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Current

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Current

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  1. Current “You’ll get a charge out of this”

  2. Current is measured in Amperes (amps or A) 1 amp = rate of flow such that 1C of charge moves past a point in 1 second. or q = charge (C) I = current (A) T = time (s)

  3. If 1.6 C moves past a point in 1.2s, what is the current?

  4. Work Done Recall W = qV Since work is done to move e-, then moving e- can do work. Since q=It and W = qV Then W=ItV Or Recall that Power = So P = IV

  5. e.g. A 100 Watt bulb is connected to 120 V (household voltage) . What current flows through the bulb? P = IV 100 = I(120) I = 0.83 A Electrical work is measured in kilowatt hours. 1 kWhr = power consumed in 1 hour

  6. e.g. How much does it cost to operate a 100 W bulb for 8 hours at 5¢ / kWhr? 100 W = 0.1 kWhr P = 0.1 kW x 8 hours P = 0.8 kWhr Cost = 0.8 x 5 = 4¢

  7. Current is measured with an ammeter. To properly measure current, the ammeter must be placed in series. To measure very small currents, a galvanometer is used. An ammeter in a diagram is symbolized by a Voltage in a circuit is often provided by a battery (dry or wet cell). Batteries produce a force which moves e-. This force is known as an electromotive force (EMF). If the battery is 12 V then it produces an EMF of 12 volts. Dry cell – chemicals in the form of a paste (AA, D,etc) Wet cell – liquid chemicals – (car battery)

  8. A battery is actually a collection of cells. A battery with 3 cells should be drawn like this can be shown as EMF is measured with a voltmeter These are placed in parallel with the circuit to be measured.

  9. EMF – Electromotive Force EMF – the ability of certain devices to produce a potential difference or electric current. Lesser known sources of EMF are friction and heat. Useful sources of EMF share the following characteristics: - produce a fairly constant voltage - cause a flow of electric current when connected to a circuit - exhibit polarity (+ & - terminals)

  10. Sources include 1. Electromagnetism – current is produced by a conducting loop rotating in a magnetic field. (generator) 2. Photo-electric cell – electrons are released from the surface of a photosensitive metal when illuminated by light (silicon solar cell) 3. Thermoelectric – when different metals are placed in two different temperatures a current is produced (thermocouple) 4. Piezoelectric – certain crystals will when subjected to mechanical stress produce a current (e.g. crystal microprobes) 5. Chemical – REDOX reactions will produce a current (batteries)

  11. Note: in Physics the terminals of a battery are defined on their electric charge rather than based on the REDOX reaction (pg. 419) SO: cathode (-) anode (+)