Flashcards
Physics- Unit 4- Chapter 8: Electromagnetic induction
Cards in this deck (32)
- 1What is Electromagnetic induction?When a wire cuts magnetic flux lines an Emf is induced which induces a current if connected in a complete circuit
- 2How can induced emf be increased?Induced emf can be increased by increasing the rate of flux slicing:
- Moving the wire faster in the field lines
- Stronger magnet
-Greater number of coils
- moving magnet inside coil instead of other way around - 3Fleming's right hand ruleThumb= Force
Forefinger= Field lines (North to south)
Second finger= Induced current - 4Methods of generating an induced emf1) Using a motor in reverse: Moving a coil between the poles of a magnetic field
2) Cycle dynamo: When a magnet spins inside a coil - 5Right hand grip ruleMethod of figuring out which way the current flows around a solenoid and the magnetic field produced. The curled fingers in direction of current and thumb in direction of North.
- 6Solenoid ruleMethod of figuring out which way the current flows around a solenoid and the magnetic field produced. If looked at through one end, clockwise current= South and aNticlockwise= North
- 7Lenz's LawThe direction of an induced current is always such that it opposes the change that induced it. This is in order to obey conservation of energy
- 8Explanation of Lenz's lawWhen a magnet, North end, is pushed into one end of a solenoid, flux slicing occurs which induces a current in the solenoid. The direction of the current is such that the magnetic field produced around the wire opposes the magnet. The end becomes north pole to repel the movement of magnet into the solenoid. Energy is not created or destroyed and so this means the energy from the magnet is transferred as electrical energy in the wire.
- 9What happens when a magnet falls through a solenoid?It slows down as it enter because the force of gravity is opposed by the repulsive force of the solenoid field. This causes acceleration to decrease. The magnet travels at constant speed. when it leaves, solenoid field switches to attract the magnet so it continues to travel at constant speed
- 10Regenerative brakingBattery powered cars and hybrids have alternators which act as motors and generators. When brake is applied, alternator uses this force to generate a magnetic field. The magnetic field opposes the alternator which creates a braking force. The kinetic energy is converted to chemical energy in the battery.
- 11Flux density, BMagnetic field strength
The flux lines passing through a unit area
Units: Tesla/ T - 12Flux, ΦTotal flux lines in an area
Units: Wb or Tm^2
Equation: B x A
[B= Flux density A= area] - 13Flux linkage, NΦFlux through a coil with N turns
Units: Wb turn - 14Flux linkage equationN Φ= B*A*N cos θ
θ is the angle between the normal of the object and field lines. - 15Max flux linkageWhen θ equals 0, when the normal is parallel with the field lines
- 16Min Flux linkagewhen θ equals 90 degrees. The wire is parallel to the field lines. Because the fleming's right hand rule is not obeyed.
- 17Faraday's LawThe induced emf in a circuit is equal to the rate of change of flux linkage through a circuit.
- 18Derivation of Faraday's lawIn the notes
- 19A.C. generator: How to calculate emfℰmax= BLv * 2 * N
*2 because both sides of the coil feels the emf
Max emf at angle 90 degrees between normal and field lines
Min emf at angle 0 degrees between normal and field lines - 20Connection between flux linkage and emfThe magnetic flux must be changing to induce an emf. Therefore the parts of the flux vs. time graph with a gradient have an emf. But at the peaks of the flux, gradient =0 therefore emf = 0 at that time. This is why the two are 90 degrees out of phase.
- 21Derivation of ℰ =ℰ. * Sin θ ℰ.= max emfin the notes
- 222 features of emf that change if speed is increasedℰ. = 2NBLv
- The maximum emf value increases
therefore the amplitude of graph increases as ℰ. is higher. and frequency increases as there are more cycles per second due to faster spin. - 23D.C. generator: Use of split ring commutatorreverses the connection so the current travels in one direction.
- 24Power station alternator3 sets of coils 120 degrees out of phase with each other but in phase with opposite coils. Coils are stationary and central electromagnet spins which means no commutator is needed.AC current produced in coils.
- 25Back emfA motor has all the conditions needed to induce an emf. This emf acts against the p.d. It cannot add to p.d. as it would go against energy conservation laws.
V - ℰ = IR
IV - Iℰ = I^2 R
I^2R= power. This gives you the power wasted.
IV= Power supplied
Iℰ= Power of motor (power converted to mechanical power)
At high speed, current is low as induced emf is high, so power wasted is low. - 26Transformers Step up transformerPrimary coil= less coils
Secondary coil= more coils
Voltage is stepped up
current is stepped down to reduce energy loss through transmittance
Vs/Vp=Ns/Np - 27Efficiency of a transformer(IsVs/IpVp) * 100%
- 28How a step up transformer worksA transformer changes the peak value of alternating potential difference. The primary coil carries an alternating current which creates an alternating flux. The iron core increases the flux so it passes through the secondary coil. This causes flux slicing which induces an emf in the secondary coil and thus a current. Because the secondary coil has more turns, it has greater flux slicing so greater emf is induced.
- 29Transformer ruleFaraday's law states that emf is proportional to number of turns on a coil. B is same and the rate of change of flux should be same. Therefore emf is proportional to number of turns.
Rule: Vs/Vp=Ns/Np
- 30Why is V used instead of ℰ in the transformer rule?Because a back ℰ is produced by the primary coil so an emf is induced in the primary coil which opposes the primary supply voltage. BUT the back emf acts on the source not the coil so it does not cancel the p.d.
- 31How is power wasted in a transformer and how can it be reduced?1) Eddy currents are caused as the alternating magnetic field around the metal core induces an emf in the core which causes heat loss. Reduced using thin layers of insulator which have high resistance so current is not induced. so heat is not lost
2) regular magnetizing and demagnetizing of the metal core can cause loss of heat energy. This can be reduced by using a material which can be magnetised easily, i.e. iron
3) When a transformer is in operation, there are ac currents in the primary
and secondary coils. The coils have some resistance and the currents
cause heating of the coils, causing some energy to be lost. This loss
may be reduced by using low resistance wire for the coils. This is
most important for the high current winding (the secondary coil of a
step-down transformer). Thick copper wire is used for this winding,
because thick wire of low resistivity has a low resistance. - 32Why is the primary current smaller if the secondary current is switched off?-Vp induces Ip inducing a back emf in the primary coil
- Back emf opposes Vp which means lower Ip
IV - Iℰ = I^2 R
- Ip induces an emf in the secondary coil and if there is a break in the secondary coil, Ip remains same.
- However if S coil is connected, Ip induces Is
- Is induces a magnetic flux in the secondary core and due to lenz's law opposes original magnetic field
- This reduces back emf in primary so reduces opposition to current so Ip is greater.
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- a level Physics: Unit 4 Flashcards on Physics- Unit 4- Chapter 8: Electromagnetic induction, created by Sindoora Jayaprakash on 01/04/2015.
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- Sindoora Jayaprakash
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