Electromagnetic effects PowerPoint Presentation

A teachers tool for igcse physics

1. ELECTROMAGNETIC EFFECTS PHYSICS YEAR 11

2. objectives By the end of this session, learner should be able to: Describe the pattern of the magnetic field(including direction) due to currents in straight wires and in solenoids Describe the effect on the magnetic field of changing the magnitude and direction of the current

3. Key terms Solenoid Conductor Right-hand grip rule Oesterd’s experiment Magnetic field Motors and generators. Transformers. Relays. Electric bells and buzzers. Loudspeakers and headphones. Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks. MRI machines. Maglev trains

4. Oersted’s EXPERIMENT In 1819 Oersted accidentally discovered the magnetic effect of an electric current. Introduction video. Magnetic effects of electric current Class 10.mp4

5. Field due to a straight wire When a current flows through a conductor, a magnetic field is created around the conductor. The magnetic field around a straight wire is made up of concentric circles perpendicular to the wire, with the wire in the centre . You can see this by placing a compass near a wire that is carrying a current. As you move the compass, it will trace the direction of the magnetic field. The direction of the field is given by the right-hand grip rule: If a straight conductor carrying current is held in the right hand such that the thumb is pointed in the direction of the current, then the direction in which your fingers encircle the wire gives the direction of the magnetic lines of force around the wire.

6. Field due to a straight wire The closer you get to the current-carrying wire, the stronger the magnetic field is. This means that closer to the current-carrying wire, the magnetic field lines are closer together Increasing the size (magnitude) of the current makes the magnetic field stronger. Changing the direction of the current changes the direction of the magnetic field — use the right-hand grip rule to work out which way it goes.

7. A solenoid If you wrap a current-carrying wire into a coil, it is called a solenoid. Inside a solenoid, the magnetic field lines around each loop of wire line up with each other. This results in lots of straight field lines pointing in the same direction, therefore, the magnetic field inside the solenoid is strong and uniform. Outside the solenoid, the magnetic field is just like the one round a bar magnet. This means one end of the solenoid has a north pole and the other end has a south pole.

8. Right-hand grip rule

9. Variation of magnetic field strength Magnetic field strength around a current-carrying straight wire becomes less as the distance from the wire increases. Field lines becoming further apart. When the current through the wire is increased, the strength of the magnetic field around the wire increases and the field lines become closer together. When the direction of the current changes, the magnetic field acts in the opposite direction. The magnetic field is stronger inside a solenoid than outside it. When the direction of the current changes in the solenoid, the magnetic field acts in the opposite direction. The field inside a solenoid can be made very strong if it has a large number of turns or a large current.

10. Force on a current-carrying conductor OBJECTIVES By the end of this session, learner should be able to: Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing: the current the direction of the field State and use the relative directions of force, field and current

11. Key terms Fleming's left-hand rule

12. introduction Passing an electric current through a wire produces a magnetic field around the wire. If you put that wire into a magnetic field, the two fields interact and a force is exerted on the wire. The motor effect A wire carrying a current in a magnetic field experiences a force. If the wire can move, it does so.

13. DEMONSTRATION https://youtu.be/hYKPDxdEcT8?si=xvXzgeaKDbYtlpw_ When a current-carrying wire (or any other conductor) is put in a magnetic field, the wire experiences a force. This can cause the wire to move and is called the motor effect. To experience the full force, the wire must be 90° to the magnetic field. If the wire runs parallel to the magnetic field, it won’t experience any force. At angles in between, it will experience some force. The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire. Reversing the direction of the current reverses the direction of the force. Reversing the direction of the magnetic field (switching the positions of the north and south poles) also causes the direction of the force to reverse.

14. Fleming’s Left-Hand Rule You can find the direction of the force with Fleming’s left-hand rule. Using your left hand, point your First finger in the direction of the Field. Point your se Cond finger in the direction of the Current. Your thu Mb will then point in the direction of the force ( Motion).

15. The d.c. motor OBJECTIVES By the end of this lesson learners should be able to: State that a current-carrying coil in a magnetic field experiences a turning effect and that the effect is increased by: increasing the number of turns on the coil increasing the current increasing the strength of the magnetic field Relate this turning effect to the action of an electric motor including the action of a split-ring commutator

16. A Current-Carrying Coil of Wire Rotates in a Magnetic Field Electric motors use the motor effect to get them moving (and to keep them moving). A current-carrying conductor in a magnetic field experiences a force. If you twist a current-carrying wire into a coil and put it into a magnetic field, the forces cause a turning effect. The turning effect on the coil is increased by: Increasing the current in the coil Increasing the number of turns on the coil Increasing the strength of the magnetic field The larger the turning effect on the coil, the faster it will turn.

17. Simple d.c. electric motor

18. Simple d.c. electric motor A simple d.c. motor consists of a rectangular coil of wire mounted on an axle that can rotate between the poles of a C-shaped magnet. Each end of the coil is connected to half of a split ring of copper, called a split-ring commutator, which rotates with the coil. Two carbon blocks, the brushes, are pressed lightly against the commutator by springs. The brushes are connected to an electrical supply. If Fleming’s left-hand rule is applied to the coil, we find that side ab experiences an upward force and side cd a downward force. (No forces act on ad and bc since they are parallel to the field.) These two forces produce a turning effect which rotates the coil in a clockwise direction until it is vertical.

19. Simple d.c. electric motor The brushes are then in line with the gaps in the commutator and the current stops. However, because of its inertia, the coil overshoots the vertical and the commutator halves change contact from one brush to the other. The current through the coil is reversed and so also the directions of the forces on the sides. Side ab is on the right now, acted on by a downward force, while cd is on the left with an upward force. The coil thus carries on rotating clockwise.

20. quiz How would the turning effect on a current carrying coil in a magnetic field change if the size of the magnetic field is increased the direction of the magnetic field is reversed? In the simple d.c. electric motor of Figure 4.5.29, the coil rotates anticlockwise as seen by the eye from the position X when current flows in the coil. Is the current flowing clockwise or anticlockwise around the coil when viewed from above?

21. Electromagnetic induction OBJECTIVES By the end of the lesson, learners should be able to: Show understanding that a conductor moving across a magnetic field or a changing magnetic field linking with a conductor can induce an e.m.f. in the conductor. State the factors affecting the magnitude of an induced e.m.f.

22. EM INDUCTION Electromagnetic induction is the creation of an e.m.f. (and a current if there’s a complete circuit) in a conductor that is experiencing a change in a magnetic field or which is moving relative to a magnetic field. Key terms Induction Coil

23. Em induction The effect of producing electricity from magnetism was discovered in 1831 by Faraday and is called electromagnetic induction. It led to the construction of generators for producing electrical energy in power stations.

24. Electromagnetic induction experiments The effect of producing electricity from magnetism was discovered in 1831 by Faraday and is called electromagnetic induction. It led to the construction of generators for producing electrical energy in power stations. Two ways of investigating electromagnetic induction follow: Straight wire and U-shaped magnet Bar magnet and coil

25. Straight wire and U-shaped magnet First the wire is held at rest between the poles of the magnet. It is then moved in each of the six directions shown in Figure 4.5.1 and the meter is observed. Only when it is moving upwards (direction 1) or downwards (direction 2) is there a deflection on the meter, indicating an induced current in the wire. The deflection is in opposite directions in these two cases and only lasts while the wire is in motion.

26. Bar magnet and coil The magnet is pushed into the coil, one pole first (Figure 4.5.2), then held still inside it. It is then withdrawn. The meter shows that current is induced in the coil in one direction as the magnet is moved in and in the opposite direction as it is moved out. There is no deflection when the magnet is at rest. The results are the same if the coil is moved instead of the magnet, i.e. only relative motion is needed. This experiment indicates that an e.m.f. is induced in a conductor when it is linked by a changing magnetic field or when it moves across a magnetic field.

27. Factors affecting the magnitude of an induced e.m.f. E.m.f. increases with increases of the speed of motion of the magnet or coil, the number of turns on the coil, the strength of the magnet

28. a.c . generator OBJECTIVES By the end of this lesson, learners should be able to: Distinguish between direct current ( d.c ) and alternating current ( a.c ) Describe and explain the operation of a rotating coil generator and the use of slip rings Sketch a graph of voltage output against time for a simple a.c . generator

29. a.c . generator Key Terms Slip rings A.c . D.c. Frquency

30. Alternating current vs Direct current Alternating current ( a.c .) is a current that constantly changes direction. Alternating current flows back and forth around a circuit — it reverses its direction many times each second. It is produced by an alternating potential difference in which the positive and negative keep switching. Direct current ( d.c. ) is a current that always flows in the same direction. Direct current is created by a direct potential difference. Cells and batteries supply d.c. .

31. Simple a.c . generator

32. Simple a.c . generator The simplest alternating current ( a.c .) generator ( alternator) consists of a rectangular coil between the poles of a C-shaped magnet (Figure 4.5.6a). The ends of the coil are joined to two slip rings on the axle and against which carbon brushes press. When the coil is rotated it cuts the field lines and an e.m.f. is induced in it. Figure 4.5.6b shows how the e.m.f. varies over one complete rotation. As the coil moves through the vertical position with ab uppermost , ab and cd are moving along the lines ( bc and da do so always), and no cutting occurs. The induced e.m.f. is zero.

33. Simple a.c . generator

34. Simple a.c . generator During the first quarter rotation the e.m.f. increases to a maximum when the coil is horizontal. Sides ab and dc are then cutting the lines at the greatest rate. In the second quarter rotation the e.m.f. decreases again and is zero when the coil is vertical with dc uppermost . After this, the direction of the e.m.f. reverses because, during the next half rotation, the motion of ab is directed upwards and dc downwards . The frequency of an a.c . is the number of complete cycles it makes each second and is measured in hertz (Hz), i.e. 1 cycle per second = 1 Hz. If the coil rotates twice per second, the a.c . has a frequency of 2Hz. The mains supply is a.c . of frequency 50Hz.

35. quiz Which feature of the rotating coil of an a.c . generator allows the induced e.m.f. to be connected to fixed contacts? a Sketch the output of an a.c . generator against time. b At what position of the coil in an a.c . generator is the output a maximum zero?

36. Starter quiz 2/3/2024

37. Starter quiz 2/3/2024

38. Transformer OBJECTIVES By the end of this lesson, learners should be able to: Describe the construction of a basic transformer with a soft-iron core, as used for voltage transformations Describe the principle of operation of a transformer Use the terms step-up and step-down Recall and use the equation( V p / V s ) = ( N p / N s ) (for 100% efficiency) Describe the use of the transformer in high-voltage transmission of electricity Recall and use the equation I p V p = I s V s (for 100% efficiency) Explain why power losses in cables are lower when the voltage is high

39. Transformer Transformers are used to change the size of the voltage of an alternating electricity supply. They consist of two coils wound around a core made from soft iron. An input voltage is applied across a primary coil. This produces an output voltage across a secondary coil. The primary and secondary coils, can either be on top of one another (Figure a) or on separate limbs (Figure b).

40. Transformer There are two types of transformer, step-up and step-down:

41. Mutual induction When the current in a coil is switched on or off or changed in a simple iron-cored transformer, a voltage is induced in a neighbouring coil. The effect, called mutual induction, is an example of electromagnetic induction . When an alternating voltage is applied across the primary coil, the current that flows induces a changing magnetic field in the iron core. Because the core is made from soft iron, it magnetises and demagnetises quickly. This changing magnetic field induces an alternating voltage in the secondary coil. a.c . because a changing magnetic field is needed to induce a voltage.

42. The Transformer Equation The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils. You can calculate the output voltage of a transformer from the input voltage and the number of turns on each coil. =  

43. The Transformer symbols

44. quiz The main function of a step-down transformer is to: decrease current decrease voltage change a.c . to d.c. change d.c. to a.c . A transformer has 1000 turns on the primary coil. The voltage applied to the primary coil is 230 V a.c . How many turns are on the secondary coil if the output voltage is 46 V a.c .? 20 2000 200 4000

45. EXAM STYLE quiz

46. Transmission of electrical power

47. Transmission of electrical power The National Grid is a network of cables, mostly supported on pylons, that connects all the power stations in a country to consumers. The huge amounts of power are transmitted at a very high voltage. To get the electricity to this voltage, step-up transformers are used between power stations and the transmission cables. The voltage is then decreased to safe, usable levels once the electricity has reached consumers. For this, step-down transformers are used. As the voltage is stepped up, the power remains at the same high level but the current decreases.

48. Advantages of high-voltage transmission of electricity Reducing the amount of thermal energy lost in the transmission cables Allowing wires with small cross-sectional areas to be used; are cheaper and easier to handle than the thicker wires required to carry large currents.

49. Energy losses in a transformer Transformers are almost 100% efficient. This means you can assume that the input power equals the output power. Power is given by P = IV . So input power = output power can also be written as:

50. example A transformer has a potential difference of 15 V and a current of 10 A in its secondary coil. The current in the transformer’s primary coil is 25 A. Assuming the transformer is 100% efficient, calculate the potential difference across the transformer’s primary coil.

51. quiz A transformer steps down the mains supply from 230 V to 10 V to operate an answering machine. What is the turns ratio, of the transformer windings? How many turns are on the primary if the secondary has 100 turns? What is the current in the primary if the transformer is 100% efficient and the current in the answering machine is 2 A? A transformer is 100% efficient. The current in the primary is 0.05A when the p.d. is 240 V. Calculate the current in the secondary where the p.d. is 12 V.  

52. EXAM STYLE quiz