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Understanding Static Electricity and Electrical Resistance

Explore the concept of static electricity and resistance in this informative guide. Learn about static experiments, uses of electrostatics, defibrillators, and electrical safety precautions. Understand the principles of electrical current and resistance, including Ohm's Law and practical examples. Discover the importance of fuses, circuit breakers, and earth wires in maintaining electrical safety in appliances.

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Understanding Static Electricity and Electrical Resistance

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  1. Radiation for Life OCR Additional Science W Richards

  2. P4a: Sparks

  3. Static Electricity 08/01/2020 An introduction – click here

  4. Static Electricity + - - + Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + + - - + + + - - - + + - + - -

  5. Short Static Experiments 08/01/2020 Try the following quick static electricity experiments: • Rubbing a balloon on your jumper and “sticking” it to the wall • Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap • Charging a plastic rod and trying to pick up small pieces of paper (or someone else’s hair!) with it • Rubbing a balloon on someone else’s head – you might want to ask their permission first… Can you explain what you saw in each of these experiments?

  6. Static Electricity - - + - - + + - - - - -

  7. Static Electricity in Lightning 08/01/2020 e- e- e- e-

  8. Van de Graaf generators 08/01/2020 08/01/2020 When a charge is neutralised by the movement of electrons either from the Earth or to the Earth we call this “earthing”

  9. Dangers of Static – fuelling lines 08/01/2020

  10. P4b: Uses of Electrostatics

  11. Using Static in Paint Sprayers 08/01/2020 Connected to negative voltage Connected to positive voltage • Why is the paint sprayer given a negative charge? • Why is the car given a positive charge?

  12. Uses of Static – Smoke Precipitators Chimney Negatively charged plates - - - - - - Positively charged grid + + +

  13. Uses of Static - Defibrillators How do defibrillators work?

  14. P4c: Safe Electricals

  15. Electric Current 08/01/2020 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” and it is larger through shorter, wider wires

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

  17. 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?

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

  19. Resistance 08/01/2020 08/01/2020 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 3 4 6 20

  20. Wiring a plug 1. 4. 5. 2. 6. 3. Earth wire Live wire Fuse Neutral wire Cable grip Insulation The neutral wire of a plug is used to complete the circuit. The Earth wire of a plug keeps the device safe by stopping the appliance becoming “live”

  21. Fuses Fuses are _______ devices. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______ and protect the appliance and user from further _____. Words – large, harm, safety, melt, live, circuit, fuse

  22. Circuit breakers Residual Current Circuit Breakers (RCCBs) are often used with fuses. They have some advantages over fuses: • They are safer – they don’t get hot • They react more quickly • They can be switched off for repairs • They are easy to reset • Each RCCB is attached to a certain circuit, so if one switches off you can see which circuit has a fault

  23. Earth wires Earth wires are always used if an appliance has a _____ case. If there is a _____ in the appliance, causing the live wire to ______ the case, the current “_______” down the earth wire and the ______ blows. Earth wires are not needed if a device is “double insulated”. Words – fuse, fault, metal, surges, touch

  24. Power and fuses 08/01/2020 P V I Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: Power = voltage x current in W in V in A Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

  25. Power and fuses 08/01/2020 Copy and complete the following table:

  26. Power and fuses 08/01/2020 Copy and complete the following table:

  27. P4d: Ultrasound

  28. Waves- Some definitions… 1) Amplitude – this is “how high” the wave is: 2)Wavelength ()– this is the distance between two corresponding points on the wave and is measured in metres: 3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz)

  29. Transverse vs. longitudinal waves Displacement Direction Direction Displacement Transverse waves are when the displacement is at right angles to the direction of the wave (e.g. light and other electromagnetic waves)… Longitudinal waves are when the displacement is parallel to the direction of the wave (e.g. sound waves)… Where are the compressions and rarefactions?

  30. Ultrasound Ultrasound is the region of sound above 20,000Hz – it can’t be heard by humans. It can be used in pre-natal scanning: How does it work? Ultrasonic waves are partly _________ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a __________ of the object. Words – depth, reflected, picture, medium

  31. Other uses of ultrasound 1) Breaking down kidney stones Ultrasonic waves break kidney stones into much smaller pieces 2) Cleaning (including teeth) Ultrasound causes dirt to vibrate dirt off without damaging the object Why is ultrasound better than X-rays? Ultrasound can be used instead of X-rays because they are able to produce images of soft tissue and they do not damage living cells.

  32. P4e: What is Radioactivity?

  33. The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”)

  34. Introduction to Radioactivity Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactice decay” and the activity is measured in “Becquerels (Bq)”.

  35. Types of radiation New nucleus 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) Unstable nucleus New nucleus Alpha particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. Beta particle Unstable nucleus 3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Words – frequency, proton, energy, neutrons, helium, beta Unstable nucleus New nucleus Gamma radiation

  36. Changes in Mass and Proton Number 241 237 4 α Am Np + 95 93 2 90 90 0 β Sr Y + -1 38 39 Alpha decay: Beta decay:

  37. Ionisation Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons: Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer.

  38. Half life = radioisotope = new atom formed The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… After 2 half lives another half have decayed (12 altogether) After 3 half lives another 2 have decayed (14 altogether) After 1 half life half have decayed (that’s 8) At start there are 16 radioisotopes

  39. A radioactive decay graph 08/01/2020 “1 Becquerel” means “1 radioactive count per second” Activity (Bq) Time

  40. A radioactive decay graph 1 half life 1 half life 1 half life Count Time

  41. P4f: Uses of Radioisotopes

  42. Background Radiation 08/01/2020 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power

  43. Background Radiation by Location In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event: Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc

  44. Uses of radioisotopes - tracers Gamma source A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.

  45. Uses of radioactivity – smoke detectors Alpha emitter +ve electrode -ve electrode If smoke enters here a current no longer flows Ionised air particles Smoke detectors Alarm

  46. Uses of radioactivity – dating rocks 1 half life later… 1 half life later… 1 half life later… 2 1 8 4 8 8 8 8 Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer: The sample was originally completely uranium… …of the sample was uranium Now only 4/8 of the uranium remains – the other 4/8 is lead Now only 2/8 of uranium remains – the other 6/8 is lead Now only 1/8 of uranium remains – the other 7/8 is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.

  47. An exam question… Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years

  48. Carbon Dating The amount of the isotope Carbon-14 present in the air has not changed for thousands of years. Therefore scientists can use it to help calculate how old something is. Consider, for example, a fossilised tree: 1) When the tree died it stopped exchanging carbon-14 with the atmosphere 2) As time went on the amount of carbon-14 decreased as it decayed 3) The amount of current activity can then be compared to a living tree to work out how old it is

  49. P4g: Treatment

  50. X-Rays and Gamma Rays X-ray images are possible because the absorption of X-rays depends on the material’s thickness and density. X-rays and gamma rays are similar in that they have similar wavelengths and they are both ionising but they are produced in different ways. Gamma rays are given out from the nucleus of radioactive materials whereas X-rays are made by firing high-speed electrons at metal targets and are therefore easier to control.

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