Which bulb draws the most current

The total power dissipated in the circuit is three times this, or With fresh light bulbs, direct measurement with an ammeter shows that the actual current flowing in the parallel circuit is 0. So the current, and thus the dissipated power If the light bulbs behaved this way, the measured current in the series circuit would agree with the estimate above.

Even though they do not, this demonstration gives a good sense of the difference in behavior between a series and parallel circuit made with three identical resistors. What happens if the light bulbs are not all of the same wattage rating?

An interesting variation of this demonstration is to show what happens when we put light bulbs of three different wattages in each circuit. A good choice is to keep one W light bulb in each circuit, and then add a W bulb and a W bulb. In the parallel circuit, as noted above, the voltage across each bulb is the same V , so each bulb draws the current that it would if it alone were connected to the wall, and the intensities of the bulbs thus vary as you would expect from the wattage ratings.

The W bulb is the brightest, the W bulb is the dimmest, and the W bulb is somewhere in between. When we put the same combination of bulbs in series, an interesting thing happens.

Because both the W bulb and the W bulb have lower resistance than the W bulb, the current through the circuit is somewhat higher than for the three W light bulbs in series, and the W bulb glows more brightly than it did when it was in series with two other W bulbs. The current through this circuit measures 0. At this current, the W bulb lights fairly brightly, the W bulb just barely glows, and the W bulb does not light at all. The photograph below shows the operation of these two circuits:.

The bulbs in each circuit, from left to right, are a W, W and a W light bulb. In the parallel circuit, the bulbs obviously increase in brightness from left to right. In the series circuit, the brightness decreases from left to right. The measured voltages in the circuit are V across all three bulbs, V across the and the W bulbs, and 78 V across the Watt bulb. Figure 3. On-demand electric hot water heater.

Heat is supplied to water only when needed. What would be the maximum cost of a CFL such that the total cost investment plus operating would be the same for both CFL and incandescent W bulbs? Calculate the cost for hours, as in the cost effectiveness of CFL example. Some makes of older cars have 6. Alkaline batteries have the advantage of putting out constant voltage until very nearly the end of their life.

How long will an alkaline battery rated at 1. A cauterizer, used to stop bleeding in surgery, puts out 2. The average television is said to be on 6 hours per day. Estimate the yearly cost of electricity to operate million TVs, assuming their power consumption averages W and the cost of electricity averages An old lightbulb draws only By what factor is its diameter reduced, assuming uniform thinning along its length? Neglect any effects caused by temperature differences.

Calculate the power loss in a kilometer of such wire when it carries 1. Cold vaporizers pass a current through water, evaporating it with only a small increase in temperature. One such home device is rated at 3.

See Figure 4. Figure 4. This cold vaporizer passes current directly through water, vaporizing it directly with relatively little temperature increase. Integrated Concepts a What energy is dissipated by a lightning bolt having a 20,A current, a voltage of 1.

Integrated Concepts What current must be produced by a Integrated Concepts How much time is needed for a surgical cauterizer to raise the temperature of 1. Ignore heat transfer to the surroundings.

Integrated Concepts Hydroelectric generators see Figure 5 at Hoover Dam produce a maximum current of 8. How many cubic meters per second are needed, assuming Figure 5. Hydroelectric generators at the Hoover dam. Integrated Concepts a Assuming See Figure 6. Figure 6. Integrated Concepts a An aluminum power transmission line has a resistance of 0. What is its mass per kilometer? A lower resistance would shorten the heating time. Discuss the practical limits to speeding the heating by lowering the resistance.

Integrated Concepts a An immersion heater utilizing V can raise the temperature of a 1. Find its resistance, assuming it is constant during the process. Unreasonable Results a What current is needed to transmit 1. Construct Your Own Problem Consider an electric immersion heater used to heat a cup of water to make tea. Construct a problem in which you calculate the needed resistance of the heater so that it increases the temperature of the water and cup in a reasonable amount of time.

Also calculate the cost of the electrical energy used in your process. Among the things to be considered are the voltage used, the masses and heat capacities involved, heat losses, and the time over which the heating takes place.

Your instructor may wish for you to consider a thermal safety switch perhaps bimetallic that will halt the process before damaging temperatures are reached in the immersion unit.

Long-distance transmission lines are kept at much higher voltages often hundreds of kilovolts to reduce power losses. Skip to main content.

Search for:. Electric Power and Energy Learning Objectives By the end of this section, you will be able to: Calculate the power dissipated by a resistor and power supplied by a power supply.

Calculate the cost of electricity under various circumstances. Example 1. The cold resistance was 0. Discussion for a The 30 W dissipated by the hot headlight is typical. Strategy and Solution for b The current when the bulb is cold can be found several different ways.

Discussion for b The cold current is remarkably higher than the steady-state value of 2. The energy your body uses in exercise is related to the power level and duration of your activity, for example.

The amount of heating by a power source is related to the power level and time it is applied. Even the radiation dose of an X-ray image is related to the power and time of exposure. Example 2. Calculating the Cost Effectiveness of Compact Fluorescent Lights CFL If the cost of electricity in your area is 12 cents per kWh, what is the total cost capital plus operation of using a W incandescent bulb for hours the lifetime of that bulb if the bulb cost 25 cents?

Strategy To find the operating cost, we first find the energy used in kilowatt-hours and then multiply by the cost per kilowatt-hour. Explain why something like a toaster has a higher rating than a digital clock. Estimate the energy consumed by these appliances in an average day by estimating their time of use. Some appliances might only state the operating current. You might need to assume the long fluorescent lights in use are rated at 32 W.

Suppose that the building was closed all weekend and that these lights were left on from 6 p. Friday until 8 a. What would this oversight cost? How about for an entire year of weekends? Conceptual Questions 1.

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