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Electricity The following formulae and symbols may be useful. vaverage d t F ma Ep mgh Fgravity mg Work Fd P g 10 m s –2 g 10 N kg –1 E t a change in speed change in time Ek 1 2 mv 2 V IR P IV QUESTION ONE: ELECTRICITY Study the circuit diagram below. The two lamps are NOT identical. When an ammeter is placed in this circuit a reading of 1.2 A is recorded. (a) Using the equation V = IR, calculate the voltage across the 4 Ω lamp. Voltage = (b) V Calculate the power output of the 4 Ω lamp. Give an appropriate unit. Power = ( ) unit (c) In terms of energy, explain what the power output in (b) means. _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ (d) _________________________________________________________________________ Calculate the resistance of lamp B. Resistance = Ω (e) Describe how the brightness of lamp B compares with the 4 Ω lamp. _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ The circuit is now altered so that the same lamps are in parallel, as shown below. (f) On the diagram above, draw an ammeter in the correct place to record the total current supplied by the power supply. When this ammeter is placed in the circuit, a reading of 5 A is recorded. (g) Explain why the current is larger in the parallel circuit than the series circuit. _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ __________________________________________________________________________________ (h) State which lamp will be brighter, the 4 Ω lamp or lamp B, and discuss the reasons for this lamp being brighter. You should consider current, resistance and power output. _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ QUESTION TWO: TORCHES This is a diagram of the inside of a standard torch. Metal conductors Light bulb Metal slide switch Two “C” cells Reflector (a) In the box below, draw a circuit diagram of the torch. Metal conductors (b) Each battery in this torch delivers 1.5 V to the light bulb. (i) Determine the voltage across the light bulb. ________________________________________________________________________ ________________________________________________________________________ (iii) The current flowing through the light bulb is 6.5 A. Calculate the resistance of the bulb. ________________________________________________________________________ ________________________________________________________________________ __________________________________________ Resistance = __________________ Ω LED (light emitting diode) torches use less power to generate the same brightness as a regular light bulb. The following is a circuit diagram showing three identical LEDs in a torch. (c) The LED lamps are connected in parallel. Give two reasons why the manufacturers of the LED lamps chose to connect the led lamps in parallel rather than series. Reason 1 _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ Reason 2 _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ (d) The set of LED’s are connected to a 4.5 V battery. Determine the voltage across each of the three LED’s. _____________________________________________________________________ _____________________________________________________________________ (e) Each LED has a power rating of 1.5 W. Calculate the resistance of one of the LED’s. _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ _____________________________________________________________________ ______________________________________ Resistance = ________________ Ω