Presentation on theme: "Caroline Chisholm College"— Presentation transcript:
1 Caroline Chisholm College Physics2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage
2 Caroline Chisholm College Electromagnetic induction Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnetCaroline Chisholm CollegePhysicsOersted - Electric current produces a magnetic fieldFaraday - current carrying conductor in a magnetic field experiences a forceThe motor effectFaraday - electric current or emf can be generated through the use of a magnetic fieldElectromagnetic induction
3 Electromagnetic induction CONCLUSION: Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnetCaroline Chisholm CollegePhysicsElectromagnetic inductionFaraday - electric current or emf can be generated through the use of a magnetic fieldFaraday wanted to see if the magnetic field from one coil would set up a current in another.Faraday wound coils of wire around wood and attached a battery (PRIMARY COIL)GThen he wound another circuit of coils which was attached to a galvanometer (SECONDARY COIL)He noticed that a temporary current registered when the battery was switched on or off but no current registered while the battery was left onM.Edwards 6/5/02THE GALVANOMETER MOVED IN OPPOSITE DIRECTIONS FOR OFF AND ONWhen he placed a needle in a secondary coil, he found that the needle became magnetisedCONCLUSION:WHEN THE MAGNETIC FIELD OF THE PRIMARY COIL IS CHANGING, A CURRENT IS INDUCED IN THE SECONDARYHis experiment with coils on opposite sides of an iron ring produce even more noticeable currents when switching off and onGM.Edwards 6/5/02
4 Caroline Chisholm College Physics Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnetCaroline Chisholm CollegePhysicsFaraday also showed thatwhen a magnet was moved near a coil,an electric current was set up in the coilOpposite polesproduced opposite currentsGNOpposite movement directionsproduced opposite currentsPerform an investigation to model the generation of an electric current by moving a magnet in a coil or a coil near a magnetFaster movementsproduced larger currentsJacaranda Experiments 7.1 & 7.2
5 Define magnetic field strength B as magnetic flux density Caroline Chisholm CollegePhysicsDescribe the concept of magnetic flux in terms of magnetic flux density and surface areaMagnetic flux, F, is the amount of magnetic field that passes through an areaM.Edwards 23/5/02Magnetic field strength, B, is the amount of flux per unit area, or ‘magnetic flux density’M.Edwards 23/5/02So if the same number of field lines pass through a smaller area, the magnetic field is stronger; like this:The closer (more dense)the field lines are,the stronger the magnetic field isMagnetic flux is measured in Webers (Wb)‘Magnetic field strength’ or ‘magnetic flux density’ is measured in Tesla (T)
6 Caroline Chisholm College Physics Describe generated potential difference as the rate of change of magnetic flux through a circuitCaroline Chisholm CollegePhysicsReviewText PFaraday's Law of Induction"The induced emf in a circuit is equal in magnitudeto the rate at which the magnetic fluxthrough the circuit is changing with time"
7 Caroline Chisholm College Physics Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when: - the distance between the coil and magnet is varied - the strength of the magnet is varied - the relative motion between the coil and the magnet is variedExperiment using the bar magnet and the solenoidGNIt eventually worked when the solenoid was fixed!
8 Caroline Chisholm College PhysicsA student wanted to determine what factors would influence the current generated when a permanent magnet was moved inside a coil of wire as shown in the diagram below.(a) Outline an experimental procedure that the student could use to determine what factors influence the induced current.(b) Design a table that clearly shows and includes a quantitative sample of expected results for the experiment in part (a).
10 Caroline Chisholm College "motors and generators links" PhysicsGather, analyse and present information to explain how induction is used in cooktops in electric rangesInduction heating uses changing magnetic fields to induce eddy currents which cause an increase in temperature of the conductor (e.g. saucepan), due to collisions between moving electrons and atoms, and agitation of the atoms due to rapidly changing B field.The induction coils and the saucepan are separated by a ceramic top plateThe eddy currents cause the saucepan to heat up directly (which then cooks the food), without the loss of energy to the air that occurs with gas heating. Induction cookers are 80% efficient, compared to 43% for gas.Click here and find therelevant links in"motors and generators links"
11 Caroline Chisholm College PhysicsThe diagram below shows an induction cooktop (a) Explain how the coil induces a current in a saucepan placed on the cooktop. (b) Explain how this would reduce fire hazards in the kitchen.
13 Caroline Chisholm College Physics Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motorsCaroline Chisholm CollegePhysicsLenz's Law"An induced emf always gives rise to a current that creates a magnetic fieldthat opposes the original change in flux through the circuit"This means that if a North pole of a magnet is brought near a solenoid, current will flowin the solenoid to produce a North pole that opposes the approaching North pole.GNNOr if a conducting ring is moving into or out of a field, a current will be set up in the ring that produces a magnetic field that opposes the change in flux.IIIIcurrent will stop when the ring has completely left the fielde.g. this ring has a decreasing number of field lines into the page as it moves right. So the current will be clockwise to produce more field into the page.
14 Caroline Chisholm College Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motorsCaroline Chisholm CollegePhysicsLenz’s Law must be true in terms of conservation of energy, otherwise if a change in flux produced a flux in the SAME direction as the original change, this would produce an even greater change in that direction, producing an even GREATER change etc. etc.... The current would keep increasing as this happened, thereby CREATING ENERGY WITHOUT DOING ANY WORK......This clearly contradicts CONSERVATION OF ENERGY:Energy cannot be created nor destroyed,but it can be transformed from one form to anotherTo create electrical energy, work must be done -e.g. moving the magnet or the ring from the examples on the previous pageExplain that, in electric motors, back emf opposes the supply emfA rotating coil in a motor also sets up a current that opposes the change in flux. This will be in the opposite direction to the supply emf, so is called back emf. The back emf increases as the motor armature speed increases until the back emf equals the supply emf. Then the armature rotates at a constant rate.
15 Explain the production of eddy currents in terms of Lenz’s Law Caroline Chisholm CollegePhysicsThe magnetic fields set up by eddy currents oppose the changein flux that is occurring in that region of the conductor.For example if an aluminium disc is moving to the left out of a B fieldwhich is directed into the page.....What change is happening to the flux through the disc?A. The amount of magnetic flux into the page (through the disc) is DECREASINGSo how will the disc OPPOSE this change?A. A current will be induced that produces amagnetic field into the page!Which way will it go? Which way will it go?USE YOUR RIGHT HAND GRIP RULE!!! POINT THE FIELD INTO THE PAGE!!Which way will it go? Which way will it go?CLOCKWISE!
16 Caroline Chisholm College "motors and generators links" Gather secondary information to identify how eddy currents have been utilised in switching devices and electromagnetic brakingCaroline Chisholm CollegePhysicsReviewText P.141“GATHER”Click here and find therelevant links in"motors and generators links"
17 Caroline Chisholm College PhysicsThe following diagram represents a disk spinning in a magnetic field.Explain what will happen to the disk and how this may be overcome.