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Electricity

Electricity. W Richards The Weald School. Electric Current. Note that electrons go from negative to positive. +. -. e -. e -. Electric current (I) is a flow of negatively charged particles (i.e. electrons). We call them “charge carriers”. Charge is denoted by Q. I = ∆Q / ∆t

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Electricity

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  1. Electricity W Richards The Weald School

  2. Electric Current Note that electrons go from negative to positive + - e- e- Electric current (I) is a flow of negatively charged particles (i.e. electrons). We call them “charge carriers”. Charge is denoted by Q.

  3. I = ∆Q / ∆t • Current is defined as the rate of flow of charge.

  4. Conventional Current + - As we said, technically electrons go from negative to positive. However, we usually talk about “conventional current” and we say that current moves from positive to negative:

  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 ____. To make electric charge (Q) flow through a conductor you need to do work (W) on it. 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. V = W / Q • Voltage is defined as the energy converted per unitcharge moved.

  7. R = V / I • Resistance is a measure of how difficult it is for a current to flow through something.

  8. 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

  9. DC and AC V DC stands for “Direct Current” – the current only flows in one direction: Time 1/50th s AC stands for “Alternating Current” – the electrons change direction 50 times every second (frequency = 50Hz) 240V T V

  10. Charge and Current PROTON – positively charged ELECTRON – negatively charged Recall the structure of an atom: • Notice: • Atoms have the same number of protons and electrons – they are NEUTRAL overall • Because electrons are on the outside of the atoms they can move around (this is what causes electrical effects)

  11. Charge and Current Consider two different rods being rubbed with a cloth:

  12. Two different rods will _____ each other if they have a ______ charge: Two rods made of the same material will _______ each other due to having ___ _____ charge:

  13. Measuring Charge The charge on an electron is very small, so we measure charge using units called “coulombs” (C). One electron has a charge of 1.6 x 10-19 C. Charge can be measured using a coulombmeter, and they usually measure in nanocoloumbs (1nC = 10-9 C). For example, a charged polythene rod may carry a charge of a few hundred nanocoulombs

  14. Calculating Charge (Q) Current = rate of flow of charge I =ΔQ ΔT By definition, current is the rate of flow of charge. In other words, its how much charge flows per second. One amp (1 A) is equal to one coulomb per second (1 Cs-1). Charge and current are related by the equation: • A battery supplies 10 C over a period of 50 seconds. What is the current? • Another battery is connected for 2 minutes and provided a current of 0.4 A. How much charge flowed? • A car battery has a capacity of 24 Ah (amp hours). If it provides a current of 48A how long can it be used for? How much charge (in coulombs) does it contain?

  15. Current in a series circuit 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.

  16. Current in a parallel circuit 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”

  17. Current in a parallel circuit 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

  18. Some example questions… 3A 6A

  19. Kirchoff’s First Law 6A Gustav Kirchoff (1824-1887) … then the current here will be 6A If the current through here is 4A... …and the current through here is 2A… “The sum of the currents leaving a point is the same as the sum of the currents entering that point.” For example:

  20. Voltage + - e- e- Earlier on we said that current is when electrons move: “Voltage” is the force that pushes the electrons. For electrons to move there must be a “voltage difference”, sometimes called a “potential difference” (p.d.). A higher p.d. means a stronger push, which causes an increase in current.

  21. Voltage in a series circuit 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

  22. Voltage in a series circuit V If the voltage across the battery is 6V… …what will the voltage across two bulbs be? V 4V

  23. Voltage in a parallel circuit If the voltage across the batteries is 4V… What is the voltage here? V V And here? 4V 4V

  24. Summary 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”

  25. An example question: 6V A3 3A A1 V1 A2 V2 V3

  26. Electromotive force and p.d. The sum of these EMFs… Is equal to the sum of the p.d.s Components like batteries and power supplies provide a force that pushes the current around a circuit: we call this the “electromotive force” (e.m.f). Other components like bulbs and motors have work done to them by the current – the voltage across them is called the “potential difference” (p.d.)

  27. Kirchoff’s Second Law Gustav Kirchoff (1824-1887) If the e.m.f of the batteries is 3V The voltage across each bulb will be 1V “Around any closed loop, the sum of the e.m.f.s is equal to the sum of the p.d.s.” For example:

  28. Voltage at a point The voltage here is 6V The voltage here is 4.5V The voltage here is 3V The voltage here is 1.5V Take this point as being 0V

  29. Voltage-position graphs 6V 5.9V 4.5V 1.5V 0.1V 0V

  30. Work done V = W Q Voltage = work done charge Definition of a volt: The voltage between two points is the work done per coulomb travelling between the two points We can see that 1V = 1JC-1

  31. Example Questions • A battery does 9J of work. If it transfers 6C of charge what is the battery’s voltage? • A powerpack does 100J of work in transferring 20C of charge. What is the voltage? • A 9V battery transfers 20C of charge. How much work did it do? • If the current of the battery is 0.2A how long was it used for? • 240J of work is done to a 12V motor. How much charge flowed through it? • If this motor was used for 40 seconds how much current did it draw?

  32. Electrical Power Voltage = work done charge 1) Recall: W = QV P = W T 2) Also, recall that power = rate of doing work Power = work done time Power = charge x voltage time P = Q x V T 3) Therefore 4) But I = Q T so Power = current x voltage P = IV

  33. 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:

  34. 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?

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

  36. 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

  37. Resistors in Series I V1 V2 VT R1 R2 “In a series circuit current stays the same but voltage splits up” VT = V1 + V2 VT = IRT But V1 = IR1 and V2 = IR2 IRT = IR1 + IR2 RT = R1 + R2

  38. Resistors in Parallel IT I1 I2 IT V IT = V RT R1 R2 V = V + V RT R1 R2 1 = 1 + 1 RT R1 R2 “In a parallel circuit voltage stays the same but current splits up” IT = I1 + I2

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

  40. Power through a resistor Recall: 1) P = IV 2) V = IR Putting these two equations together gives us: Power = I x IR = I2R • A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor. • A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor. • A 2KW heater has a resistance of 20 Ω. Calculate the current through it.

  41. Using voltmeters and ammeters A V The resistance of an ammeter is assumed to be very small – this ammeter will only have a very small voltage across it. The resistance of a voltmeter is assumed to be very large, so only a small current will go through it.

  42. Resistivity Resistance = resistivity x length area R = ρL A The resistance of a wire depends on 3 things: the length of the wire, the width of the wire and what the wire is made of: • Calculate the following: • The resistance of a copper wire of length 2m, area 2mm2 and resistivity 1.7x10-8Ωm-1 • The resistance of an iron wire of length 100m, area 5mm2 and resistivity 1x10-7Ωm-1 • A copper wire has a resistance of 5Ω. If the wire is 20m long and the wire is cylindrical what is the radius of the wire?

  43. Electron Drift Electrons Ions What happens inside a conducting material? The following model of a metal wire could help: At normal temperatures, with no current flowing, electrons hurtle around continuously. They collide with ions but because their movement is random there is no net energy transfer.

  44. Electron Drift Electrons Ions Now apply a voltage: Negative Positive This time we can see that the electrons are accelerated from negative to positive. This movement is superimposed on top of the random velocities and is responsible for electrical effects.

  45. Current-voltage graphs I R V V Consider a resistor: Current increases in proportion to voltage Resistance stays constant

  46. Current-voltage graphs I R V V Now consider a bulb: As voltage increases the bulb gets hotter and resistance increases Resistance increases as the bulb gets hotter

  47. Current-voltage graphs I I V V Now consider a diode: Now consider a thermistor: A diode only lets current go in the “forward” direction Resistance decreases as the (“negative-temperature-coefficient”) thermistor gets hotter

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

  49. Internal Resistance + - V The voltage across the terminals of a battery is called the “terminal p.d.”

  50. Internal Resistance + - V This voltage DECREASES when more components are added…

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