WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:07.329 Unit S P 10: Electricity and circuits. 00:00:10.000 --> 00:00:14.842 Atoms contain positive protons, neutral neutrons and negative electrons. 00:00:14.992 --> 00:00:20.218 Metals have mobile delocalised electrons, allowing charge to move through a circuit. 00:00:22.900 --> 00:00:27.475 A complete conducting loop and potential difference are needed for a steady current. 00:00:27.625 --> 00:00:32.415 Opening a switch breaks the circuit; an insulator prevents free charge flow. 00:00:35.067 --> 00:00:40.910 Conventional current travels from the positive supply terminal to the negative through the external circuit. 00:00:41.060 --> 00:00:47.267 Electrons in metal drift in the opposite direction; show conventional current unless asked otherwise. 00:00:49.933 --> 00:00:55.691 Current is the rate of charge flow: I equals Q divided by T or Q equals I times T. 00:00:55.841 --> 00:00:59.749 Current is in amperes, charge in coulombs and time in seconds. 00:01:02.400 --> 00:01:06.657 An ammeter measures current and is connected in series with the component. 00:01:06.807 --> 00:01:11.087 It has very low resistance; do not put it directly across a supply. 00:01:13.767 --> 00:01:23.888 Potential difference (voltage) is the energy transferred for each coulomb of charge passing through a component: V equals E divided by Q or E equals Q times V. 00:01:24.038 --> 00:01:28.147 It is measured in volts; 1 volt equals 1 joule per coulomb. 00:01:30.800 --> 00:01:35.993 A voltmeter measures potential difference and is connected in parallel across a component. 00:01:36.143 --> 00:01:39.066 It has high resistance so draws little current. 00:01:39.216 --> 00:01:42.295 A is in the current path; V spans the resistor. 00:01:46.967 --> 00:01:50.628 Charge is conserved: current is not “used up” by a lamp. 00:01:50.778 --> 00:01:54.211 The charges transfer energy as they pass through components. 00:01:56.867 --> 00:02:03.881 Use standard circuit symbols for cells, batteries, lamps, switches, resistors, diodes, ammeters and voltmeters. 00:02:04.031 --> 00:02:06.772 A longer cell line is the positive terminal. 00:02:06.922 --> 00:02:11.295 Longer cell line denotes positive; a battery has multiple cells. 00:02:15.967 --> 00:02:21.214 A series circuit has one route for current; the same current passes through every component. 00:02:21.364 --> 00:02:23.625 Breaking any part stops the whole loop. 00:02:26.300 --> 00:02:30.891 In series, supply voltage equals the sum of voltages across components. 00:02:31.041 --> 00:02:35.202 Equal components may share equally; different components need not. 00:02:37.867 --> 00:02:42.784 Series resistances add: R total equals R one plus R two, and so on. 00:02:42.934 --> 00:02:46.431 Adding a resistor at fixed supply voltage decreases current. 00:02:49.100 --> 00:02:52.310 A parallel circuit has branches joined at junctions. 00:02:52.460 --> 00:02:58.571 Total current into a junction equals total current out; it splits between branches and rejoins. 00:02:58.721 --> 00:03:01.824 Parallel branches share the same supply voltage. 00:03:06.500 --> 00:03:11.460 Each branch connected across the supply has the same potential difference as the supply. 00:03:11.610 --> 00:03:15.847 Components within one branch can still share that voltage in series. 00:03:18.500 --> 00:03:25.805 With a fixed supply voltage, adding a parallel branch increases total current and decreases total circuit resistance. 00:03:25.955 --> 00:03:28.451 It provides an extra conducting route. 00:03:31.133 --> 00:03:35.933 A failed lamp in one parallel branch need not stop current in the other branches. 00:03:36.083 --> 00:03:42.088 Household appliances are connected in parallel so each receives the supply voltage independently. 00:03:44.767 --> 00:03:50.857 Resistance relates voltage to current: V equals I times R and R equals V divided by I. 00:03:51.007 --> 00:03:53.226 Resistance is measured in ohms. 00:03:53.376 --> 00:03:56.330 Use the voltage and current for the same component. 00:03:59.000 --> 00:04:05.867 An ohmic resistor obeys Ohm’s law: at constant temperature, current is directly proportional to voltage. 00:04:06.017 --> 00:04:08.151 Doubling voltage doubles current. 00:04:08.301 --> 00:04:12.071 Its current, voltage graph is a straight line through the origin. 00:04:12.221 --> 00:04:16.711 A linear current voltage relation corresponds to constant resistance. 00:04:21.367 --> 00:04:27.361 Resistance of a metal wire generally increases with length and decreases with cross-sectional area. 00:04:27.511 --> 00:04:30.177 Material and temperature also affect it. 00:04:32.833 --> 00:04:36.022 Heating a metal makes its ions vibrate more strongly. 00:04:36.172 --> 00:04:41.377 Moving electrons collide more with this vibrating lattice, so resistance increases. 00:04:41.527 --> 00:04:44.969 Current then rises less quickly as voltage increases. 00:04:47.633 --> 00:04:51.671 A filament lamp's current, voltage graph curves as its filament heats. 00:04:51.821 --> 00:04:56.759 Current continues to increase with voltage; it does not suddenly become constant. 00:04:56.909 --> 00:04:59.789 Its resistance increases at higher temperature. 00:04:59.939 --> 00:05:04.511 At higher voltage, current rises less steeply; it does not become fixed. 00:05:09.167 --> 00:05:15.565 A diode lets current flow mainly in one direction (the forward direction), once the voltage is high enough. 00:05:15.715 --> 00:05:20.823 In the reverse direction, the current is very small under normal operating conditions. 00:05:20.973 --> 00:05:24.486 The precise forward-voltage region depends on the diode. 00:05:29.167 --> 00:05:33.587 An L D R's resistance decreases as light intensity increases. 00:05:33.737 --> 00:05:36.425 It can be used in a light-sensitive circuit. 00:05:39.100 --> 00:05:44.273 A typical N T C thermistor's resistance decreases as temperature increases. 00:05:44.423 --> 00:05:47.727 State the type: not every thermistor behaves this way. 00:05:50.400 --> 00:05:55.967 Use a low-voltage supply, switch, ammeter in series and voltmeter across the test component. 00:05:56.117 --> 00:06:01.077 A variable resistor or adjustable supply changes the potential difference. 00:06:03.733 --> 00:06:10.089 For a resistor, filament lamp and diode, record paired current and voltage readings over a suitable range. 00:06:10.239 --> 00:06:15.890 Reverse supply polarity to investigate negative voltage or current where appropriate. 00:06:18.567 --> 00:06:22.935 Plot current on the vertical axis and voltage on the horizontal axis. 00:06:23.085 --> 00:06:29.494 A resistor gives a straight line, a filament lamp gives a curve, and a diode conducts mainly in one direction. 00:06:32.167 --> 00:06:38.853 To investigate wire resistance, measure a known length between contacts and use R equals V divided by I. 00:06:39.003 --> 00:06:44.273 Vary length while keeping wire material, thickness and temperature as constant as possible. 00:06:46.933 --> 00:06:51.343 Use low currents and switch off between readings to limit unwanted heating. 00:06:51.493 --> 00:06:56.075 Keep connections secure, repeat readings and use suitable meter ranges. 00:06:58.733 --> 00:07:08.833 Do not infer resistance from an arbitrary tangent gradient of a non-ohmic current voltage graph: the resistance at an operating point is V divided by I there. 00:07:11.500 --> 00:07:17.099 Moving charges transfer energy to the vibrating ions in a resistive component, heating it. 00:07:17.249 --> 00:07:22.240 This is useful in heaters and kettles, but can be unwanted in cables and chargers. 00:07:24.900 --> 00:07:33.483 Energy transferred equals voltage times current times time, E equals V times I times T, in joules with volts, amperes and seconds. 00:07:33.633 --> 00:07:37.924 This follows from E equals Q times V and Q equals I times T. 00:07:40.600 --> 00:07:45.261 Power equals energy divided by time, P equals E divided by T. 00:07:45.411 --> 00:07:51.906 Electrical power also equals I times V, and for a resistive component P equals I squared times R. 00:07:52.056 --> 00:07:53.912 Power is measured in watts. 00:07:56.567 --> 00:08:04.309 A 12 volts device carrying 2 amperes has power 24 watts and transfers 240 joules in 10 seconds. 00:08:04.459 --> 00:08:09.865 A power rating tells you energy transferred each second, not total energy already used. 00:08:12.533 --> 00:08:20.009 A higher resistance does not always mean a greater heating power: at fixed current P equals I squared times R rises with R; 00:08:20.159 --> 00:08:24.810 at fixed voltage P equals V squared divided by R falls with R. 00:08:27.467 --> 00:08:32.875 Reduce cable heating with appropriate low-resistance material and sufficiently thick wire. 00:08:33.025 --> 00:08:36.945 Do not treat a dangerously hot cable as normal useful heating. 00:08:39.600 --> 00:08:45.796 Cells and batteries supply direct current: charge moves in one direction around the external circuit. 00:08:45.946 --> 00:08:48.549 D C voltage has fixed polarity. 00:08:51.200 --> 00:08:57.289 Alternating current reverses direction repeatedly as the supply voltage changes polarity. 00:08:57.439 --> 00:09:02.019 The UK mains supply is about 230 volts and 50 hertz. 00:09:04.700 --> 00:09:12.271 A frequency of 50 hertz means 50 complete cycles each second; a sinusoidal current reverses direction twice per cycle. 00:09:12.421 --> 00:09:20.163 One cycle lasts 0.02 seconds. 50 hertz means 50 cycles per second, not 50 reversals per second. 00:09:24.833 --> 00:09:33.621 Large domestic appliances use mains electricity; their energy is supplied by a network of generators and transmission or distribution lines. 00:09:33.771 --> 00:09:37.490 Battery devices instead draw from stored chemical energy. 00:09:40.167 --> 00:09:44.922 In a three-core cable, live is brown, neutral is blue and earth is green-and-yellow. 00:09:45.072 --> 00:09:52.643 Live is at about 230 volts relative to earth; neutral is close to earth potential, about 0 volts. 00:09:55.300 --> 00:10:00.931 The live wire supplies alternating potential difference; neutral completes the normal circuit. 00:10:01.081 --> 00:10:06.829 The earth wire is a protective connection to exposed metal casing, normally carrying no current. 00:10:09.500 --> 00:10:19.354 If live touches an earthed metal case, the low-resistance earth path allows a large fault current, causing a fuse or suitable circuit breaker to disconnect the supply. 00:10:19.504 --> 00:10:24.528 The earth connection helps protective disconnection; it does not replace safe wiring. 00:10:29.200 --> 00:10:33.002 A fuse melts if the current is too large, breaking the circuit. 00:10:33.152 --> 00:10:38.751 Choose a rating above normal operating current but suitable for protecting the appliance or cable. 00:10:41.433 --> 00:10:47.682 Circuit breakers disconnect automatically for specified faults and can be reset after the fault is corrected. 00:10:47.832 --> 00:10:53.186 A fuse must be replaced; operating speed depends on the protection device and fault. 00:10:55.867 --> 00:11:02.127 Put switches and fuses in the live connection so opening them disconnects the appliance from live potential. 00:11:02.277 --> 00:11:05.666 Switching only neutral could leave internal parts live. 00:11:08.333 --> 00:11:14.252 Contact between a person and live while connected to earth can drive a dangerous current through the body. 00:11:14.402 --> 00:11:18.460 Neutral is not a reason to treat any mains wiring as safe to touch. 00:11:21.133 --> 00:11:29.515 Double-insulated appliances have insulating cases and protective insulation and do not need an earth connection to an exposed metal case. 00:11:29.665 --> 00:11:34.709 Never experiment with mains circuits; practical work uses low-voltage supplies. 00:11:37.367 --> 00:11:39.861 That completes Electricity and circuits. 00:11:40.011 --> 00:11:43.357 Revisit the notes and test yourself on the revision website.