WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:07.403 Unit C P 9: Electricity and circuits. 00:00:10.067 --> 00:00:14.909 Atoms contain positive protons, neutral neutrons and negative electrons. 00:00:15.059 --> 00:00:20.285 Metals have mobile delocalised electrons, allowing charge to move through a circuit. 00:00:22.967 --> 00:00:27.541 A complete conducting loop and potential difference are needed for a steady current. 00:00:27.691 --> 00:00:32.482 Opening a switch breaks the circuit; an insulator prevents free charge flow. 00:00:35.133 --> 00:00:40.977 Conventional current travels from the positive supply terminal to the negative through the external circuit. 00:00:41.127 --> 00:00:47.334 Electrons in metal drift in the opposite direction; show conventional current unless asked otherwise. 00:00:50.000 --> 00:00:55.758 Current is the rate of charge flow: I equals Q divided by T or Q equals I times T. 00:00:55.908 --> 00:00:59.815 Current is in amperes, charge in coulombs and time in seconds. 00:01:02.467 --> 00:01:06.724 An ammeter measures current and is connected in series with the component. 00:01:06.874 --> 00:01:11.154 It has very low resistance; do not put it directly across a supply. 00:01:13.833 --> 00:01:23.955 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. 00:01:24.105 --> 00:01:28.214 It is measured in volts; 1 volt equals 1 joule per coulomb. 00:01:30.867 --> 00:01:36.060 A voltmeter measures potential difference and is connected in parallel across a component. 00:01:36.210 --> 00:01:39.133 It has high resistance so draws little current. 00:01:39.283 --> 00:01:42.362 A is in the current path; V spans the resistor. 00:01:47.033 --> 00:01:50.695 Charge is conserved: current is not “used up” by a lamp. 00:01:50.845 --> 00:01:54.278 The charges transfer energy as they pass through components. 00:01:56.933 --> 00:02:03.948 Use standard circuit symbols for cells, batteries, lamps, switches, resistors, diodes, ammeters and voltmeters. 00:02:04.098 --> 00:02:06.839 A longer cell line is the positive terminal. 00:02:06.989 --> 00:02:11.361 Longer cell line denotes positive; a battery has multiple cells. 00:02:16.033 --> 00:02:21.281 A series circuit has one route for current; the same current passes through every component. 00:02:21.431 --> 00:02:23.692 Breaking any part stops the whole loop. 00:02:26.367 --> 00:02:30.958 In series, supply voltage equals the sum of voltages across components. 00:02:31.108 --> 00:02:35.269 Equal components may share equally; different components need not. 00:02:37.933 --> 00:02:42.851 Series resistances add: R total equals R one plus R two, and so on. 00:02:43.001 --> 00:02:46.498 Adding a resistor at fixed supply voltage decreases current. 00:02:49.167 --> 00:02:52.377 A parallel circuit has branches joined at junctions. 00:02:52.527 --> 00:02:58.637 Total current into a junction equals total current out; it splits between branches and rejoins. 00:02:58.787 --> 00:03:01.891 Parallel branches share the same supply voltage. 00:03:06.567 --> 00:03:11.527 Each branch connected across the supply has the same potential difference as the supply. 00:03:11.677 --> 00:03:15.914 Components within one branch can still share that voltage in series. 00:03:18.567 --> 00:03:25.872 With a fixed supply voltage, adding a parallel branch increases total current and decreases total circuit resistance. 00:03:26.022 --> 00:03:28.518 It provides an extra conducting route. 00:03:31.200 --> 00:03:35.999 A failed lamp in one parallel branch need not stop current in the other branches. 00:03:36.149 --> 00:03:42.155 Household appliances are connected in parallel so each receives the supply voltage independently. 00:03:44.833 --> 00:03:50.924 Resistance relates voltage to current: V equals I times R and R equals V divided by I. 00:03:51.074 --> 00:03:53.293 Resistance is measured in ohms. 00:03:53.443 --> 00:03:56.397 Use the voltage and current for the same component. 00:03:59.067 --> 00:04:05.934 An ohmic resistor obeys Ohm’s law: at constant temperature, current is directly proportional to voltage. 00:04:06.084 --> 00:04:08.217 Doubling voltage doubles current. 00:04:08.367 --> 00:04:12.138 Its current, voltage graph is a straight line through the origin. 00:04:12.288 --> 00:04:16.777 A linear current voltage relation corresponds to constant resistance. 00:04:21.433 --> 00:04:27.427 Resistance of a metal wire generally increases with length and decreases with cross-sectional area. 00:04:27.577 --> 00:04:30.244 Material and temperature also affect it. 00:04:32.900 --> 00:04:36.089 Heating a metal makes its ions vibrate more strongly. 00:04:36.239 --> 00:04:41.443 Moving electrons collide more with this vibrating lattice, so resistance increases. 00:04:41.593 --> 00:04:45.035 Current then rises less quickly as voltage increases. 00:04:47.700 --> 00:04:51.738 A filament lamp's current, voltage graph curves as its filament heats. 00:04:51.888 --> 00:04:56.825 Current continues to increase with voltage; it does not suddenly become constant. 00:04:56.975 --> 00:04:59.855 Its resistance increases at higher temperature. 00:05:00.005 --> 00:05:04.577 At higher voltage, current rises less steeply; it does not become fixed. 00:05:09.233 --> 00:05:15.631 A diode lets current flow mainly in one direction (the forward direction), once the voltage is high enough. 00:05:15.781 --> 00:05:20.889 In the reverse direction, the current is very small under normal operating conditions. 00:05:21.039 --> 00:05:24.553 The precise forward-voltage region depends on the diode. 00:05:29.233 --> 00:05:33.654 An L D R's resistance decreases as light intensity increases. 00:05:33.804 --> 00:05:36.492 It can be used in a light-sensitive circuit. 00:05:39.167 --> 00:05:44.340 A typical N T C thermistor's resistance decreases as temperature increases. 00:05:44.490 --> 00:05:47.794 State the type: not every thermistor behaves this way. 00:05:50.467 --> 00:05:56.034 Use a low-voltage supply, switch, ammeter in series and voltmeter across the test component. 00:05:56.184 --> 00:06:01.144 A variable resistor or adjustable supply changes the potential difference. 00:06:03.800 --> 00:06:10.155 For a resistor, filament lamp and diode, record paired current and voltage readings over a suitable range. 00:06:10.305 --> 00:06:15.957 Reverse supply polarity to investigate negative voltage or current where appropriate. 00:06:18.633 --> 00:06:23.001 Plot current on the vertical axis and voltage on the horizontal axis. 00:06:23.151 --> 00:06:29.561 A resistor gives a straight line, a filament lamp gives a curve, and a diode conducts mainly in one direction. 00:06:32.233 --> 00:06:38.920 To investigate wire resistance, measure a known length between contacts and use R equals V divided by I. 00:06:39.070 --> 00:06:44.340 Vary length while keeping wire material, thickness and temperature as constant as possible. 00:06:47.000 --> 00:06:51.409 Use low currents and switch off between readings to limit unwanted heating. 00:06:51.559 --> 00:06:56.141 Keep connections secure, repeat readings and use suitable meter ranges. 00:06:58.800 --> 00:07:08.900 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. 00:07:11.567 --> 00:07:17.165 Moving charges transfer energy to the vibrating ions in a resistive component, heating it. 00:07:17.315 --> 00:07:22.307 This is useful in heaters and kettles, but can be unwanted in cables and chargers. 00:07:24.967 --> 00:07:33.549 Energy transferred equals voltage times current times time, E equals V times I times T, in joules with volts, amperes and seconds. 00:07:33.699 --> 00:07:37.991 This follows from E equals Q times V and Q equals I times T. 00:07:40.667 --> 00:07:45.328 Power equals energy divided by time, P equals E divided by T. 00:07:45.478 --> 00:07:51.973 Electrical power also equals I times V, and for a resistive component P equals I squared times R. 00:07:52.123 --> 00:07:53.979 Power is measured in watts. 00:07:56.633 --> 00:08:04.375 A 12 volts device carrying 2 amperes has power 24 watts and transfers 240 joules in 10 seconds. 00:08:04.525 --> 00:08:09.932 A power rating tells you energy transferred each second, not total energy already used. 00:08:12.600 --> 00:08:20.075 A higher resistance does not always mean a greater heating power: at fixed current P equals I squared times R rises with R; 00:08:20.225 --> 00:08:24.877 at fixed voltage P equals V squared divided by R falls with R. 00:08:27.533 --> 00:08:32.941 Reduce cable heating with appropriate low-resistance material and sufficiently thick wire. 00:08:33.091 --> 00:08:37.011 Do not treat a dangerously hot cable as normal useful heating. 00:08:39.667 --> 00:08:45.863 Cells and batteries supply direct current: charge moves in one direction around the external circuit. 00:08:46.013 --> 00:08:48.615 D C voltage has fixed polarity. 00:08:51.267 --> 00:08:57.356 Alternating current reverses direction repeatedly as the supply voltage changes polarity. 00:08:57.506 --> 00:09:02.086 The UK mains supply is about 230 volts and 50 hertz. 00:09:04.767 --> 00:09:12.338 A frequency of 50 hertz means 50 complete cycles each second; a sinusoidal current reverses direction twice per cycle. 00:09:12.488 --> 00:09:20.230 One cycle lasts 0.02 seconds. 50 hertz means 50 cycles per second, not 50 reversals per second. 00:09:24.900 --> 00:09:33.687 Large domestic appliances use mains electricity; their energy is supplied by a network of generators and transmission or distribution lines. 00:09:33.837 --> 00:09:37.557 Battery devices instead draw from stored chemical energy. 00:09:40.233 --> 00:09:44.989 In a three-core cable, live is brown, neutral is blue and earth is green-and-yellow. 00:09:45.139 --> 00:09:52.710 Live is at about 230 volts relative to earth; neutral is close to earth potential, about 0 volts. 00:09:55.367 --> 00:10:00.998 The live wire supplies alternating potential difference; neutral completes the normal circuit. 00:10:01.148 --> 00:10:06.896 The earth wire is a protective connection to exposed metal casing, normally carrying no current. 00:10:09.567 --> 00:10:19.421 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. 00:10:19.571 --> 00:10:24.595 The earth connection helps protective disconnection; it does not replace safe wiring. 00:10:29.267 --> 00:10:33.069 A fuse melts if the current is too large, breaking the circuit. 00:10:33.219 --> 00:10:38.817 Choose a rating above normal operating current but suitable for protecting the appliance or cable. 00:10:41.500 --> 00:10:47.749 Circuit breakers disconnect automatically for specified faults and can be reset after the fault is corrected. 00:10:47.899 --> 00:10:53.253 A fuse must be replaced; operating speed depends on the protection device and fault. 00:10:55.933 --> 00:11:02.193 Put switches and fuses in the live connection so opening them disconnects the appliance from live potential. 00:11:02.343 --> 00:11:05.733 Switching only neutral could leave internal parts live. 00:11:08.400 --> 00:11:14.319 Contact between a person and live while connected to earth can drive a dangerous current through the body. 00:11:14.469 --> 00:11:18.527 Neutral is not a reason to treat any mains wiring as safe to touch. 00:11:21.200 --> 00:11:29.582 Double-insulated appliances have insulating cases and protective insulation and do not need an earth connection to an exposed metal case. 00:11:29.732 --> 00:11:34.776 Never experiment with mains circuits; practical work uses low-voltage supplies. 00:11:37.433 --> 00:11:39.928 That completes Electricity and circuits. 00:11:40.078 --> 00:11:43.423 Revisit the notes and test yourself on the revision website.