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07/01/2020

07/01/2020. Unit 2 – Physics for your Future. (EdExcel). N Smith St. Aidan’s. Topic 1 – Static and Current Electricity. 07/01/2020. The structure of the atom. ELECTRON – negative, mass nearly nothing. PROTON – positive, same mass as neutron (“1”).

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07/01/2020

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  1. 07/01/2020 Unit 2 – Physics for your Future (EdExcel) N Smith St. Aidan’s

  2. Topic 1 – Static and Current Electricity 07/01/2020

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

  4. The structure of the atom MASS NUMBER = number of protons + number of neutrons 4 He SYMBOL 2 PROTON NUMBER = number of protons (obviously)

  5. Static Electricity 07/01/2020 + - - + 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 and the insulator can be charged up by friction by “transferring electrons”: + + - - + + + - - - + + - + - -

  6. Static Electricity 07/01/2020 - - + - - + + - - - - -

  7. Short Static Experiments 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?

  8. Static Electricity in Lightning e- e- e- e-

  9. Van de Graaf generators 07/01/2020 When a charge is neutralised by the movement of electrons either from the Earth or to the Earth we call this “earthing”

  10. Using Static in Paint Sprayers 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?

  11. Dangers of Static – fuelling lines

  12. Electric Current 07/01/2020 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” Notice that the electrons from this battery only went in one direction around the circuit – this is called “direct current” (d.c.).

  13. Charge (Q) 07/01/2020 Q I T As we said, electricity is when electrons move around a circuit and carry energy with them. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). We can work out how much charge flows in a circuit using the equation: Charge = current x time (in C) (in A) (in s)

  14. Example questions 07/01/2020 • A circuit is switched on for 30s with a current of 3A. How much charge flowed? • During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? • A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

  15. Example questions 07/01/2020 • A circuit is switched on for 30s with a current of 3A. How much charge flowed? • During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for? • A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed? 90C 200s 300C

  16. Topic 2 – Controlling and Using Electric Current 07/01/2020

  17. Circuit Symbols 07/01/2020 Variable resistor A V Diode Switch Bulb Ammeter Voltmeter LDR Resistor Cell Fuse Thermistor Battery

  18. Basic ideas… 07/01/2020 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

  19. More basic ideas… 07/01/2020 If a battery is added the current will ________ because there is a greater _____ on the electrons caused by a greater potential difference If a bulb is added the current will _______ because there is greater ________ in the circuit

  20. Current in a series circuit 07/01/2020 If the current here is 2 amps… The current here will be… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point

  21. Current in a parallel circuit 07/01/2020 Here comes the current… Half of the current will go down here (assuming the bulbs are the same)… And the rest will go down here… A PARALLEL circuit is one where the current has a “choice of routes”. Notice how current is “conserved” at each junction:

  22. Current in a parallel circuit 07/01/2020 And the current here will be… The current here will be… The current here will be… The current here will be… If the current here is 6 amps

  23. Some example questions… 07/01/2020 3A 6A 4A 2A 1A each

  24. Voltage in a series circuit 07/01/2020 V If the voltage across the battery is 6V… …and these bulbs are all identical… V V …what will the voltage across each bulb be? 2V

  25. Voltage in a series circuit 07/01/2020 V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? V 4V

  26. Voltage in a parallel circuit 07/01/2020 If the voltage across the batteries is 4V… What is the voltage here? V V And here? 4V 4V

  27. Summary 07/01/2020 In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”

  28. An example question: 07/01/2020 3A 6V A1 6V 3A A2 2A V1 1A A3 V2 V3 3V 3V

  29. Another example question: 07/01/2020 3A 10V A1 1.2A 3A A2 V1 6.7V A3 1.8A V2 V3 5V 5V

  30. Energy and charge 07/01/2020 W V Q The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around: Energy transferred = charge x voltage (in J) (in C) (in V) By definition then, voltage means “energy transferred per unit charge” and 1V = 1J/C

  31. Example questions 07/01/2020 • In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred? • In the above circuit the radio drew a current of 0.5A. How long was it on for? • A motor operates at 6V and draws a current of 3A. The motor is used for 5 minutes. Calculate: a) the charge flowing through it, b) the energy supplied to it • A lamp is attached to a 12V circuit and a charge of 1200C flows through it. If the lamp is on for 10 minutes calculate a) the current, b) the energy supplied to the bulb. 600J 200s 900C, 5400J 2A, 14,400J

  32. Resistance 07/01/2020 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:

  33. An example question: 07/01/2020 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?

  34. More examples… 07/01/2020 3A 3A 2A 4V 2V 1A 6V 12V What is the resistance of these bulbs?

  35. Resistance 07/01/2020 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

  36. Varying Resistance 07/01/2020 Recall our earlier idea that if you increase the number of bulbs in a circuit you increase the resistance and therefore decrease the current: The same effect is seen when using a variable resistor: Increase the resistance:

  37. Resistors, bulbs and diodes 07/01/2020

  38. Current-Voltage Graphs 07/01/2020

  39. Current-voltage graphs 07/01/2020 I I I V V V 3. Diode 1. Resistor 2. Bulb A diode only lets current go in one _______ – it has very _____ resistance in the other direction Current increases in proportion to _______, provided the temperature doesn’t change As voltage increases the bulb gets ______ and _______ increases due to increased vibrations in the ions in the filament Words – resistance, high, voltage, hotter, direction

  40. LDRs and Thermistors 07/01/2020

  41. Two simple components: 07/01/2020 Resistance Resistance Amount of light Temperature 1) Light dependant resistor – resistance DECREASES when light intensity INCREASES 2) Thermistor – resistance DECREASES when temperature INCREASES

  42. 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. This effect causes the metal to heat up.

  43. Using this heating effect This heating effect can have its advantages and its disadvantages. For example, consider an old-fashioned light bulb: This heating effect causes the filament to emit light… …but it also causes a lot of energy to be wasted to the environment

  44. Electrical Power revision P V I Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W). The amount of power being transferred in an electrical device is given by: Power = voltage x current in W in V in A • How much power is transferred by a 230V fire that runs on a current of 10A? • An electric motor has a power rating of 24W. If it runs on a 12V battery what current does it draw? • An average light bulb in a home has a power rating of 60W and works on 230V. What current does it draw?

  45. Energy and Power The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second Energy transferred (J) = power (W) x time (s) OR Energy (J) = current (A) x voltage (V) x time (s)

  46. Some example questions • A battery gives out a current of 0.2A and has a voltage of 1.5V. If it is used for 30 seconds how much energy has it transferred? • An electric fire runs at a voltage of 230V and a current of 8A. If it is left on for 2 hours how much electrical energy has it transferred? • A toaster transfers 20,000J of electrical energy. If it runs at a voltage of 230V and a current of 2A how long was it on for? • A light bulb is left on overnight for 8 hours. If it transfers 1,000,000J of energy and runs on a voltage of 230V what current did it draw? 9J 13.2MJ 43.5s 0.15A

  47. Topic 3 – Motion and Forces 07/01/2020

  48. Some subtle differences… Start -1 metre 1 metre “Distance” is how far you have gone, “displacement” is how far you are from a point and can be positive or negative: Distance = Displacement = Distance = Displacement = Distance = Displacement = Distance = Displacement =

  49. Some subtle differences… Start -1 metre 1 metre “Speed” means “how fast you are going”, “velocity” means “how far you are going in a certain direction”. If the following journeys take 1 second then work out: Speed = Velocity = Speed = Velocity = Speed = Velocity = Speed = Velocity =

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