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The law of magnets Like poles repel unlike poles attract TRIPLE ONLY.

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Presentation on theme: "The law of magnets Like poles repel unlike poles attract TRIPLE ONLY."— Presentation transcript:

1 The law of magnets Like poles repel unlike poles attract TRIPLE ONLY

2 Magnetic fields A magnetic field is a volume of space where magnetic force is exerted. All magnets are surrounded by magnetic fields. The shape of a magnetic field can be shown by iron filings or plotting compasses.

3 Magnetic field around a bar magnet magnetic field line Arrows 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.

4 Magnetic fields between two bar magnets

5 The Earth’s magnetic field The earth’s magnetic field is similar in shape to that around a bar magnet. It is thought to be caused by electric currents flowing through the molten outer core of the Earth. At the present the field pattern is like that with a magnetic SOUTH pole situated somewhere below northern Greenland

6 Electromagnetism In 1820 Hans Ørsted noticed that a wire carrying an electric current caused a compass needle to deflect. No current, compass points to north Current, compass deflected

7 Magnetic field patterns around wires 1. Straight wire TRIPLE ONLY The magnetic field 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.

8 The right-hand grip rule (for fields) TRIPLE ONLY Grip the wire with the RIGHT hand. The thumb is placed in the direction of the electric current. The fingers show the direction of the circular magnetic field.

9 3. Solenoid A solenoid is 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 TRIPLE ONLY N S

10 The right-hand grip rule (for poles) TRIPLE ONLY Grip the coil with 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

11 Electromagnets An electromagnet consists of a current carrying coil wrapped around an iron core. TRIPLE ONLY

12 Uses of electromagnets 1. Scrap yard crane 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. TRIPLE ONLY

13 Charge deflection by a magnetic field Electric charges are deflected by magnetic fields provided they are not travelling parallel to the field lines. Positive and negative charges are deflected in opposite directions. TRIPLE ONLY S N + -

14 The motor effect When a current 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 + - Motor effect Motor effect - Fendt

15 The force increases if: – 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. Motor effect Motor effect - Fendt

16 Fleming’s left-hand motor rule Note: Magnetic field direction is from NORTH to SOUTH Current direction is from PLUS to MINUS Motor effect Motor effect - Fendt

17 Insert the missing information Note:means current out of the page means current into the page NSSN N S Q1. Force direction ? Q2 Current direction ? Q3 N and S poles ? Q4 Force directions ? NS Motor effect Motor effect - Fendt

18 The electric motor Electric current flowing 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. Electric motor Electric motor - Fendt

19 N + S 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 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 Electric motor Electric motor - Fendt

20 The loudspeaker The sound signal 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.

21 Electromagnetic induction If an electrical conductor 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. Generator Generator - Fendt

22 If a magnet is 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. Generator Generator - Fendt

23 The size of the 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. Generator Generator - Fendt

24 Alternating Current Generators Most electricity is produced using the ‘generator effect’. The simplest generators and the types used in power stations produce alternating current (A.C.) Generator Generator - Fendt

25 Moving Coil A.C. Generator Generator Generator - Fendt

26 Generator Generator - Fendt

27 This like an electric motor in reverse. As the coil is rotated electromagnetic induction occurs. An alternating voltage is induced in the coil. An alternating current is drawn off 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. Generator Generator - Fendt

28 Bicycle generator When the wheel 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. Generator Generator - Fendt

29 Question 1 The graph opposite shows how 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.

30 Question 2 Choose appropriate words to 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 alternatingreversed magnetic generator complete induction movement greaterinduced alternating reversed

31 The transformer A transformer is a device that is used to change one alternating voltage level to another. TransformerTransformer - eChalk circuit symbol TRIPLE ONLY

32 Structure of a transformer A transformer consists of at least two coils of wire wrapped around a laminated iron core. TransformerTransformer - eChalk laminated iron core PRIMARY VOLTAGE V p PRIMARY COIL of N p turns SECONDARY COIL of N s turns SECONDARY VOLTAGE V s TRIPLE ONLY

33 How a transformer works When an alternating voltage, V p is applied to the primary coil of N p turns it causes an alternating to flow in this coil. This current causes a changing magnetic field in the laminated iron core which cuts across the secondary coil of N s turns. Electromagnetic induction occurs in this coil which produces an alternating voltage, V s. TransformerTransformer - eChalk TRIPLE ONLY

34 Question Why can a transformer not change the level of the voltage output of a battery? –A battery produces a steady (DC) voltage. –This voltage would cause a constant direct current in the primary coil of a transformer. –This current would produce an unchanging magnetic field in the iron core. –This unchanging magnetic field would NOT cause electromagnetic induction in the secondary coil. –There would therefore be no secondary voltage. TRIPLE ONLY

35 The transformer equation The voltages or potential differences across the primary and secondary coils of a transformer are related by the equation: primary voltage = primary turns secondary voltage secondary turns V p = N p VsNsVsNs TransformerTransformer - eChalk TRIPLE ONLY

36 Question 1 Calculate the secondary voltage of a transformer that has a primary coil of 1200 turns and a secondary of 150 turns if the primary is supplied with 230V. V p = N p V s N s 230 / V s = 1200 / / V s = = 8 x V s 230 / 8 = V s Secondary voltage = 28.8 V TransformerTransformer - eChalk TRIPLE ONLY

37 Question 2 Calculate the number of turns required for the primary coil of a transformer if secondary has 400 turns and the primary voltage is stepped up from 12V to a secondary voltage of 48V. V p = N p V s N s 12 / 48 = N p / = N p / x 400 = N p Primary has 100 turns TransformerTransformer - eChalk TRIPLE ONLY

38 Complete: PRIMARYSECONDARY VoltageTurnsVoltageTurns 230 V V V50046 V V V800 9 V12072 V960 Answers V V Transformer - eChalk TRIPLE ONLY

39 Transformer power transfer equation If a transformer is 100% efficient then the power input to the primary coil is equalled by the power output from the secondary coil. as power = current x voltage then: I p x V p = I s x V s TRIPLE ONLY

40 Question 1 Calculate the primary current if when a transformer is supplied with 230V the secondary provides 4A at a voltage of 13V. Assume that the transformer is 100% efficient. I p x V p = I s x V s I p x 230V = 4A x 13V I p = 52 / 230 Primary current = A TRIPLE ONLY

41 Question 2 Calculate the secondary current from a transformer supplying a secondary voltage of 6V if the primary is supplied with a current of 0.20A at 230V. Assume that the transformer is 100% efficient. I p x V p = I s x V s 0.2A x 230V = I s x 6V I s = 46 / 6 Secondary current = 7.67 A TRIPLE ONLY

42 Complete: PRIMARYSECONDARY NpVp IpIp NsVp IsIs V0.4 A3010V8 A 10012V8 A V0.2 A 30072V0.4 A5012V2.4 A 5025V10 A250125V2 A Answers TRIPLE ONLY

43 Step-up transformers In a step-up transformer the voltage across the secondary coil is greater than the voltage across the primary coil. The secondary turns must be greater than the primary turns. Use: To increase the voltage output from a power station from 25 kV ( V) to up to 400 kV. TransformerTransformer - eChalk TRIPLE ONLY

44 Step-down transformers In a step-down transformer the voltage across the secondary coil is smaller than the voltage across the primary coil. The secondary turns must be smaller than the primary turns. Use: To decrease the voltage output from the mains supply from 230V to 18V to power and recharge a lap-top computer. TransformerTransformer - eChalk TRIPLE ONLY

45 Transformers and the National Grid The National Grid is the system of cables used to deliver electrical power from power stations to consumers. The higher the voltage used, the greater is the efficiency of energy transmission. Lower voltages result in higher electric currents and greater energy loss to heat due to the resistance of the cables. TRIPLE ONLY

46 At power stations the output voltage of the generators is stepped up by transformers from 25kV to 132kV. The voltage may be further increased to up to 400 kV for transmission over long distance pylon lines. TRIPLE ONLY

47 The voltage is reduced in stages by step-down transformers to different levels for different types of consumer. The lowest level is 230V for domestic use. The final step- down transformer will be at sub station within a few hundred metres of each group of houses. TRIPLE ONLY

48 Question 1 Why is electrical energy transmitted over the National Grid in the form of alternating current? –To maximise efficiency high voltages must be used. –Voltage therefore needs to be changed in level. –Transformers are needed to change voltage levels. –Transformers only work with alternating current. TRIPLE ONLY

49 Question 2 Choose appropriate words to fill in the gaps below: Transformers are used to change one ___________ voltage level to another. They do not work with ____________current. Step-up transformers _________ the voltage because their ___________ coil has more turns than the primary. Transformers are used in the __________ Grid. The _______ output of a power station is increased to up to _______. A high voltage reduces the ________ lost to heat due to the _________ of the power lines. alternating 400 kVincrease energysecondary 25 kV WORD SELECTION: directNationalresistance alternating 400 kV increase energy secondary 25 kV direct National resistance TRIPLE ONLY

50 Electromagnetism Simulations Motor effect - Fendt Electric motor - Fendt Faraday Electromagnetic Lab – PhET Play with a bar magnet and coils to learn about Faraday's law. Move a bar magnet near one or two coils to make a light bulb glow. View the magnetic field lines. A meter shows the direction and magnitude of the current. View the magnetic field lines or use a meter to show the direction and magnitude of the current. You can also play with electromagnets, generators and transformers! Faraday's Law - PhET - Light a light bulb by waving a magnet. This demonstration of Faraday's Law shows you how to reduce your power bill at the expense of your grocery bill. Generator - Fendt Transformer - load can be changed but not turns ration - netfirms Transformer - eChalk

51 Electric Motors and Electromagnetic Induction Notes questions from pages 187 to (a) What is the motor effect? (b) What factors determine the size of the force exerted on a conductor in a magnetic field? (c) With the aid of a diagram show how Fleming’s left-hand rule can be used to find the direction of the force on a conductor. 2.Copy figures 22.5 and 22.6 and explain how a moving coil loudspeaker and electric motor work. 3.(a) Draw diagrams and explain what is meant by ‘electromagnetic induction’? (b) What factors determine the size of the voltage produced? 4.Copy figure and use it to explain how a simple generator works. 5.Copy figure and use it to explain how a transformer works. 6.Copy the two transformer equations on pages 193 and 194 and find the secondary current and voltage for a 100% efficient transformer that has a primary coil of 800 turns supplied with 2A at 40V if the secondary coil has 100 turns. 7.Explain what is meant by step-up and step-down transformers and how they are used in the UK’s National Grid system. 1.Answer the questions on pages 195 and Verify that you can do all of the items listed in the end of chapter checklist on page 195. TRIPLE ONLY

52 Electric Motors and Electromagnetic Induction Notes questions from pages 187 to (a) What is the motor effect? (b) What factors determine the size of the force exerted on a conductor in a magnetic field? (c) With the aid of a diagram show how Fleming’s left-hand rule can be used to find the direction of the force on a conductor. 2.Copy figures 22.5 and 22.6 and explain how a moving coil loudspeaker and electric motor work. 3.(a) Draw diagrams and explain what is meant by ‘electromagnetic induction’? (b) What factors determine the size of the voltage produced? 4.Copy figure and use it to explain how a simple generator works. 1.Answer questions 1, 2 and 3 on pages 195 and 196. DOUBLE SCIENCE ONLY


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