WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.819 Unit S P 13: Electromagnetic induction. 00:00:10.500 --> 00:00:18.424 A changing magnetic field through a coil or relative motion that cuts magnetic field lines can induce a potential difference. 00:00:18.574 --> 00:00:20.985 This is electromagnetic induction. 00:00:23.667 --> 00:00:27.945 Moving a magnet into or out of a coil changes the field through the coil. 00:00:28.095 --> 00:00:33.182 A sensitive voltmeter connected across the coil can detect an induced voltage. 00:00:33.332 --> 00:00:38.013 The voltmeter measures potential difference, with polarity set by connections. 00:00:42.667 --> 00:00:48.042 If the coil is part of a closed conducting circuit, the induced voltage drives a current. 00:00:48.192 --> 00:00:52.343 An open circuit can have induced voltage but no continuing current. 00:00:55.000 --> 00:01:03.543 A stationary magnet held within a stationary coil does not produce a continuing induced voltage because the field through the coil is not changing. 00:01:06.200 --> 00:01:13.494 Faster magnet movement, a stronger magnet or more coil turns can increase induced voltage when the other factors are fixed. 00:01:16.167 --> 00:01:21.617 Reverse the motion or reverse the magnet's poles to reverse the induced voltage direction. 00:01:21.767 --> 00:01:24.922 Reversing both can keep the direction unchanged. 00:01:27.600 --> 00:01:32.325 The field associated with the induced current opposes the original change. 00:01:32.475 --> 00:01:41.999 An approaching north pole induces a facing north pole that resists approach; a withdrawing north pole induces a facing south pole that resists separation. 00:01:42.149 --> 00:01:45.931 Both induced interactions resist the change that created them. 00:01:50.600 --> 00:01:57.095 This opposition means mechanical work is needed to maintain motion when electrical energy is transferred. 00:01:57.245 --> 00:01:59.698 Induction does not produce free energy. 00:02:02.367 --> 00:02:06.806 Connect a coil to a sensitive centre-zero voltmeter or data logger. 00:02:06.956 --> 00:02:12.587 Move a bar magnet in and out and record size and sign of voltage; compare with holding it still. 00:02:15.267 --> 00:02:20.087 Change one factor at a time: magnet speed, magnet strength or number of turns. 00:02:20.237 --> 00:02:25.357 Keep coil size, movement path, starting positions and other variables consistent. 00:02:28.033 --> 00:02:34.293 For fair comparisons, use repeatable movement and compare the same part of the movement or peak voltage. 00:02:34.443 --> 00:02:38.026 Repeat trials because hand movement gives varying speed. 00:02:40.700 --> 00:02:45.637 Investigate direction by reversing magnet motion or poles individually. 00:02:45.787 --> 00:02:49.923 Label which end of the coil is connected to the positive voltmeter terminal. 00:02:52.600 --> 00:02:59.318 A voltage pulse depends on the rate of field change, not simply how much magnetic material is sitting inside a coil. 00:03:02.000 --> 00:03:07.003 A transformer has a primary coil and a secondary coil linked by a magnetic core. 00:03:07.153 --> 00:03:10.828 The coils are electrically separate in the usual ideal model. 00:03:10.978 --> 00:03:14.505 A changing magnetic field transfers energy between them. 00:03:14.655 --> 00:03:18.773 Coils are magnetically coupled, not joined by an electrical wire. 00:03:23.433 --> 00:03:28.627 An alternating current in the primary creates a changing magnetic field in the core. 00:03:28.777 --> 00:03:33.309 The changing field through the secondary induces an alternating voltage. 00:03:35.967 --> 00:03:39.410 A secondary current flows if a load completes its circuit. 00:03:39.560 --> 00:03:45.415 An unloaded secondary can have voltage with little output power because there is little load current. 00:03:48.067 --> 00:03:54.647 A steady D C input does not provide the continuously changing field needed for normal transformer operation. 00:03:54.797 --> 00:04:00.599 Switching D C on or off can produce brief changes, not a sustained transformed output. 00:04:03.267 --> 00:04:08.621 A step-up transformer increases A C voltage; a step-down transformer decreases it. 00:04:08.771 --> 00:04:14.231 More secondary than primary turns gives step-up behaviour; fewer gives step-down behaviour. 00:04:16.900 --> 00:04:19.947 The core helps the magnetic field link both coils. 00:04:20.097 --> 00:04:28.265 Real transformers transfer some energy to the surroundings, including by heating, so their useful output power is less than their input power. 00:04:30.933 --> 00:04:36.256 For a transformer assumed one hundred percent efficient, input power equals output power. 00:04:36.406 --> 00:04:45.364 Use primary voltage times primary current equals secondary voltage times secondary current, with voltages in volts and currents in amperes. 00:04:48.033 --> 00:04:53.599 A primary of 230 volts and 2 amperes supplies 460 watts. 00:04:53.749 --> 00:05:01.193 If the ideal secondary is 23 volts, its current is 460 divided by 23 equals 20 amperes. 00:05:01.343 --> 00:05:05.083 Lower output voltage permits greater current at the same power. 00:05:09.733 --> 00:05:15.897 Increasing voltage reduces current for the same transferred power; reducing voltage increases current. 00:05:16.047 --> 00:05:22.157 A transformer cannot increase both output voltage and output current at unchanged input power. 00:05:24.833 --> 00:05:29.229 Rearrange the ideal equation to find a missing voltage or current. 00:05:29.379 --> 00:05:36.437 For example, secondary current equals primary voltage times primary current divided by secondary voltage. 00:05:36.587 --> 00:05:40.686 Check whether the question explicitly assumes an ideal transformer. 00:05:43.367 --> 00:05:50.203 If a real transformer has known efficiency, useful output power equals efficiency fraction times input power. 00:05:50.353 --> 00:05:53.901 Account for losses rather than claiming exact equality. 00:05:56.567 --> 00:06:04.021 The National Grid links generation and electricity transmission; local distribution networks deliver energy to consumers. 00:06:04.171 --> 00:06:07.975 Generation includes power stations and renewable sources. 00:06:10.633 --> 00:06:14.831 Step-up transformers raise voltage before long-distance transmission. 00:06:14.981 --> 00:06:18.252 For the same power, higher voltage means smaller current. 00:06:20.933 --> 00:06:24.673 Cable heating loss is power loss equals I squared times R. 00:06:24.823 --> 00:06:31.531 Reducing current greatly reduces heating losses for the same cable resistance, improving transmission efficiency. 00:06:31.681 --> 00:06:35.357 Halving current reduces I squared times R loss to a quarter. 00:06:40.033 --> 00:06:45.397 Step-down transformers reduce voltage for distribution and again near domestic users. 00:06:45.547 --> 00:06:54.559 UK domestic supply is approximately 230 volts A C; high transmission voltages would be unsuitable for direct household use. 00:06:54.709 --> 00:07:00.195 Illustrates the purpose of voltage changes rather than a universal list of grid voltages. 00:07:04.867 --> 00:07:10.477 Some transmission lines operate at hundreds of kilovolts, for example 400 kilovolts. 00:07:10.627 --> 00:07:17.484 Intermediate and generator voltages vary; the source's numbers are examples rather than one compulsory sequence. 00:07:20.167 --> 00:07:25.158 High-voltage equipment requires insulation, separation and controlled access. 00:07:25.308 --> 00:07:29.525 Lower domestic voltage still presents a serious electrical hazard. 00:07:32.200 --> 00:07:35.574 The grid transfers energy; it does not create it. 00:07:35.724 --> 00:07:41.930 Demand, generation and network operation must be managed, with some energy inevitably dissipated. 00:07:44.600 --> 00:07:50.807 (Higher tier) Moving a conductor relative to a magnetic field can produce (induce) a potential difference. 00:07:50.957 --> 00:07:53.539 Current flows if the circuit is complete. 00:07:53.689 --> 00:08:00.097 A stationary conductor in an unchanging magnetic field does not produce a continuing induced output. 00:08:02.767 --> 00:08:09.345 (Higher tier) An alternator rotates a coil in a magnetic field and uses slip rings to maintain connections. 00:08:09.495 --> 00:08:16.694 As the coil moves through successive half-turns, the induced output changes direction, producing alternating current. 00:08:19.367 --> 00:08:26.587 (Higher tier) A simple D C dynamo uses a split-ring commutator to reverse the coil’s connections every half-turn. 00:08:26.737 --> 00:08:30.807 This keeps the current in the external circuit flowing in one direction. 00:08:30.957 --> 00:08:36.065 Its size can still rise and fall (pulsate), rather than staying perfectly constant. 00:08:38.733 --> 00:08:49.665 (Higher tier) In a moving-coil microphone, sound pressure variations move a diaphragm and attached coil in a magnetic field, inducing a varying voltage that represents the sound. 00:08:49.815 --> 00:08:55.509 A loudspeaker reverses the conversion, using current variations to produce motion and sound. 00:09:00.167 --> 00:09:10.267 (Higher tier) A loudspeaker works by the reverse energy transfer: a varying current in a coil produces a varying force in a magnetic field, moving the cone to create sound. 00:09:10.417 --> 00:09:13.520 Headphones use the same principle on a smaller scale. 00:09:16.200 --> 00:09:23.953 (Higher tier) Generators transfer kinetic energy to electrical energy; microphones transfer sound into electrical signals. 00:09:24.103 --> 00:09:30.032 Energy is conserved and friction, electrical resistance and other losses reduce useful output. 00:09:32.700 --> 00:09:35.132 (Higher tier) For an ideal transformer, 00:09:35.282 --> 00:09:48.389 primary voltage divided by secondary voltage equals primary turns divided by secondary turns: primary voltage divided by secondary voltage equals primary turns divided by secondary turns. 00:09:48.539 --> 00:09:53.827 Use alternating input; the changing core field induces the secondary voltage. 00:09:56.500 --> 00:10:05.491 (Higher tier) A step-up transformer has more secondary turns and a higher secondary voltage; a step-down has fewer secondary turns and lower voltage. 00:10:05.641 --> 00:10:09.461 Turns ratio sets the voltage ratio, not the power gain. 00:10:12.133 --> 00:10:20.153 (Higher tier) With ideal power balance, primary voltage times primary current equals secondary voltage times secondary current. 00:10:20.303 --> 00:10:26.713 Raising voltage lowers current for the same transferred power; a transformer does not create extra energy. 00:10:29.367 --> 00:10:34.189 (Higher tier) Power dissipated in a transmission cable is I squared times R. 00:10:34.339 --> 00:10:42.263 For fixed transmitted power and cable resistance, doubling transmission voltage halves current and reduces cable loss to one quarter. 00:10:42.413 --> 00:10:46.890 These relative values assume unchanged transmitted power and resistance. 00:10:51.567 --> 00:10:59.043 (Higher tier) The National Grid uses step-up transformers before long-distance transmission and step-down transformers for consumers. 00:10:59.193 --> 00:11:06.145 High-voltage transmission reduces current-related heating, but requires suitable insulation and safety systems. 00:11:08.800 --> 00:11:14.005 (Higher tier) Real transformers have energy losses, so output power is less than input. 00:11:14.155 --> 00:11:20.585 Use ideal assumptions only when stated, and distinguish useful output power from the heat lost in cables. 00:11:23.267 --> 00:11:25.922 That completes Electromagnetic induction. 00:11:26.072 --> 00:11:29.417 Revisit the notes and test yourself on the revision website.