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Electricity for Gadgets

Electricity for Gadgets. OCR Gateway. W Richards. P6a Resisting. Circuit Symbols. 20/12/2019. Variable resistor. A. V. Diode. Switch. Bulb. Ammeter. Voltmeter. LDR. Resistor. Cell. Fuse. Thermistor. Battery. Electric Current – A Definition. 20/12/2019.

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Electricity for Gadgets

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  1. Electricity for Gadgets OCR Gateway W Richards

  2. P6a Resisting

  3. Circuit Symbols 20/12/2019 Variable resistor A V Diode Switch Bulb Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

  4. Electric Current – A Definition 20/12/2019 Note that electrons go from negative to positive + - e- e- Electric current is a flow of negatively charged particles (i.e. electrons). By definition, current is “the rate of flow of charge”

  5. 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

  6. 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

  7. Using Variable Resistors (Rheostats) A V • What will happen to the ammeter reading when the resistance is increased? • How will this affect the brightness of the bulb?

  8. Resistance Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. Georg Simon Ohm 1789-1854 V Resistance = Voltage (in V) (in ) Current (in A) I R The resistance of a component can be calculated using Ohm’s Law:

  9. An example question: Ammeter reads 2A A V Voltmeter reads 10V • What is the resistance across this bulb? • Assuming all the bulbs are the same what is the total resistance in this circuit?

  10. More examples… 3A 3A 2A 4V 2V 1A 6V 12V What is the resistance of these bulbs?

  11. Resistance Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) Resistance is anything that opposes an electric current. What is the resistance of the following: • A bulb with a voltage of 3V and a current of 1A. • A resistor with a voltage of 12V and a current of 3A • A diode with a voltage of 240V and a current of 40A • A thermistor with a current of 0.5A and a voltage of 10V

  12. Current-Voltage graphs for conductors V V High resistance Low resistance I I 1) An Ohmic conductor 2) A non-Ohmic conductor 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).

  13. Understanding Resistance Electrons Ions When a voltage is applied it basically causes electrons to move towards the positive end of the battery: Negative Positive 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?

  14. P6b Sharing

  15. Potential Dividers VIN R1 VOUT R2 0V 0V The Potential Divider equation: (R2) VOUT VIN x (R1 + R2)

  16. Some example questions 12V 3V 1.5V 50V 100  10  50  75  VOUT VOUT VOUT VOUT 75  45  100  25  0V 0V 0V 0V 0V 0V 0V 0V

  17. Potential Dividers VIN R1 VOUT R2 0V 0V Notice the following: • If R2 is much greater than R1 then the output voltage is more or less equal to VIN. • If R2 is much less than R1 then the output voltage is more or less zero. • If two variable resistors are used in place of R1 and R2 then the output voltage will have an adjustable threshold.

  18. LDRs and Thermistors 20/12/2019

  19. Two simple components: 20/12/2019 Resistance Resistance Amount of light Temperature 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES

  20. Using Thermistors and LDRs in circuits A V • What will happen to the resistance of the thermistor when it gets hotter? • How will this affect the brightness of the bulb and the reading on the ammeter? • 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

  21. Practical applications Vin VOUT 0V 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 VOUT 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

  22. Resistors in Series and Parallel 1 = 1 + 1 RT R1 R2 The following equations can be used to calculate the total resistance in a circuit: For resistors in series: RT = R1 + R2 For resistors in parallel:

  23. Example questions Calculate the equivalent resistance: 40Ω 1) 10Ω 2) 20Ω 10Ω 20Ω 100Ω 50Ω 100Ω 3) 4) 20Ω 100Ω 50Ω

  24. P6c It’s Logical

  25. Computers Apple 1 (1976) – 1Mhz processor, 4K RAM, cost $670 IBM 5100 portable PC (1975), 25kg, 1.9MHz processor, 64K RAM, 200K internal tape storage, cost up to $20,000 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.

  26. Computers before Transistors The “Harwell Dekatron” a relay-based computer (first run in 1951):

  27. Transistors A transistor acts like a switch: Collector When a SMALL current flows through the base-emitter part of the transistor a different current is switched on through the collector-emitter part. Base Emitter Ie = Ib + Ic

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

  29. Using a transistor as a switch for an LED A high resistor is placed here – why? 9V 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. 0V

  30. Combining Transistors 6V A B Out Consider this circuit: Off Off 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”)

  31. Logic gate symbols NOT gate: AND gate: OR gate:

  32. Logic gates NOT gate – “the output is NOT what the input is” Logic gates are the basics behind any kind of processor. Here are the three basic ones: AND – “the output is on if A AND B are both on” OR – “the output is on if A OR B are on”

  33. NAND and NOR gates NAND gate: NOR gate:

  34. P6d Even More Logical

  35. Complex Logic Gate problems A J O B Work out the truth tables for the following arrangements:

  36. Complex Logic Gate problems A J O B Work out the truth tables for the following arrangements:

  37. Complex Logic Gate problems A J B K O C Work out the truth tables for the following arrangements:

  38. Complex Logic Gate problems A J B K O C Work out the truth tables for the following arrangements:

  39. Complex Logic Gate problems A J O B K L C D Work out the truth tables for the following arrangements:

  40. Complex Logic Gate problems A J O B K L C D Work out the truth tables for the following arrangements:

  41. Using LDRs and Thermistors as inputs 5V 6V Output 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

  42. 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.

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

  44. Relays M Symbol for relay: 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: Words – motor, larger, electromagnet, small

  45. P6e Motoring

  46. Comparing magnets and solenoids N S Magnet: Solenoid:

  47. Magnetic Field around a current-carrying wire “Right hand corkscrew” or “right hand grip” rule

  48. Magnetic field around a rectangular coil

  49. The Motor Effect N S • What will happen to this wire? • How can you make it move faster? • How can you make it move in a different direction?

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

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