Ch 25: Current, Resistance, and EMF
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Problem 21
Lightning occurs when there is a flow of electric charge (principally electrons) between the ground and a thundercloud. The maximum rate of charge flow in a lightning bolt is about 20,000 C/s; this lasts for 100 ms or less. How much charge flows between the ground and the cloud in this time? How many electrons flow during this time?Problem 25
An idealized ammeter is connected to a battery as shown in Fig. E25.28
. Find (a) the reading of the ammeter.
Problem 25
An idealized ammeter is connected to a battery as shown in Fig. E25.28
. Find (b) the current through the 4.00-Ω resistor.
Problem 25
An idealized ammeter is connected to a battery as shown in Fig. E25.28
. Find (c) the terminal voltage of the battery.
Problem 25
Consider the circuit of Fig. E25.30
. (a) What is the total rate at which electrical energy is dissipated in the 5.0-Ω and 9.0-Ω resistors?
Problem 25
Consider the circuit of Fig. E25.30
. (b) What is the power output of the 16.0-V battery?
Problem 25
Consider the circuit of Fig. E25.30
. (c) At what rate is electrical energy being converted to other forms in the 8.0-V battery?
Problem 25
Electric eels generate electric pulses along their skin that can be used to stun an enemy when they come into contact with it. Tests have shown that these pulses can be up to 500 V and produce currents of 80 mA (or even larger). A typical pulse lasts for 10 ms. What power and how much energy are delivered to the unfortunate enemy with a single pulse, assuming a steady current?Problem 25
A heart defibrillator is used to enable the heart to start beating if it has stopped. This is done by passing a large current of 12 A through the body at 25 V for a very short time, usually about 3.0 ms. (b) how much energy is transferred?Problem 25
The battery for a certain cell phone is rated at 3.70 V. According to the manufacturer it can produce 3.15 * 10^4 J of electrical energy, enough for 5.25 h of operation, before needing to be recharged. Find the average current that this cell phone draws when turned on.Problem 25
A typical small flashlight contains two batteries, each having an emf of 1.5 V, connected in series with a bulb having resistance 17 Ω. (a) If the internal resistance of the batteries is negligible, what power is delivered to the bulb?Problem 25
A typical small flashlight contains two batteries, each having an emf of 1.5 V, connected in series with a bulb having resistance 17 Ω. ((b) If the batteries last for 5.0 h, what is the total energy delivered to the bulb?Problem 25
A cylindrical tungsten filament 15.0 cm long with a diameter of 1.00 mm is to be used in a machine for which the temperature will range from room temperature (20°C) up to 120°C. It will carry a current of 12.5 A at all temperatures. (c) What will be the maximum potential drop over the full length of the filament?Problem 25
A ductile metal wire has resistance R. What will be the resistance of this wire in terms of R if it is stretched to three times its original length, assuming that the density and resistivity of the material do not change when the wire is stretched? (Hint: The amount of metal does not change, so stretching out the wire will affect its cross-sectional area.)Problem 25
In household wiring, copper wire 2.05 mm in diameter is often used. Find the resistance of a 24.0-m length of this wire.Problem 25
A hollow aluminum cylinder is 2.50 m long and has an inner radius of 2.75 cm and an outer radius of 4.60 cm. Treat each surface (inner, outer, and the two end faces) as an equipotential surface. At room temperature, what will an ohmmeter read if it is connected between (a) the opposite faces and (b) the inner and outer surfaces?Problem 25
(b) What is the resistance of a carbon rod at 25.8°C if its resistance is 0.0160 Ω at 0.0°C?Problem 25
A copper transmission cable 100 km long and 10.0 cm in diameter carries a current of 125 A. (b) How much electrical energy is dissipated as thermal energy every hour?Problem 25
Pure silicon at room temperature contains approximately 1.0 * 10^16 free electrons per cubic meter. (a) Referring to Table 25.1, calculate the mean free time t for silicon at room temperature. (b) Your answer in part (a) is much greater than the mean free time for copper given in Example 25.11. Why, then, does pure silicon have such a high resistivity compared to copper?Problem 25
When a resistor with resistance R is connected to a 1.50-V flashlight battery, the resistor consumes 0.0625 W of electrical power. (Throughout, assume that each battery has negligible internal resistance.) (a) What power does the resistor consume if it is connected to a 12.6-V car battery? Assume that R remains constant when the power consumption changes.Problem 25
Consider the circuit of Fig. E25.30 (d) Show that the power output of the 16.0-V battery equals the overall rate of consumption of electrical energy in the rest of the circuit.Problem 26
The heating element of an electric dryer is rated at 4.1 kW when connected to a 240-V line. (b) What is the resistance of the dryer's heating element at its operating temperature?Problem 26
A 1500-W electric heater is plugged into the outlet of a 120-V circuit that has a 20-A circuit breaker. You plug an electric hair dryer into the same outlet. The hair dryer has power settings of 600 W, 900 W, 1200 W, and 1500 W. You start with the hair dryer on the 600-W setting and increase the power setting until the circuit breaker trips. What power setting caused the breaker to trip?
