Flashcards
Unit 14: Electromagnetism
Flashcards do conjunto (24)
- 1Describe the magnetic field around a wire carrying a currentThe magnetic field lines around a wire are circles centred on the wire in a plane perpendicular to the wire.
- 2Describe the magnetic field around a solenoid carrying a currentThe magnetic field of a solenoid is uniform inside the solenoid and like that of a bar magnet outside.
- 3State how the strength and direction of the above fields varies with positionIncreasing the current increases the strength of the magnetic fields around a wire and around a solenoid.
Reversing the current reverses the magnetic field lines. - 4Describe applications of the solenoidThe uniform magnetic field inside a solenoid is used in cathode ray TV tubes and 'magnetic resonance' (MR) brain scanners.
The magnetic field outside a solenoid is used in large electromagnets used in scrap yards and small electromagnets used in relays. Their iron core makes the magnetic field much stronger. - 5Describe the motor effect and relate the force to the current and the magnetic fieldthe motor effect = the force exerted on a current-carrying wire when it is in a magnetic field aligned at right angles (or at any non-zero angle) to the magnetic field lines.
In the motor effect, the force is increased if the current or the strength of the magnetic field is increased. - 6Describe an experiment to show the effect of a magnetic field on an electron beamThe force on each electron is at right angles to the direction of the magnetic field and to the direction in which the electron is moving.
- 7Explain why a current-carrying coil in a magnetic field turnsSee picture.
When a current passes round the coil, a force acts as shown on each of the two long sides of the coil. As these forces are in opposite directions, they act to make the coil turn.
- 8Describe how an electric motor worksThe simple motor in the picture consists of a rectangular coil of insulated wire (the armature coil) that is forced to rotate. The coil is connected via two metal or graphite 'brushes' to the battery. The brushes press onto a metal 'split ing' commutator fixed to the coil.
When a coil is passed through the coil, the coil spins because:
> a force acts on each side of the coil due to the motor effect
> the force on one side is in the opposite direction to the force on the other side.
The split ring commutator reverses the current round the coil every half-turn of the coil. Because the sides also swap over each half-turn, the coil is pushed in the same direction every half-turn.
- 9Recall that cathode rays are electrons and describe their production by means of thermionic emissionCathode rays = electrons in a discharge tube or a vacuum tube attracted towards the anode
Thermionic emission = process in which free electrons in a heated metal filament in a vacuum are emitted from the filament
A beam of electrons can be produced by attracting electrons from a heated filament towards a positive electrode in a vacuum tube.
- 10Describe the effect of an electric field on cathode raysA beam of cathode rays can be deflected by an electric field.
The tube in the picture contains two deflecting plates P and Q. When one plate is made positive and the other negative, the beam of electrons from the anode is attracted to the positive plate (P) and is repelled by the negative plate (Q). The electrons are deflected towards plate P.
- 11Describe the structure and action of a cathode ray oscilloscopeA cathode ray oscilloscope is an instrument used to display electrical signals on a screen. An electron beam passes between two pairs of deflecting plates and hits the screen at the end of the tube, producing a spot of light.
See picture:
The Y-deflecting plates receive a signal that makes the spot move vertically.
The X-deflecting plates receive a signal that makes the spot move horizontally.
- 12Use an oscilloscope to display waveforms and describe such use> Connect a low voltage ac supply unit to an oscilloscope.
> The trace on the oscilloscope screen shows that the pd increases and decreases continuously.
> The highest pd is reached at each peak. Increasing the pd of the ac supply makes the waves on the screen taller.
> Increasing the frequency of the ac supply increases the number of cycles you see on the screen. - 13Describe how to induce emf in a circuitA generator contains coils of wire that spin in a magnetic field. A pd is created/ induced in the wire when it cuts across the magnetic field lines. We refer to this source of pd as an induced emf/ induced voltage. If the wire is part of a complete circuit, the induced emf makes an electric current pass round the circuit.
- 14State the factors that affect the magnitude of the induced emfThe current is increased if the wire moves faster or a stonger magnet is used.
- 15Recall that the direction of an induced current opposes the change that it causesThe direction of an induced current opposes the change that causes it.
- 16Describe a simple alternating current generator and explain the use of the slip ringsThe simple ac generator consists of a coil that spins in a uniform magnetic field.
The slip rings and brush enable the coil to stay connected to the external circuit.
- 17Sketch a graph of the variation of the induced emf of an ac generator with time and relate the position of the generator coil in the magnetic field to above graphThe peak value of the induced emf is when the sides of the coil cut directly across the magnetic field lines.
- 18Describe the construction of a transformerA transformer consists of a primary coil and a secondary coil wrapped on the same iron core.
- 19Recall and use the transformer equation relating voltage ratio to the turns ratioprimary voltage / secondary voltage = no. of turns on primary / no. of turns on secondary
- 20Describe the principle of operation of the transformerTransformers only work using alternating current.
The alternating current in the primary coil creates an alternating magnetic field in the iron core which induces an alternating voltage in the secondary coil. - 21Describe the use of the transformer in high-voltage transmission of electricityTransformers are used to step voltages up or down in the electricity grid.
- 22State the advantages of high-voltage transmissionThe higher the grid voltage, the greater the efficiency of transferring electrical power through the grid.
- 23Explain why energy loss in cables are lower at high voltage than at low voltageThe grid voltage is at least 132,000 V. If the grid voltage was much lower, much more current would be needed to deliver the same amount of power. The grid cables would therefore heat up more and waste more power.
- 24Recall and use the power equation relating the current and pd in each coil for a transformer that is 100% efficientprimary current x primary voltage = secondary current x secondary voltage
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- GCSE Physics (Unit 14: Electromagnetism) FlashCards sobre Unit 14: Electromagnetism, criado por cmharrisuk em 19-04-2015.
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