20.1 Current  (Page 5/8)

Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.

Car batteries are rated in ampere-hours ( $\text{A}\cdot \text{h}$ ). To what physical quantity do ampere-hours correspond (voltage, charge, . . .), and what relationship do ampere-hours have to energy content?

If two different wires having identical cross-sectional areas carry the same current, will the drift velocity be higher or lower in the better conductor? Explain in terms of the equation $v_{\text{d}}=\frac{I}{\text{nqA}}$ , by considering how the density of charge carriers $n$ relates to whether or not a material is a good conductor.

Why are two conducting paths from a voltage source to an electrical device needed to operate the device?

In cars, one battery terminal is connected to the metal body. How does this allow a single wire to supply current to electrical devices rather than two wires?

Why isn’t a bird sitting on a high-voltage power line electrocuted? Contrast this with the situation in which a large bird hits two wires simultaneously with its wings.

What is the current in milliamperes produced by the solar cells of a pocket calculator through which 4.00 C of charge passes in 4.00 h?

A total of 600 C of charge passes through a flashlight in 0.500 h. What is the average current?

What is the current when a typical static charge of $0\text{.}\text{250}\mu \text{C}$ moves from your finger to a metal doorknob in $1.00\mu \text{s}$ ?

Find the current when 2.00 nC jumps between your comb and hair over a $0\text{.}\text{500 -}\mu \text{s}$ time interval.

A large lightning bolt had a 20,000-A current and moved 30.0 C of charge. What was its duration?

The 200-A current through a spark plug moves 0.300 mC of charge. How long does the spark last?

(a) A defibrillator sends a 6.00-A current through the chest of a patient by applying a 10,000-V potential as in the figure below. What is the resistance of the path? (b) The defibrillator paddles make contact with the patient through a conducting gel that greatly reduces the path resistance. Discuss the difficulties that would ensue if a larger voltage were used to produce the same current through the patient, but with the path having perhaps 50 times the resistance. (Hint: The current must be about the same, so a higher voltage would imply greater power. Use this equation for power: $P=I^{2}R$ .)

During open-heart surgery, a defibrillator can be used to bring a patient out of cardiac arrest. The resistance of the path is $5\text{00 Ω}$ and a 10.0-mA current is needed. What voltage should be applied?

(a) A defibrillator passes 12.0 A of current through the torso of a person for 0.0100 s. How much charge moves? (b) How many electrons pass through the wires connected to the patient? (See figure two problems earlier.)

A clock battery wears out after moving 10,000 C of charge through the clock at a rate of 0.500 mA. (a) How long did the clock run? (b) How many electrons per second flowed?

The batteries of a submerged non-nuclear submarine supply 1000 A at full speed ahead. How long does it take to move Avogadro’s number ( $6\text{.}\text{02}\times {\text{10}}^{\text{23}}$ ) of electrons at this rate?

Electron guns are used in X-ray tubes. The electrons are accelerated through a relatively large voltage and directed onto a metal target, producing X-rays. (a) How many electrons per second strike the target if the current is 0.500 mA? (b) What charge strikes the target in 0.750 s?

A large cyclotron directs a beam of ${\text{He}}^{\text{++}}$ nuclei onto a target with a beam current of 0.250 mA. (a) How many ${\text{He}}^{\text{++}}$ nuclei per second is this? (b) How long does it take for 1.00 C to strike the target? (c) How long before 1.00 mol of ${\text{He}}^{\text{++}}$ nuclei strike the target?

Repeat the above example on [link] , but for a wire made of silver and given there is one free electron per silver atom.

Using the results of the above example on [link] , find the drift velocity in a copper wire of twice the diameter and carrying 20.0 A.

A 14-gauge copper wire has a diameter of 1.628 mm. What magnitude current flows when the drift velocity is 1.00 mm/s? (See above example on [link] for useful information.)

SPEAR, a storage ring about 72.0 m in diameter at the Stanford Linear Accelerator (closed in 2009), has a 20.0-A circulating beam of electrons that are moving at nearly the speed of light. (See [link] .) How many electrons are in the beam?