05/09/2015 Electricity for Gadgets W Richards The Weald School OCR Gateway

05/09/2015 P6a Resisting

05/09/2015 Circuit Symbols VA Battery Cell Fuse Resistor LDR Voltmeter Ammeter Variable resistor Diode Switch Bulb Thermistor

05/09/2015 Electric Current – A Definition Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive -+ e-e- e-e- By definition, current is “the rate of flow of charge”

05/09/2015 Basic ideas… Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____. Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta. Resistance is anything that resists an electric current. It is measured in _____. Words: volts, amps, ohms, voltage, ammeter, voltmeter

05/09/2015 More basic ideas… If a battery is added the current will ________ because there is a greater _____ on the electrons so they move ______ If a bulb is added the current will _______ because there is greater ________ in the circuit, so the electrons move _____ Words – faster, decrease, slower, increase, push, resistance

05/09/2015 Using Variable Resistors (Rheostats) V A 1)What will happen to the ammeter reading when the resistance is increased? 2)How will this affect the brightness of the bulb?

05/09/2015 Georg Simon Ohm Resistance 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 = Voltage (in V) (in  )Current (in A) V RI

05/09/2015 An example question: V A 1)What is the resistance across this bulb? 2)Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V Ammeter reads 2A

05/09/2015 More examples… 12V 3A 6V 4V 2A 1A 2V What is the resistance of these bulbs?

05/09/2015Resistance 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. 2)A resistor with a voltage of 12V and a current of 3A 3)A diode with a voltage of 240V and a current of 40A 4)A thermistor with a current of 0.5A and a voltage of 10V

05/09/2015 Current-Voltage graphs for conductors 1) An Ohmic conductor2) A non-Ohmic conductor High resistance Low resistance V I V I In Ohmic conductors the resistance of the conductor stays the same. The gradient gives us the resistance. In non-Ohmic conductors (like filament bulbs) the resistance of the conductor increases as the current increases (when it gets hot).

05/09/2015 Understanding Resistance When a voltage is applied it basically causes electrons to move towards the positive end of the battery: Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. What would happen if we increased the temperature of the metal? IonsElectrons NegativePositive

05/09/2015 P6b Sharing

05/09/2015 Potential Dividers 0V V IN V OUT 0V R1R1 R2R2 (R 1 + R 2 ) V IN x (R 2 ) V OUT The Potential Divider equation:

05/09/2015 Some example questions 0V 12V V OUT 0V 100  0V 1.5V V OUT 0V 50  45  0V 50V V OUT 0V 10  75  0V 3V V OUT 0V 75  25 

05/09/2015 Potential Dividers 0V V IN V OUT 0V R1R1 R2R2 Notice the following: 1)If R 2 is much greater than R 1 then the output voltage is more or less equal to V IN. 2)If R 2 is much less than R 1 then the output voltage is more or less zero. 3)If two variable resistors are used in place of R 1 and R 2 then the output voltage will have an adjustable threshold.

05/09/2015 LDRs and Thermistors

05/09/2015 Two simple components: 2) Thermistor – resistance DECREASES when temperature INCREASES 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES Resistance Amount of light Resistance Temperature

05/09/2015 Using Thermistors and LDRs in circuits V A 1)What will happen to the resistance of the thermistor when it gets hotter? 2)How will this affect the brightness of the bulb and the reading on the ammeter? 3)Try designing your own circuit that warns you when an intruder enters your home, maybe by standing between a source of light and an LDR

05/09/2015 Practical applications 0V V in V OUT Here’s a potential divider that is used to control light-activated switches… When the light intensity on the LDR decreases its resistance will ________. This causes V OUT to _______ so the processor and output will probably turn _____. The variable resistor can be adjusted to change the ________ of the whole device. Words – decrease, sensitivity, increase, off

05/09/2015 Resistors in Series and Parallel The following equations can be used to calculate the total resistance in a circuit: 1 = R T R 1 R 2 R T = R 1 + R 2 For resistors in series: For resistors in parallel:

05/09/2015 Example questions Calculate the equivalent resistance: 1) 3) 2) 4) 10 Ω 40 Ω 20 Ω 100 Ω 20 Ω 100 Ω 50 Ω

05/09/2015 P6c It’s Logical

05/09/2015Computers IBM 5100 portable PC (1975), 25kg, 1.9MHz processor, 64K RAM, 200K internal tape storage, cost up to $20,000 Apple 1 (1976) – 1Mhz processor, 4K RAM, cost $670 Commodore 64 (1982) – 1Mhz processor, 64K RAM, 16 colours, cost $600, 17m sold Apple Macintosh (1984) – 8Mhz processor, 512K RAM, cost $2500 All of these computers (and modern ones) are based on transistors, which have enabled computers to get smaller over the years.

05/09/2015 Computers before Transistors The “Harwell Dekatron” a relay-based computer (first run in 1951):

05/09/2015Transistors A transistor acts like a switch: Base Collector Emitter When a SMALL current flows through the base- emitter part of the transistor a different current is switched on through the collector- emitter part. I e = I b + I c

05/09/2015 Drawing electronic circuits 1) Two resistors in series 4) An ammeter measuring the current through a resistor 3) A voltmeter measuring the voltage across two bulbs in series 2) Two resistors in series with one bulb in parallel V IN 0V V IN 0V V IN 0V V V IN 0V A

05/09/2015 Using a transistor as a switch for an LED 9V 0V Basically, a small current through the base-emitter part of the transistor will “switch on” a larger current through the LED and cause it to emit light. A high resistor is placed here – why?

05/09/2015 Combining Transistors Consider this circuit: Input AInput BOutput Off On Off On Off On The output is only on if A AND B are on. This is called an “AND” gate is a basic logic gate. Other logic gates can be made from transistor combinations and they usually take voltages of 5V (“on”) or 0V (“off”) A B Out 6V

05/09/2015 Logic gate symbols NOT gate: AND gate: OR gate:

05/09/2015 Logic gates Logic gates are the basics behind any kind of processor. Here are the three basic ones: 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” InputOutput Input AInput BOutput Input AInput BOutput

05/09/2015 NAND and NOR gates NAND gate: NOR gate: Input AInput BOutput Input AInput BOutput

05/09/2015 P6d Even More Logical

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: A B J O ABJO

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: A B J O ABJO

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: A B C J KO ABCJKO

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: A B C J KO ABCJKO

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: ABCDJKLO A B C D J KL O

05/09/2015 Complex Logic Gate problems Work out the truth tables for the following arrangements: ABCDJKLO A B C D J KL O

05/09/2015 Using LDRs and Thermistors as inputs Thermistors and LDRs can be used as the input to a logic gate: 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 5V 6V Output

05/09/2015 Some problems to solve 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.

05/09/2015 Using LEDs with Logic Gates LEDs work on low power so they can be used to indicate the status of the output from a logic gate: A B J O

05/09/2015Relays Circuits such as those containing logic gates only take very ______ currents. These circuits may be needed to operate a device that takes a much _____ current, e.g. a _________. To do this the circuit would need a RELAY switch, a device made of an _______ 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: Symbol for relay: Words – motor, larger, electromagnet, small M

05/09/2015 P6e Motoring

05/09/2015 Comparing magnets and solenoids N S Magnet: Solenoid:

05/09/2015 Magnetic Field around a current- carrying wire “Right hand corkscrew” or “right hand grip” rule

05/09/2015 Magnetic field around a rectangular coil

05/09/2015 The Motor Effect S N 1)What will happen to this wire? 2)How can you make it move faster? 3)How can you make it move in a different direction?

05/09/2015 Current-carrying wire in a magnetic field S N F = Force B = Magnetic field I = Current Q. Where will this wire go?

05/09/2015 Revision of DC and AC DC stands for “Direct Current” – the current only flows in one direction: AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz) 1/50 th s 230V V V Time T

05/09/2015 Electric Motors

05/09/2015 Electric Motors The “split ring commutator” changes the current every half rotation (hence the red- blue colour change) Notice the curved poles – these will give a greater magnetic field strength near the coil

05/09/2015 P6f Generating

05/09/2015 Electromagnetic Induction 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

05/09/2015 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

05/09/2015 AC Generators Magnetic Field Slip rings and brushes

05/09/2015 AC Generators SN Voltage Time

05/09/2015 Questions on the AC Generator 1)How does the generator work? 2)How would you increase its output? Give two answers 3)How would you increase the frequency?

05/09/2015 P6g Transforming

05/09/2015Transformers A transformer is basically a device made of two coils of wire around an iron core: The circuit diagram for a transformer:

05/09/2015Transformers Time Current through primary Magnetic field Voltage induced in secondary

05/09/2015Transformers Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” 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 (V p ) No. of turns on secondary (N s )Voltage across secondary (V s ) No. of turns on primary (N p ) …and the current changes using this formula: V P I P = V S I S

05/09/2015 Simple transformer questions Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 12V24V100?? 400V200V20?? 25,000V50,000V1,000?? 23V230V150??

05/09/2015 More example questions Primary voltage V p Secondary voltage V s No. of turns on primary N p No. of turns on secondary N s Step up or step down? 6V24V100?? 400,000V200V?1,000? 25,000V?20,00020? ?230V1501,500? 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? 2)A current of 0.5A is supplied to a transformer that steps down a voltage from 230V to 12V. What is the current from the secondary coil?

05/09/2015 Isolating Transformers Words – electrocuted, shaver, voltage, contact, coils An isolating transformer is used in some mains circuits (for example, a bathroom _____ socket). Isolating transformers do not change the _____; they simply consist of two ______ that don’t make _____ with each other. This stops the user from getting ___________ from the mains supply.

05/09/2015 Transformers in the National Grid Electricity reaches our homes from power stations through the National Grid: If electricity companies transmitted electricity at 240 volts through overhead power lines there would be too much ______ loss by the time electricity reaches our homes. This is because the current is ___. To overcome this they use devices called transformers to “step up” the voltage onto the power lines. They then “____ ____” the voltage at the end of the power lines before it reaches our homes. This way the voltage is _____ and the current and power loss are both ____. Words – step down, high, power, low, high Power station Step up transformer Step down transformer Homes

05/09/2015 Power through the National Grid The following equation can be used to calculate the amount of power loss in the National Grid’s cables: Power = I 2 R 1)A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor. 2)A motor takes a current of 10A. If its resistance is 2.2M Ω calculate the power dissipated by the motor. 3)A 2KW heater has a resistance of 20 Ω. Calculate the current through it.

05/09/2015 Transformers and Power If this transformer is 100% efficient then the power output from the secondary coil is the same as the power input to the primary coil. Power = voltage x current in W in V in A In previous units we came across this equation: If we apply this equation to transformers and assume 100% efficiency we conclude: V p x I p = V s x I s

05/09/2015 Example questions 1)A transformer connected to the mains supply in the UK runs on a voltage of 230V and a current of 0.1A. If its output voltage is 12V what is its output current? 2)Another step down transformer outputs 120V and 2A. If it runs from the mains what is its input current? 3)A step up transformer near a power station would convert 20,000V and 10A into 300,000V and what current? 1.9A 1.04A 0.07A

05/09/2015 P6h Charging

05/09/2015Diodes I V A diode is a device that only allows current to flow in one direction: If alternating current is passed through a diode it becomes “half-wave rectified”: V T V T The current flows easily in the forwards direction but there is very high resistance to the reverse current.

05/09/ HolesElectrons How Diodes work Diodes work by having areas of extra electrons and areas of extra “holes”: When the battery is connected like this the electrons move towards the holes and jump from one hole to another, which makes it seem like the holes are moving.

05/09/2015 Full-Wave Rectification A group of 4 diodes can be used to make a “bridge rectifier” circuit to make full-wave rectification: AC supply Load V T V T

05/09/2015 The Capacitor A capacitor is a device that can store charge (it has a “capacity”). Here’s how they work: e

05/09/2015 Charging and discharging a capacitor P.d. across capacitor Time Current in circuit Time P.d. across capacitor Time Current in circuit

05/09/2015Smoothing Many devices need a constant voltage supplied to them, rather than AC. Capacitors can be used in “smoothing” circuits to “smooth” out a supply: AC supply Input Output without capacitor Output with capacitor