symbol Units charge Q coulombs C time t secondsscurrentIamperesAvoltageVvolts
69How can we measure voltage? Voltage (or p.d.) can be measured using avoltmeter.An ammeter is connected in the circuitbut a voltmeter must be connected acrossthe component.V
70You can’t measure voltage… in a circuitthrough a circuitthrough a componentflowing
71Build a series circuit Build a series circuit which contains a 6V battery, two 6V lamps, and a meterused for measuring potential differenceacross each lamp.What is the meter called?Where is it positioned in the circuit?
72Drawing a circuit diagram Now draw a circuit diagram of the seriescircuit which you built.Remember to use a ruler and pencil, drawcomponents carefully, draw wires asstraight lines (with corners as rightangles!), and make sure all components arecorrectly draw and joined in the circuit.
73Series Circuits and Voltage We are measuring the potential difference (V) in a series circuit.What have we observed?We find that theHow can this be written mathematically?
74Parallel Circuit Now use the same components to construct a parallel circuit.This is a circuit with different branches.
75Parallel Circuits and Voltage We are measuring the potentialdifferences in a parallel circuit.What have we observed?How can this be written mathematically?
76Tasks & Homework Yellow Practice Questions: 2.10, 2.11 Numerical Questions: p33-36 qu 5-14Complete for homework for Tuesday 27thNovember
80Key words: electrical resistance, voltage, current, Ohm’s law, ohms, resistor,variable power supplyBy the end of this lesson you will be able to:State that V/I for a resistor remainsapproximately constant for different currents.State that an increase in resistance of a circuitleads to a decrease in the current in thatcircuit.draw the symbol or a variable power supply andresistor.
81Key words: electrical resistance, voltage, current, Ohm’s law, ohms, resistor,variable power supplyBy the end of this lesson you will havepractised:building a series circuitusing an ammeter and a voltmeter to findcurrent and voltage.graphing results21//11 and 22/11.
83Resistors Resistors oppose (or resist) the flow of electric current. They have aproperty called resistance (R) whichis measured in ohms (Ω).
84What is the relationship between current and voltage in a resistor? Current is measured using an ammeter.Voltage is measured using a voltmeter.Investigation: relationship betweencurrent and voltage in a resistor.
85Relationship between current and voltage in a resistor I / AmpsStraight line throughthe origin tells us thatcurrent isdirectly proportional to voltageThe ratio V/I is constant and is equal to resistance in the circuit.p.d. / Volts
86Relationship between current and voltage in a resistor
87Relationship between current and voltage in a resistor Ohm’s Law
88Resistors cell A lamp resistor What do you expect to happen to the current if you increase the value of the resistor in the circuit shown?AlampresistorDemonstration
89Calculate For a voltage of 12V, calculate the current for a resistant of1 Ω2 Ω4 Ω24 Ω1 k Ω
90What can you say about current and resistance for a fixed voltage? Completethe sentences.As resistance increases, the currentAs resistance decreases, the current
91Varying Resistance The opposition to current or resistance of a material (measured in Ω) dependson several things.Think and discuss what some of thesemight be.
92Varying Resistance The opposition to current or resistance of a material (measured in Ω) depends ontype of material (the better the conductor, the lower the resistance)length of material (the longer the material, the higher the resistance)thickness of material (the thinner the material, the higher the resistance)temperature of material (the higher the temperature, the higher the resistance)
93Varying Resistance variable resistor (or rheostat). The relationship between length of thematerial and resistance allows us to makeavariable resistor (or rheostat).
95Variable Resistors In the above diagram, if the slider is moved in the directionA→B the resistance willincrease because the length ofwire through which the currentpasses increases.
96Uses of Variable Resistors? Variable resistors can be usedas volume or brightness controls ontelevisionsvolume control on MP3 playerslight dimmer switches.
97Key words: resistance, series, parallel, ohms, ohmmeter By the end of this lesson you will be ableto:State the relationships between totalresistance and individual resistances inseries and parallel circuitsCarry out calculations involving therelationships between resistors in seriesand in parallelWednesday 28th / Thursday 29th
98Key words: resistance, series, parallel, ohms, ohmmeterBy the end of this lesson you will havepractised:building a series circuitbuilding a parallel circuitdrawing circuit diagramsusing an ohmmeter to measure resistancein a circuit
99Variation of Resistance and Current for a Lamp Filament Look at the circuit diagram below:Handout
100Name each of the components Is this a series or parallel circuit?As the voltage across the lamp increases, what do you expect to happen to the current?Sketch a graph of your prediction of the relationship between current and voltage.
101In the resistor, current and voltage are directly proportional.But in a filament lamp, heat is generated.We know that resistance increases astemperature increases. So we see that asvoltage increases, temperature increases,resistance increases and currentincreases – but more slowly than we mightpredict.
102Measuring Resistance We can find the resistance of a component by measuringvoltage across the component usinga voltmetercurrent through the component usingan ammeter
103Ω Measuring Resistance ohmmeter or we can measure it directly using an Demonstration & experiment
104Series and Parallel Circuits Voltage, Current and Resistance VsI3I2I1R3+-R2R1V1V2V3What type of circuit is this?
105One electrical path from negative to positive therefore series. VsI3I2I1R3+-R2R1V1V2V3One electrical path from negative to positive therefore series.
106What is the relationship between the three currents? VsI3I2I1R3+-R2R1V1V2V3What is the relationship between the three currents?The current is the same at each point.
107What is the relationship between the four voltages? VsI3I2I1R3+-R2R1V1V2V3What is the relationship between the four voltages?They add to equal the supply voltage.
108Disadvantages of Series Circuits? When one component fails the whole circuitfails.The current is the same at all points and thevoltage is divided between the bulbs. Themore bulbs added the dimmer each one is.
109How do you find total resistance in series? VsI3I2I1R3+-R2R1V1V2V3How do you find total resistance in series?Add each resistance together.
110What type of circuit is this? VsI3I2I1R3+-R2R1V1V2V3What type of circuit is this?
111ITVsI3I2I1R3+-R2R1V1V2V3More than one electrical path – components connected on different branches therefore parallel.
112What is the relationship between the four currents? Vs-+What is the relationship between the four currents?ITV1ITR1I1V2The four currents add to give the total current.R2I2V3R3I3
113What is the relationship between the four voltages? Vs-+What is the relationship between the four voltages?ITV1ITR1I1V2Each voltage is equal to the supply voltage.R2I2V3R3I3
114The resistance in parallel? Vs-+ITV1ITThe resistance in parallel?R1I1V2R2I2V3R3I3
115If more resistors are connected in parallel the total resistance will always decreaseThis is because there are more branches through which the electricity can flow.
116Advantages of the Parallel Circuit? When one bulb fails the rest of the circuitcontinues to work.The more components, the lower theresistance. The total current drawnincreases. Voltage in each branch is the same asthe supply voltage therefore bulbs in parallelwill each be as bright as a single bulb.What have you learned today?
117Key words: resistor, resistance, series, potential, potential divider Handout 3Key words: resistor, resistance, series,potential, potential dividerBy the end of this lesson you will be ableto:State that a potential divider circuitconsists of a number of resistors, or avariable resistor, connected across apower supply.Carry out calculations involving potentialdifferences and resistance in a potentialdivider.Thursday 29th / Friday 30th
118Name each component.What type of circuit is this?VV
119The supply voltage is 6V. What is voltage V1? V2? 10Ω10ΩV1V2
120The supply voltage is 10V. What is voltage V1? V2? 10Ω10ΩV1V2
121The supply voltage is 5V. What is voltage V1? V2? 10Ω10ΩV1V2
122The supply voltage is 6V. What is voltage V1? V2? 5Ω10ΩV1V2
123A series circuit with two resistor and a power supply is known as a potential divider. Why is it called a potential divider?V1V2
124The potential difference of the supply is divided between the two resistors.When the two resistors are identical (i.e.have the same value of resistance), thepotential difference is split equally.
125Investigating Potential Dividers Worksheet – review values for next year. This year completed using Croc Physics rather than practical – this seemed to work better than with practical values.
126Potential Divider Circuits A voltage divider consists of two devices, usually resistors, connected in series.V1V2R1=100 Ω6VR2=100 ΩV1V2R1=4.5 kΩ6VR2=9 kΩ
127The current in each resistor is calculated using Ohm’s Law:
128V2 V1 I2 = I1 = = R2 R1 What can we say about the current in a series circuit?It stays the same throughout the circuit.I2 =V2R2I1 = =V1R1
131If the resistance of one resistor is increased, the voltage across thisresistor willThis means the other voltage must
132Potential Dividers What do the symbols mean? V1 is the voltage across resistor R1V2 is the voltage across resistor R2VS is the supply voltageRT is the total resistance
133Potential Dividers Look again at the worksheet. Use the formula to calculate V1 and V2 for each circuit.The answers found using the formula match the values measured using the voltmeter.
134Potentiometer The potentiometer is a special type of voltage divider. It is a variable resistor with a slidingcontact.
135What range of output is it possible to obtain from a potentiometer?Range of output voltages 0V to supplyvoltage.
136Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be ableto:State that when there is an electrical currentin a component there is an energytransformation and give some examples.State the relationship between energy andpower.Carry out calculations using E = PtState that in a lamp electrical energy istransformed into heat and light.State that the energy transformation in anelectrical heater occurs in the resistance wire.Thursday 29th / Friday 30th
137What is electricity? What is a voltage? What is a volt?
138What is “potential difference” ? What is voltage? It is the same thing!The potential difference (p.d.), or voltage,of a battery is a measure of the electricalenergy given to one coulomb of chargepassing through the battery.
139What is the energy change which takes place in a battery? Chemical to Electrical
140When a battery is in a circuit… The electrical energy is carried by theelectrons that move round the circuit.It is converted into others forms ofenergy.
141If there is a bulb in the circuit, it is converted from toVirtual Int 2 Physics -> Electricity ->Electrical Energy & Power ->Energy Transformation in a Lamp
142Filament lampsFilament of tungsten wireGlassHow does it work?
143Filament Lamp Tungsten (metal) filament becomes so hot it glows. Why isn’t oxygen used inside the bulb?
144Filament lamps Electric current passes through the resistance wire whichis made of tungsten.Electrical energy ischanged into heatenergy and thewire glows white hot.Filament lampsproduce both heat andlight.
145In an electric fire, energy is converted fromto
146Resistance in a wireWe have learned that when a voltage is applied across a lamp, the resistance increases.What happens to the temperature?
147Resistance in a wire As current passes through a resistance wire, the wire gets hot.This is how electric fires and filament lightswork.The filament becomes hot enough to glow andemit light. The bar of the electric fire is alength of wire which also glows when hot.
148Electrical appliances change electrical energy into other forms. What are the energy changes taking place in these appliances?Notes p22
149Power and EnergyElectrical energy has the symboland is measured in
150Power the amount of energy used by the component / appliance in one The power rating of an appliance or acomponent is defined asthe amount of energy used by thecomponent / appliance in onesecond
151Power The power rating tells us the rate at which energy is transformed, that is theenergy transformed each second.
152Power For example, an appliance with a power rating of 250 W converts 250 Joules ofelectrical energy into another form eachsecond.
153Power How can this be written as a formula? Power in Watts (W) Energy in Joules (J)time in seconds (s)Demonstration / experiment
154Investigating Energy and Power Connect the joule meter to the voltage supply and a ray box bulb to the joule meter.Set the supply voltage at 6V and switch on. You’ll see the counter on the joule meter increasing (note each time the counter increases by 1, this is 100J of energy).Record the number of joules used in 50s and 100s. Calculate the number of joules used per second.Power is energy used per second, in watts. Write the formula:If the supply voltage was increased to 12V, what would you expect to happen?Increase supply voltage to 12V and repeat the experiment.Worksheet / experiment
155Power and Energy Were your results as expected? Ray box bulb, 6V supplyRay box bulb, 12V supplyNumber of joules used in 50 s?Number of joules used in 100 s?Number of joules used eachsecond?Power (W)Were your results as expected?1 watt is equivalent to the transfer of 1 joule per second.
156Power & Energy ExampleIf an electric fire uses 1.8 MJ of energy in a time of 10 minutes, calculate the power output of the fire.
157Power & Energy ExampleP = ?E = 1.8 MJ = 1.8x106 Jt=10 minutes = 600 s
159Power Ratings of Appliances Different appliances have differentpower ratings.What is meant by power?
160Watt’s my power rating? 500 W, 150 W, 1200 W, 100 W, 3000 W, 300 W, As a group – use tablet to write on power ratings – score off as go.What do you notice about those with the highest power rating?500 W, 150 W, 1200 W,100 W, 3000 W, 300 W,800 W, 1500 W, 30 W, 60 W, 11 W
161Watt’s my power rating? 300 W 500 W 60 W, 1500 W 1200 W 30 W 150 W As a group – use tablet to write on power ratings – score off as go.What do you notice about those with the highest power rating?11 W800 W
163Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be ableto:State that the electrical energytransformed each second = VICarry out calculations using P=IV and E=PtExplain the equivalence between VI, I2Rand V2/R.Carry out calculations involving therelationships between power, current,voltage and resistance.Thursday 29th / Friday 30th
164Watt’s my power rating? 500 W, 150 W, 1200 W, 100 W, 3000 W, 300 W, As a group – use tablet to write on power ratings – score off as go.What do you notice about those with the highest power rating?500 W, 150 W, 1200 W,100 W, 3000 W, 300 W,800 W, 1500 W, 30 W, 60 W, 11 W
165Watt’s my power rating? 300 W 500 W 60 W, 1500 W 1200 W 30 W 150 W As a group – use tablet to write on power ratings – score off as go.What do you notice about those with the highest power rating?11 W800 W
166Current through Appliances Different appliances have differentpower ratings.P = IVFor appliances which use the mains supplyV =
167Current through Appliances As power increases for a fixed voltage,what happens to the current?As power increases the currentincreases
169Even withthe switch open and zero current the lamp is still at 9V.NeutralLive
170This time, when the switch is open, the lamp is at 0V and is safe to touch. NeutralLive
171The red flags indicate that voltage at these points is 9V.
172Closing the third switch results in a current greater than 1A, blowing the fuse.
173Inserting a voltmeter across a bulb shows that the bulbs are at zero volts. If you touch them, you won’t receive an electric shock as they are isolated from the voltage supply.
174The red flags indicate that voltage at these points is 9V The red flags indicate that voltage at these points is 9V. The fuse is now in the neutral wire.
175Closing the third switch results in a current greater than 1A, blowing the fuse. The red flags show that at these points the voltage is still at 9V.If you touch this now, you’ll complete the circuit and receive an electric shock – you become the “neutral wire” and allow electricity to flow through you.
176Why can a bird sit safely on this high voltage power line?What will happen if the bird spreads its wings and touches the pylon?
178Which fuse to use?How would you calculate which fuse is required for an appliance?An appliance operating from the mains supply has a supply voltage of 230V.The rating plate gives you information on the power of the appliance.
179The formula which links voltage, power and current: P = VI
180The general rule for fuses The fuse value needs to be just above the normal operating currentIf the appliance has a power rating of:Fuse value should be:Less than 700W3AMore than 700W13A
181Example What is the appropriate choice of fuse for a mains appliance with a power rating of330 W?
182Example What is the appropriate choice of fuse for a mains appliance with a power rating of330 W?
183Power Ratings of Appliances Which type of appliances tend to have thehighest power ratings?Generally, appliances which produce heat.
184Power Ratings of Appliances Which type of appliances draw thehighest current?Generally, appliances which produce heat.
185Power Ratings of Appliances Which type of appliances need the largestvalue of fuse?Generally, appliances which produce heat.
188Key words: electrical energy, power, voltage, current, resistance By the end of this lesson you will be ableto:State that the electrical energytransformed each second = VICarry out calculations using P=IV and E=PtExplain the equivalence between VI, I2Rand V2/R.Carry out calculations involving therelationships between power, current,voltage and resistance.Thursday 29th / Friday 30th
189power, voltage, current and resistance. What do you notice about Investigating…power, voltage, current andresistance.What do you notice aboutWorksheet / experiment
190Power can be calculated from the voltage across the appliance and the current flowing through it. Written as an equation:P = IV
191Relationship between power, current, voltage and resistance Our experiments showed that
192Relationship between power, current, voltage and resistance
196Key words: alternating current, direct current, mains supply, frequencyBy the end of this lesson you will be ableto:Explain in terms of current the terms a.c. and d.c.State that the frequency of the mains supply is 50Hz.State that the quoted value of an alternating voltage isless than its peak value.State that a d.c. supply and an a.c. supply of thesame quoted value will supply the same power toa given resistor.Thursday 29th / Friday 30th
197Direct Current (d.c.)The voltage drives a steady or direct current.The electrons move in one direction.The current (or voltage) does not change with time.
199Alternating Current (AC) An alternating current is continually changing directionThe alternating voltage and current has a distinctive waveform
200Alternating CurrentUsing the oscilloscope, we can measure the peak voltage of the a.c. supply.The declared, quoted or “effective”, voltage is always less than the peak voltage.
201Calculating Declared Voltage The declared (or effective) voltage can be calculated from the peak voltage.The quoted voltage is ~ 0.7 x peak voltage.The declared voltage is the value of a.c. voltage which gives the same heating or lighting effect as d.c. voltage.
202Mains SupplyWhat is the frequency of the mains supply?50 Hz
203Mains Supply What is meant by the frequency of the supply? Alternating current flows one way then the other. It is continually changing direction. The rate of the changing direction is called the frequency and it is measured in Hertz (Hz) which is the number of forward-backward cycles in one second.
204Mains Supply Why does the current change direction? Voltage pushes the current. The voltage changes polarity causing the current to change direction.
205Mains SupplyWhat is the declared value of the mains supply voltage?What is meant by the voltage of the supply?230VThe voltage of a power supply or battery is a measure of how much “push” it can provide and how much energy it can give to the electrical charge.
206Measuring effective voltage / current in an a.c. circuit The effective voltage or current in an a.c.circuit can be measured using a.c.voltmeter or ammeter.
207Measuring peak a.c. voltage using an oscilloscope Adjust the position so the trace is central on the screen.Adjust the volts/div so the trace fills the screen.Count the number of boxes from the axis to the peak.Multiply the number of boxes by the volts / div.
209Key words: electromagnetism, induced voltage, field strength, turns. By the end of this lesson you will be able to:State that a magnetic field exists arounda current carrying wire.Identify circumstances in which a voltagewill be induced in a conductor.State the factors which affect the sizeof the induced voltage i.e. field strength,number of turns on a coil, relativemovement.18th December / 8th / 9th January
210Permanent Magnets A magnetic field is the region around a magnet in which a magnetic force can bedetected.
212Magnetic Field Around a Current Carry Wire What happens when the direction of thecurrent is reversed?The direction of the magnetic field isreversed.Plotting magnetic field. Solenoid (on OHP)
213Electromagnets When an electric current passes through a wire which is coiled around an iron core, thecore becomes magnetised and anelectromagnet is produced.When an a.c. current is used, the currentchanged direction and so the magnetic fieldchanges direction.Coil without core (10V); coil with core (10V) (pick up paper clips)Magnetic knifeCoils with different number of turns (balance to find mass of paperclips collected).e-m demo
214Electromagnets Strength of electromagnet with/without iron core? Effect of increasing current through the coil?Effect of increasing number of turns in thecoil (while keeping current constant)?
215How is an electromagnet constructed? A current through a wire can be used tocreate an electromagnet.
216How is an electromagnet constructed? A conducting wire is wound round an iron core.When a current passes through theconductor there is a magnetic field aroundthe conductor. By wrapping it round a softiron core, the magnetic field is concentrated.
218How can the strength of an electromagnet be increased? By increasing the current through thecoil.By increasing the number of turns on thecoil of wire.
219What are the advantages of an electromagnet over a permanent magnet? The electromagnet can be switched off.The magnetic field strength can be varied(how?)The electromagnet provides a much strongermagnet field for the same size than apermanent magnet.
220Electromagnetic Induction What happens when a wire is moved in amagnetic field?A voltage is created – or induced. For thisreason we call this electromagneticinduction.
221Electromagnetic Induction What happens when a permanent magnetis moved towards or away from a coil ofwire?
222ELECTROMAGNETIC INDUCTION Got to this point 9th Jan – continue on 10th
223What do we know so far? When a current passes through a coil of wire, there is a magnetic field around thewire.Changing direction of the current changesthe direction of the magnetic field.
224What do we know so far? When we move a wire in a magnetic field, voltage is induced.When we move a magnet in a coil of wire,a voltage is induced.What do we have in common? Changingmagnetic field leading to electricity!
236DIRECTLY PROPORTIONAL TO THE RATE OF CHANGE OF MAGNETIC FIELD THE INDUCED VOLTAGE ISDIRECTLY PROPORTIONAL TOTHE RATE OF CHANGE OF MAGNETIC FIELD
237What is observed when… the magnet is stationary next to the coil? Nothing! No voltage is induced.The magnet is moved in the oppositedirection (towards the coil instead ofaway from it)?The voltage produced has opposite polarity.
238What is observed when… the magnet is moved backwards and forwards? Voltage induced which has a changing polarity.What does this mean for the current?The current will change direction – it isa.c.!
239Generating Electricity A voltage can be induced in a coil of wireif a magnet is moved towards (or awayfrom the coil).This effect is known as induction.What does the induced voltage dependon?
240Generating Electricity Induced voltage depends on:strength of the magnetic field (the stronger the greater the induced voltage)speed of movement (the faster the greater the induced voltage).number of turns in the coil (the more turns of wire on the coil the greater the induced voltage).Virtual Int 2 Physics – Electricity & Electronics – em induction
241Generating Electricity To summarise:A voltage is induced across the ends of a wirecoil is the coil experiences a changing magneticfield.
244Now I understand!VOLTAGE IS ONLY INDUCED WHEN THERE IS RELATIVE MOTION BETWEEN A CONDUCTOR AND A MAGNETIC FIELD
245DIRECTLY PROPORTIONAL TO THE RATE OF CHANGE OF MAGNETIC FIELD THE INDUCED VOLTAGE ISDIRECTLY PROPORTIONAL TOTHE RATE OF CHANGE OF MAGNETIC FIELD
246Generating Electricity How do we “create” electricity?
247A Simple Generator A current can be passed through a wire to result in movement (a motor!).Electrical energy was changed to kineticenergy.
248A Simple Generator The motor can work “in reverse”. Kinetic energy can be used to createelectricity in a dynamo or simplegenerator.
249TransformersWhat is a transformer?Demonstration.
250Transformers A transformer consists of two separate coils of wire wound on the same iron core.The first coil, the primary, is connected to an a.c.voltage supply. There is therefore a changingmagnetic field around the core.This changing field induces a voltage across theother coil, the secondary. A current flows as aresult of the induced voltage.
251Transformer Terms We talk about Primary coil (the first one – connected toa.c. voltage)Secondary coil (the second one – voltageis induced)Number of turns – number of “loops” ofwire in coil
252Transformer Terms We talk about Np – the number of turns on the primary coilNs – the number of turns on the secondary coilVp – the voltage applied to the primary coilVs – the voltage induced across secondary coilIp – the current in the primary coilIs – the current in the secondary coil
253THE TRANSFORMER Laminated soft iron core PRIMARY Np Turns AC input VP VoltsSECONDARY NS TurnsAC output VS Volts
254Equipment Set your power supply to 2V. YOU MUST NOT EXCEED 2V as the 2 coils1 x a.c. voltmeterFour wiresA variable power supply.Set your power supply to 2V. YOUMUST NOT EXCEED 2V as theprimary voltage.
255Measure the output voltage for a 2V input for each of the combinations of numberof turns in the primary and secondary.Record your results in your table.
256Calculate andand record your results in your table.
259TransformersA step-up transformer is one in which the secondary voltage is greater than the primary.A step-up transformer has more turns on the secondary coil than the primary coil.Which of the transformers are step-up?
260TransformersA step-down transformer is one in which the secondary voltage is less than the primary.A step-down transformer has fewer turns on the secondary coil than the primary coil.Which are step-down transformers?
261What would happen if a d.c. supply was connected to a transformer? At the moment of switching on, there is achanging magnetic field which would inducea voltage in the secondary coil. The same atthe moment of switching off.Once switched on, no changing magneticfield (since steady current) and therefore noinduced voltage.
262Step-Up Transformer What is a step up transformer? What can you say about the relationshipbetween the number of turns in the secondaryand primary?and the voltage in the secondary andprimary?
263Step-Down Transformer What is a step down transformer?What can you say about the relationshipbetween the number of turns in the secondaryand primary?and the voltage in the secondary andprimary?
264Energy Losses in Transformers For calculations, we often assume thatthe transformer is 100% efficient.however in reality they are about 95%efficient.What causes the energy losses?
265Energy Losses in Transformers Heating effect of current in coils (coils are long length of wire with resistance hence electrical energy changed to heat)Iron core being magnetised and demagnetisedTransformer vibrating -> soundMagnetic field “leakage”
266Voltage and Current in Transformers Assuming an ideal transformer with noenergy losses total energy input mustequal total energy output.Since rate of energy input is power:power input = power output
267Voltage and Current in Transformers Power is given asP = V Isowhich can be rearranged as
268Voltage and Current in Transformers In a step-up transformer, the voltage in the secondary is greater than the primary. What happens to the current?10TH January – pupils having difficulty with using this formula. Left with problem to solve overnight – pick up tomorrow 11th.
269Voltage and Current in Transformers The current in the coils is in the reverse ratio to the voltage therefore as voltage increases, current decreases.
270Voltage and Current in Transformers In a step-down transformer, the voltage in the secondary is less than the primary. What happens to the current?
271Voltage and Current in Transformers The current in the coils is in the reverse ratio to the voltage therefore as voltage decreases, current increases.
272Transformers np = number of turns on primary coil ns = number of turns on secondary coilVp = voltage across primary coilVs = voltage across secondary coilIp = current in primary coilIs = current in secondary coil
273Type oftransformerTurns ratio?Effect on VOLTAGE?Effect on CURRENT?Step-upStep-down
275Key words: electromagnetism, induced voltage, field strength, turns. By the end of this lesson you will be able to:State that high voltages are used in thetransmission of electricity to reducepower loss.Carry out calculations involving power lossin transmission lines.18th December / 8th / 9th January
276Transmitting Electrical Energy Transformers are used by the NationalGrid system through which electricalenergy is transmitted.Demonstration
277Electricity Transmission Electrical energy is transferred from the power station tothe consumer via the National Grid.Electricity is sent for many kilometres along transmissionlines on pylons.
278Transformers in Electrical Transmission What happens as current flows throughthe wires?The length of the wires means largeresistance and hence heating in the wires.
279Transformers in Electrical Transmission Energy is changed from electrical to heat resulting in large power losses in the wires.Relationship between power, current and resistance?
280Transformers in Electrical Transmission At the power station, a step-up transformer is used to increase the voltage.Why?
281Transformers in Electrical Transmission As voltage stepped up, current stepped down by the same factor. And since by reducing current the power losses due to heating are reduced.
282Transformers in Electrical Transmission This stepping up of the voltage and hence stepping down of the current makes the transfer much more efficient. The losses due to heating are reduced.
283Transformers in Electrical Transmission At the consumer end, a step-down transformer reduces the voltage to 230V, increasing the current.