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Electric currents

Electric currents. +. -. -. +. 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:. +. +. -. -. +. +. +. -. -. -. +. +. -. +. -. -. -. -. +. -. -. +. +. -.

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Electric currents

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  1. Electric currents

  2. + - - + 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: + + - - + + + - - - + + - + - -

  3. - - + - - + + - - - - - Static Electricity

  4. Static shock

  5. 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

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

  7. Define electric current. It is sufficient for students to know that current is defined in terms of the force per unit length between parallel current-carrying conductors.

  8. Answers

  9. Current = rate of flow of charge I =Δq Δt Calculating Charge (Q) 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?

  10. + - 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:

  11. How long will it take for the electrons to complete 1 circuit?

  12. Electrons Ions Electron Drift 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.

  13. Electrons Ions Electron Drift 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.

  14. What does an electric current look like?

  15. Conductors….. How many free electrons?

  16. Hyperlink

  17. 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 _____. Basic ideas… Words: volts, amps, ohms, voltage, ammeter, voltmeter

  18. + - e- e- Voltage Earlier on we said that current is when electrons move: “Voltage” is the energy that allows the electrons to move. 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.

  19. Define electric potential difference The potential difference between 2 points in a circuit is…….. (1 volt = 1 joule per coulomb)

  20. 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 Resistance Define resistance. Students should be aware that R = V/I is a general definition of resistance. It is not a statement of Ohm’s law. Students should understand what is meant by resistor. The resistance of a component can be calculated using

  21. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) Resistance 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

  22. Resistivity Apply the equation for resistance in the form where ρ is the resistivity of the material of the resistor. What effect does doubling the diameter have on the resistance?

  23. A V Compare ohmic and non-ohmicbehaviour. For example, students should be able to draw the I–V characteristics of an ohmic resistor and a filament lamp. Now determine the I-V graphs for a resistor and a bulb.

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

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

  26. I I V V Current-voltage graphs 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

  27. LDR and Thermistor How does their resistance vary with light and heat?

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

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

  30. 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… Kirchoff’s First Law “The sum of the currents leaving a point is the same as the sum of the currents entering that point.” For example: 6A

  31. What happens if you have a choice?

  32. 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… Current in a parallel circuit A PARALLEL circuit is one where the current has a “choice of routes”

  33. And the current here will be… The current here will be… The current here will be… Current in a parallel circuit If the current here is 6 amps The current here will be…

  34. 6A What is the current in each bulb? 3A

  35. 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

  36. 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? Voltage in a series circuit 2V

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

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

  39. 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”

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

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

  42. Solve problems involving potentialdifference, current and resistance. 10V A3 3A A1 V1 A2 V2 V3

  43. An example Calculate the missing values (from A-level June 2006) 6V ? ? A ? A R ? 4Ω V 0.24A A 15Ω

  44. 3A 3A 2A 4V 2V 1A More examples… 6V 12V What is the resistance of these bulbs?

  45. I V1 V2 VT R1 R2 Resistors in Series “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

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

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

  48. Question 1  Calculate the total resistance of the circuit. Question 2  Calculate the current I. Question 3  Calculate the voltage across the points A and B. Question 4  Calculate the current I1. Question 5  How much charge will pass through the cell in 20s?

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