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Magnetism and Electromagnetism.pptx
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- 2. Outline • Magnetism andElectromagnetism • Electric Motors and Electromagnetic Induction
- 4. What is Magnet? Ifa material is a magnetic, it has the ability to exert forces on magnets or other magnetic materials
- 5. Poles of amagnet • Magnet have a North and South Pole. • Like poles repel. • Unlike poles attract. • What happens if you break a magnet in half?
- 7. Properties of Magnets •Magnets have two opposite poles --North --South • Magnets exert forces on each other • The forces depend on the alignment对 齐 of the poles
- 8. Properties of Magnets •Plastics, wood ,and most insulating materials are virtually transparent to magnetic forces. • Conducting metals, like aluminum, also allow magnetic forces to pass through, but may change the forces.
- 9. The Magnetic Field Allmagnets create a magnetic field in the space around them, and the magnetic field creates forces on other magnets
- 10. Magnetic field lineArrows on the field lines show the direction of the force on a free to move north pole The stronger the magnetic field the denser the magnetic field lines. The Magnetic Field
- 11. The Magnetic Fieldsbetween two bar magnets
- 12. Magnetism can be inducedin a magnetic material if it is placed within a magnetic field. iron bar Induced magnetism
- 13. Hard vs. softmagnetic materials Permanent magnets are made of magnetically hard materials such as steel. These materials retain their magnetisation once magnetised. Magnetically soft materials, such as iron, lose their magnetisation easily. They suitable for temporary magnet such as electromagnets.
- 14. Choose appropriate wordsto fill in the gaps below: Magnetic materials are either hard or ______. Hard magnetic materials such as ______ retain their magnetisation once magnetised. A magnetic _____ is a region where the magnetic force is greatest. Magnetic poles always occur in ______. Like poles _______, unlike attract. A magnetic ______ is a region where magnetic force is exerted. The ________ of the magnetic field around a bar magnet is from north to south. soft repel field steel direction pole pairs WORD SELECTION: soft repel field steel direction pole pairs
- 16. In 1820 HansØrsted noticed that a wire carrying an electric current caused a compass needle to deflect. No current, compass points to north Current, compass deflected Electromagnetism电磁
- 17. • The magneticfield consists of concentric circles centred on the wire. • The magnetic field is strongest near the wire. • This is shown by the field lines being closest together near to the wire. • The strength of the field increases if the electric current is increased. Magnetic field patterns around wires 1. Straight wire
- 18. Grab the wirewith the RIGHT hand. The thumb is placed in the direction of the electric current. The fingers show the direction of the circular magnetic field. The right-hand rule (for fields)
- 19. Complete the diagramsbelow: Electric current out of the page Electric current into the page Add field arrows Add field arrows Add current direction Add current direction Practice
- 20. Plan view Magnetic fieldpatterns around wires 2. Flat circular coil 平面圆形线圈
- 21. • A solenoidis a coil of wire carrying an electric current. • The magnetic field is similar in shape to that around a bar magnet. • The strength of the field increases with: 1. the electric current 2. the number of turns in the coil N S Magnetic field patterns around wires 3. Solenoid螺线管
- 22. Grab the coilwith the RIGHT hand. The fingers are placed in the direction that the eclectic current flows around the coil. The thumb points towards the north pole end of the coil. N S The right-hand grip rule (for poles)
- 23. Complete the diagramsbelow: 1. Locate north 4. Add coils 3. Add current direction 2. Locate south N S N Practice
- 24. An electromagnet consistsof a current carrying coil wrapped around an iron core. Electromagnets 电磁铁
- 25. 1. Scrap yardcrane 废料场起重机 • The iron core of the electromagnet is a SOFT magnetic material. • When current flows the iron becomes strongly magnetised and so picks up the scrap iron and steel. • When the current is turned off the iron loses its magnetisation and so releases the scrap. Uses of electromagnets
- 26. 2. The electricbell When the push switch is closed current flows around the circuit turning on the electromagnet. The soft iron armature is pulled towards the electromagnet and the hammer hits the gong. This causes the contact switch to open cutting off the electric current. The spring now pulls the armature back again closing the contact switch. Current now flows again and the hammer hits the gong again. gong hammer electromagnet push switch spring contact switch soft iron armature Uses of electromagnets
- 27. Label the diagramof the electric bell below: 1 2 3 4 5 6 Contact switch 7 8 Practice
- 28. Choose appropriate wordsto fill in the gaps below: A wire carrying an electric ______ produces a magnetic field. This field increases in ________ if the current is increased. A ________ is a coil of wire carrying an electric current. The field produced increases in strength if the number of _____ in the coil is increased or if _____ is placed inside the coil. An ____________ consists of a coil of a solenoid wrapped around an iron core. Iron is a ______ magnetic material that loses its magnetisation once the current in the coil is switched off. solenoid strength iron soft turns current electromagnet WORD SELECTION: solenoid strength iron soft turns current electromagnet
- 30. Electric charges aredeflected by magnetic fields provided they are not travelling parallel to the field lines. Positive and negative charges are deflected in opposite directions. S N + - Charge deflection by a magnetic field
- 31. + - + - + - When acurrent carrying conductor carrying an electric current is placed in a magnetic field, it will experience a force provided that the conductor is not placed parallel to the field lines. This is called the motor effect. S N + - The Motor Effect
- 32. The force increasesif: – the strength of the magnetic field is increased – the current is increased The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed. The Motor Effect
- 33. Note: Magnetic field directionis from NORTH to SOUTH Current direction is from PLUS to MINUS Fleming’s left-hand motor rule
- 34. Insert the missinginformation Note: means current out of the page means current into the page N S S N N S Q1. Force direction ? Q2 Current direction ? Q3 N and S poles ? Q4 Force directions ? N S Practice
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- 37. • Electric currentflowing around the coil of the electric motor produces oppositely directed forces on each side of the coil. • These forces cause the coil to rotate. • Every half revolution the split ring commutator causes the current in the coil to reverse otherwise the coil would stop in the vertical position. The electric motor
- 38. N + S Brushes lose contactwith the split ring commutator. Current no longer flows through the motor coil. The coil will continue to rotate clockwise due to its momentum. Brushes in contact with the split ring commutator. Current flows through the motor coil. Forces exert a clockwise turning effect on the coil Brushes regain contact with the split ring commutator. Current flows through the motor coil but in the opposite direction. Forces exert a clockwise turning effect on the coil. Brushes lose contact with the split ring commutator. Current no longer flows through the motor coil. The coil will continue to rotate clockwise due to its momentum. Brushes regain contact with the split ring commutator. Current flows through the motor coil but in the original direction. Forces exert a clockwise turning effect on the coil. split-ring commutator contact brush rotation axis
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- 40. The loudspeaker The soundsignal consists of an alternating current supplied by the amplifier. This current flows through the coil of the loudspeaker. Due to the motor effect, the magnetic field around the coil causes the coil to vibrate in step with the alternating current. The coil causes the diaphragm (speaker cone) to vibrate in step with the original sound signal. The diaphragm causes air to vibrate and so produces a sound wave.
- 41. Question Choose appropriate wordsto fill in the gaps below: The motor effect occurs when a _______ carrying wire is placed inside a ________ field. The force exerted is __________ when the wire is at 90° to the magnetic field __________ but is zero if the wire is ________ to the field. The force increases with _________ or current strength, the force __________ in direction if either are reversed. Applications include the electric motor and ___________. magnetic parallel maximum direction loudspeaker current WORD SELECTION: reverses magnetic parallel maximum direction loudspeaker current field reverses field
- 43. What is e.m.f e.m.f. isan abbreviation for electromotive force. When charge flows through a cell it is given energy by the cell. The number of joules of energy given to each coulomb of charge that passes through the cell is the e.m.f. of the cell Recall 当电荷流过电池时，电池给它能量。
- 44. Show understanding thata conductor moving across a magnetic field or a changing magnetic field linking with a conductor can induce an e.m.f. in the conductor Basically …….. A piece of wire + N S A magnetic field = V A small e.m.f is generated Movement The production of an e.m.f. by moving a wire in a magnetic field is known as electromagnetic induction.
- 45. If an electricalconductor cuts through magnetic field lines, a voltage is induced across the ends of the conductor. If the wire is part of a complete circuit, a current is induced in the wire. This is called electromagnetic induction and is sometimes called the generator effect. Electromagnetic Induction电磁感应 如果电导体穿过磁力线，导线两端就会感应电压。如果导线是完整电路的一部分，导 线中就会感应电流。这被称为电磁感应，有时也被称为发电机效应。
- 46. If a magnetis moved into a coil of wire, a voltage is induced across the ends of the coil. If the direction of motion, or the polarity of the magnet, is reversed, then the direction of the induced voltage and the induced current are also reversed. Electromagnetic induction also occurs if the magnetic field is stationary and the coil is moved. Electromagnetic Induction电磁感应 如果把磁体移动到线圈中，线圈两端就会感应电压。如果磁体的运动方向或极性相 反，那么感应电压和感应电流的方向也相反。如果磁场是静止的而线圈是移动的， 也会产生电磁感应。
- 47. The size ofthe induced voltage increases when: – the speed of the movement increases – the strength of the magnetic field increases – the number of turns on the coil increases – the area of the coil is greater. Electromagnetic Induction电磁感应
- 48. Alternating Current Generators Mostelectricity is produced using the ‘generator effect’. The simplest generators and the types used in power stations produce alternating current (A.C.)
- 49. Moving Coil A.C.Generator
- 51. This like anelectric motor in reverse. As the coil is rotated electromagnetic induction occurs. An alternating voltage is induced in the coil. An alternating current is moved out through two slip rings. The faster the coil is rotated: - the greater is the amplitude of the voltage and current - the higher is the frequency of the a.c.
- 52. Bicycle generator When thewheel turns the magnet is made to rotate next to the fixed coil of wire. Electromagnetic induction occurs and a alternating voltage is induced in the coil. This causes an alternating current to flow to the light bulb of the bicycle.
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- 54. Practice The graph opposite showshow the voltage of a generator varies in time. Using the same set of axes show how the voltage would vary if the rotational speed of the generator was doubled. V time The new voltage will have TWICE the amplitude AND frequency of the original.
- 55. Choose appropriate wordsto fill in the gaps below: The _________ effect occurs when a conductor is moved relative to a ____________ field. This is also known as electromagnetic ___________. The greater the relative __________ of the conductor and magnetic field the _______ is the voltage ________. If the conductor is part of a ________ circuit an electric current will flow. ___________ current is produced if the direction of movement is continually _________. magnetic generator complete induction movement greater WORD SELECTION: induced alternating reversed magnetic generator complete induction movement greater induced alternating reversed
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