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23/05/2017 P6 Electricity for Gadgets AGAC Circuit Symbols 23/05/2017 Variable resistor Diode Switch Bulb A V Ammeter Voltmeter LDR Resistor Power Supply Capacitor Thermistor Battery 23/05/2017 Equations for this unit (Foundation) 23/05/2017 Equations For This Unit (Higher) Basic ideas… 23/05/2017 Electric current is when electrons start to flow around a ammeter to measure it and it is circuit. We use an _________ measured in ____. amps Potential difference (also called _______) voltage is how big the push on the electrons is. We use a voltmeter to measure it and it is measured in ________ ______, volts a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. ohms Words: volts, amps, ohms, voltage, ammeter, voltmeter More basic ideas… 23/05/2017 If a battery is added the current will ________ increase because there is a greater _____ push on the electrons so they move ______ faster If a bulb is added the decrease current will _______ because there is resistance in greater ________ the circuit, so the slower electrons move _____ Words – faster, decrease, slower, increase, push, resistance Resistance 23/05/2017 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: Resistance (in ) = Georg Simon Ohm 1789-1854 Voltage (in V) V Current (in A) I is used for current because current was originally referred to as “electrical intensity”. I R An example question: 23/05/2017 Ammeter reads 2A A 1) R = V = 10 = V I 1) Voltmeter reads 10V What is the resistance across this bulb? 5 2 Assuming all the bulbs are the same what is the total resistance in this circuit? 5 x 3 = 15 More examples… 23/05/2017 3A 6V 4 12V 3A 2 2A 4V Difficult!! 2V 1A Difficult! What is the resistance of these bulbs? 2 Resistance 23/05/2017 Resistance is anything that opposes an electric current. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: 1) A bulb with a voltage of 3V and a current of 1A. 3 2) A resistor with a voltage of 12V and a current of 3A 4 3) A diode with a voltage of 240V and a current of 40A 6 4) A thermistor with a current of 0.5A and a voltage of 10V 20 23/05/2017 F 23/05/2017 Current-Voltage graphs for conductors 1) An Ohmic conductor V High resistance 2) A non-Ohmic conductor V Low resistance I In Ohmic conductors the resistance of the conductor stays the same. I In non-Ohmic conductors (like filament bulbs) the resistance of the conductor increases as the current increases (when it gets hot). LDRs and Thermistors 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES Resistance 23/05/2017 2) Thermistor – resistance DECREASES when temperature INCREASES Resistance Amount of light Temperature 23/05/2017 F Divides a value of voltage (1) by the corresponding value of current(1) d 23/05/2017 Divides a value of voltage (1) by the corresponding value of current(1) 23/05/2017 F LDR thermistor capacitor diode Potential Dividers 23/05/2017 VIN R1 VOUT R2 0V 0V The Potential Divider equation: VOUT VIN x (R2) (R1 + R2) Find the output voltage VIN R1 Example R2 0V VOUT 0V VIN x VOUT (R2) (R1 + R2) 50V 100 Q1 100 0V VOUT 0V 25V 23/05/2017 (Vout) 1 Find the output voltage VIN R1 Example R2 0V VOUT VOUT 0V VIN x (R2) (R1 + R2) 3V 200 Q2 100 0V VOUT 1V 0V 23/05/2017 (Vout) 2 Find the output voltage VIN R1 Example R2 0V VOUT 0V VIN x VOUT (R2) (R1 + R2) 12V 5 Q3 15 0V VOUT 9V 0V 23/05/2017 (Vout) 3 Practical applications Here’s a potential divider that is used to control light-activated switches… 23/05/2017 Vin VOUT 0V When the light intensity on the LDR decreases its decrease increase resistance will ________. This causes VOUT to _______ off so the processor and output will probably turn _____. The sensitivity variable resistor can be adjusted to change the ________ of the whole device. Words – decrease, sensitivity, increase, off 23/05/2017 input (2) output 0V Comparing magnets and solenoids 23/05/2017 Magnet: Solenoid: N S Magnetic Field around a currentcarrying wire 23/05/2017 “Right hand corkscrew” or “right hand grip” rule The X is like looking at a dart moving away from you i.e. into the page. This is the direction of the current in the wire. This produces a clockwise magnetic field The dot is like looking at a dart coming towards you i.e. Out of the page. This is the direction of the current in the wire. This produces an anticlockwise magnetic field 23/05/2017 Magnetic field around a rectangular coil This looks like the solenoid seen on a previous slide Revision of DC and AC 23/05/2017 V DC stands for “Direct Current” – the current only flows in one direction: Time 1/50th s AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) 230V T V The Motor Effect 23/05/2017 N S 1) What will happen to this wire? move away 2) How can you make it move faster? stronger magnetic field add battery/ more current 3) How can you make it move in a different direction? reverse current OR reverse magnet 23/05/2017 Current-carrying wire in a magnetic field N Field goes from N to S F = Force Current is coming upwards towards you B= Magnetic field I = Current S Q. Where will this wire go? 23/05/2017 Current-carrying wire in a magnetic field N F = Force B= Magnetic field Q. Where will this wire go? I = Current S 23/05/2017 downwards Electric Motors 23/05/2017 23/05/2017 F 3 Electromagnetic Induction 23/05/2017 N The direction of the induced current is reversed if… 1) The wire is moved in the opposite direction 2) The field is reversed The size of the induced current can be increased by: 1) Increasing the speed of movement 2) Increasing the magnet strength 23/05/2017 Electromagnetic induction The direction of the induced current is reversed if… 1) The magnet is moved in the opposite direction 2) The other pole is inserted first The size of the induced current can be increased by: 1) Increasing the speed of movement 2) Increasing the magnet strength 3) Increasing the number of turns on the coil DC Generators 23/05/2017 Basically, a DC generator is an electric motor in reverse: 1) Remove the battery 2) Turn the coil manually instead. A DC current will be generated in the wires AC Generators Magnetic Field 23/05/2017 Slip rings and brushes S N AC Generators 23/05/2017 Voltage Time Questions on the AC Generator 23/05/2017 1) How does the generator work? Electromagnetic induction 2) How would you increase its output? Give two answers More coils Stronger magnet 3) How would you increase the frequency? Turn it faster 23/05/2017 F increases increases increases stays the same 23/05/2017 increases increases increases stays the same 23/05/2017 Move magnet away from coil or move coil away from magnet Transformers 23/05/2017 Current through primary Time Magnetic field Time Voltage induced in secondary Time Transformers • Look back at the previous slide. • The 2 points to realise are • 1 You only get a voltage if the magnetic field is CHANGING • 2 You get a greater voltage if the magnetic field is changing more quickly. 23/05/2017 Transformers 23/05/2017 step up Transformers are used to _____ __ or step down _______. voltage They only work on AC because an alternating ________ current in the magnetic______. field primary coil causes a constantly alternating _______ This will “_____” induce an alternating current in the secondary coil. Words – alternating, magnetic field, induce, step up, voltage We can work out how much a transformer will step up or step down a voltage: Voltage across primary (Vp) No. of turns on primary (Np) Voltage across secondary (Vs) No. of turns on secondary (Ns) …and the current changes using this formula: VPIP = VSIS Some example questions Primary voltage Vp Secondary voltage Vs No. of turns on primary Np 6V 24V 100 400V 200V 250V ? 23 ? 25V 230V ? 2000 No. of turns on secondary Ns 23/05/2017 Step up or step down? ? 400 Step?up 1000 Step ?down 200 20 Step ?down 150 1500 Step?up 1) A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil? 1:3 2) A current of 0.5A is supplied to a transformer that steps down a voltage from 230V to 23V. What is the current from the secondary 5A coil? Transformers in the National Grid 23/05/2017 Electricity reaches our homes from power stations through the National Grid: Power station Step up transformer Step down transformer Homes If electricity companies transmitted electricity at 230 volts through power loss by the time overhead power lines there would be too much ______ electricity reaches our homes. This is because the current is high ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ step down ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage high and the current and power loss are both ____. low is _____ Words – step down, high, power, low, high F 23/05/2017 No output/zero step-down phone-chargers Vout 240 Vout radio laptops = 200 4000 =12 Any from: low voltage lighting 23/05/2017 Vout 240 = 200 4000 Vout = 12 Any three from: An output voltage is induced in the secondary coil when a changing magnetic field passes through the coil The changing magnetic field is produced by the primary coil DC produces a steady field AC produces a changing field Isolating Transformers 23/05/2017 An isolating transformer is used in some mains circuits (for example, a bathroom shaver _____ socket). Isolating transformers do not change the voltage _____; they simply consist of two ____ coils which have the same _______ number of turns that don’t make contact _____ with each other. This stops the user from getting ___________ electrocuted from the mains supply. Words – electrocuted, shaver, voltage, contact, coils, number Diodes A diode is a device that only allows current to flow in one direction: 23/05/2017 I V The current flows easily in the forwards direction but there is very high resistance to the reverse current. If alternating current is passed through a diode it becomes “half-wave rectified”: V V T T Full-Wave Rectification 1 23/05/2017 A group of 4 diodes can be used to make a “bridge rectifier” circuit to make full-wave rectification: AC supply Load V V T T Full Wave Rectification 2 23/05/2017 The current can only flow through the diode in the direction of the arrow First half of the a.c. wave Second half of the a.c. wave Current flows from positive to negative + AC supply - Load AC supply + Load Notice that the current flows the same way through the load resistor i.e. DC The Capacitor A capacitor is a device that can store charge (it has a “capacity”). Here’s how they work: e 23/05/2017 23/05/2017 Charging and discharging a capacitor P.d. across capacitor P.d. across capacitor Current in circuit Time Time Current in circuit Time Time Smoothing 23/05/2017 Capacitors can be used in “smoothing” circuits to “smooth” out a supply: AC supply Input Output without capacitor Output with capacitor Logic Gates 23/05/2017 Logic gates are the basics behind any kind of processor. There are 5 that you need to know for GCSE: NOT gate: AND gate: OR gate: Logic Gates NAND gate: NOR gate: 23/05/2017 Logic gates NOT gate – “the output is NOT what the input is” AND – “the output is on if A AND B are both on” OR – “the output is on if A OR B are on” 23/05/2017 Input Output 0 1 1 0 Input A Input B Output 0 0 0 0 1 0 1 0 0 1 1 1 Input A Input B Output 0 0 0 0 1 1 1 0 1 1 1 1 Logic gates NAND – “an AND gate and a NOT gate in series” 23/05/2017 Input A Input B Output 0 0 1 0 1 1 1 0 1 1 1 0 TIP: You don’t need to learn this separately. Just learn the AND gate and this gate just has the opposite outputs Logic gates NOR – “a NOR gate and a NOT gate in series” 23/05/2017 Input A Input B Output 0 0 1 0 1 0 1 0 0 1 1 0 TIP: You don’t need to learn this separately. Just learn the OR gate and this gate just has the opposite outputs Some problems to solve 23/05/2017 • The pump on a central heating system is switched on at room temperature if the system is switched on (with the slide switch). When the temperature rises the pump needs to be switched off. • Design a circuit that will sound a buzzer if the temperature of a hot radiator falls during the day ONLY. Include a test switch to check the operation of the buzzer. • Design a circuit for a gardener that will warn them of cold conditions at night. The alarm should be able to be switched off. Using LDRs and Thermistors as inputs 23/05/2017 Thermistors and LDRs can be used as the input to a logic gate: 6V 5V Output When the light intensity on the LDR decreases its resistance will ________. This causes the input to the ____ gate to turn ___ so the output will turn on. The variable resistor can be adjusted to change the ________ of the whole device. Words – AND, sensitivity, increase, on Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J B A 0 0 1 1 B 0 1 0 1 O J depends on A O depends On B and J J O 1 1 0 0 1 1 0 1 Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J O B A 0 0 1 1 B 0 1 0 1 J 1 1 0 0 O 1 1 0 1 Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J B K C A B C 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 J depends on A and B J 0 0 0 0 0 0 1 1 O K depends on C and J K 0 1 0 1 0 1 1 1 O Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J B K O C A B C J K O 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 1 1 0 1 0 1 0 0 0 Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J O B C D K L A B C D 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 J K L O Complex Logic Gate problems 23/05/2017 Work out the truth tables for the following arrangements: A J O B C D K L A B C D J K L O 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 Relays 23/05/2017 Circuits such as those containing logic gates only take very ______ small currents. These circuits may be needed to operate a device that takes a much larger _____ current, e.g. a _________. To do this the circuit would need motor a RELAY switch, a device made of an ____________ electromagnet that can operate a switch. A relay switch is activated by the small current and the switch part is placed in the circuit needing a large current: M Words – motor, larger, electromagnet, small Symbol for relay: Latch Circuits 23/05/2017 A Latch is a switch that stays on after is has been switched on (a burglar alarm, for example). Bistable Latch circuits can be made using NOR gates: Bistable latch circuits work by: input 1) A brief “on” signal at one ____ on signal results in a permanent “___” output at the latch ______ 2) A brief “on” signal at the other off signal at the input causes an “___” latch output 3) An “off” signal at both inputs leaves the latch signal _________. unchanged Words – on, off, unchanged, output, input 23/05/2017 F low high once the alarm starts it stays on (1) or even if the door is shut it is reset s 23/05/2017 0 0 0 1 Any two from: fan needs a large current logic gate only produces a small current relay can use low input to switch large fan current 23/05/2017 decreases Input increases or Goes to 1/high/on Allows the temperature at which the fan comes on to be adjusted