Ch 23

Find the cost in dollars of the operating a 60-W lamp continuously for 1 week when the power utility rate is 15 cents/kWh .

(60W/1000)= 0.06W W= PT (0.06)(168)= 10.08kWhr cost=WRate= (10.08kWhr)(15 cents/kWh)= $1.5

A. Using the formula power = current × voltage, find the current drawn by a 1500-W toaster connected to 140 V . B. Then, using the equation current = voltage/resistance, find the resistance of the toaster.

A. 1500/140= 10.7 A B. 140/10.7 = 13.1 ohms

A 4.8-W night-light is plugged into a 120-V circuit and operates continuously for 1 year. (a) the current it draws, (b) the resistance of its filament, (c) the energy consumed in a year. (d) Find the cost in dollars for a utility rate of 14 cents/kWh for a year's operation.

(a) Find the current it draws: -Use this formula: P=IV or W=AV (in units) --> P(watts)= 4.8W; V(volts)= 120V; and Current (I; amps)= ? -I=P/V or A=W/V --> I= 4.8W/120V = 0.04 and a watt divided by a volt is = to 1 ampere --> so the answer is: 0.04A is the current (b) Find the resistance of its filament: -Use this formula: I=V/R or A=V/ohms -->V(volts)= 120V; I(amps)= (4.8W/120V= 0.04A); and R=? -R=V/I --> so R= 120V/(4.8W/120V) --> = 3000 and a volt divided by an ampere is = to 1 ohm --> so the answer is: 3,000 ohms (c) Find the energy consumed in a year: -Use this formula: Power=Energy/Time or Electrical Energy = Electrical Power X Time (same thing) -->Power(watts)= 4.8W; Time= 1 year or 8,760 hours; and Energy= ? -so E=PT so E= (4.8W)(8,760hrs) = 42,048 --> divide 42,048 by 1,000 because the unit is kilowatt-hour for this formula -->so the answer is: 42.05 kWh energy consumed in one year. (d) The cost for a year's operation is $5.89: -We know that 42.05 kWh of Energy is consumed in 1 year and the rate of each kWh is 15 cents. --> so (42.05 kWh)(14 cents) = 588.7 cents/kWh --> 588.7 cents/100 = $5.89 -->so the answer is: $5.89 is the cost for a years operation

What is the current in a bulb if the resistance of its filament is 2 ohms and it is connected across a 6-volt battery?

3 amps

What single resistor would be equivalent to the resistors in this arrangement?

4 ohms

An electric current of 5 amperes in a wire carries ________.

5 coulombs per second

1. Consider the three parts of the circuit: the top branch with two bulbs; the middle branch with one bulb; the battery. A. Rank the current through each, from greatest to least. B. Rank the voltage across each from greatest to least.

A. battery, one bulb, two bulbs B. the battery, the middle branch with one bulb, the top branch with two bulbs IS ALL THE SAME

What is the relationship between current, resistance, and voltage difference?

Current = Voltage Difference / Resistance

Why are electrons, rather than protons, the principal charge carriers in metal wire?

Electrons are free to move through the metal, whereas protons are fixed in place.

When you make your household electric payment at the end of the month, are you billed for voltage, current, power, or energy?

Energy

All bulbs are identical in the following circuits. A voltmeter is connected across a single bulb to measure the voltage drop across it.

Greatest is with least light bulbs least is with more light bulbs

Rank the current readings in the ammeter, from greatest to least.

Greatest is with the most light bulbs least is with the least light bulbs

Does heating a metal wire increase or decrease its electrical resistance? Why?

It increases resistance because atoms at higher temperatures jostle into the way of moving electrons

If the resistance of a circuit remains constant while the voltage across the circuit decreases to half its former value, what change occurs in the current?

It is cut in half

Why does a wire that carries electric current become hot?

Moving electrons collide with atoms, which transfer their kinetic energy to atomic motion

What is the unit of electrical resistance?

Ohm

A flow of electric charge in a wire normally requires a _________.

Potential difference across the ends of the wire

What is the relationship among electric power, current, and voltage?

Power is current times voltage.

Rank the circuits illustrated according to the brightness of the identical bulbs, from brightest to dimmest.

SAME

The bulbs shown are identical. An ammeter is placed in different locations, as shown. Rank the current readings in the ammeter from greatest to least.

SAME

How does the current change if you increase the resistance, keeping the voltage difference the same?

The current decreases

If both are halved?

The current remains unchanged

What is the effect on the current in a wire if both the voltage across it and its resistance are doubled?

The current remains unchanged

How does the resistance of a thick piece of copper wire compare to the resistance of a thin piece of copper wire?

The resistance of the thin piece is greater than that of the thick piece

The total charge that an automobile battery can supply without being recharged is given in ampere-hours. A typical 12-V battery has a rating of 60 ampere-hours (60 A for 1 h, 30 A for 2 h, and so on). Suppose that you forget to turn off the headlights in your parked automobile. If each of the two headlights draws 2.5 A , how long will it be before your battery is "dead"?

Time = 60 A/(2 *2.5 A ) *1 h = 12 h

What factors does the resistance offered by a piece of conductor depend upon?

both the geometry and the material of the conductor

When the filament breaks in one lamp in a parallel circuit, lamps in other branches of the circuit normally _________.

continue glowing as brightly

When the filament breaks in one lamp in a series circuit, other lamps in the circuit normally _________.

go out