Electric current
is the rate at which charge flows, given by
where
is the amount of charge passing through an area in time
.
The direction of conventional current is taken as the direction in which positive charge moves.
The SI unit for current is the ampere (A), where
Current is the flow of free charges, such as electrons and ions.
Drift velocity
is the average speed at which these charges move.
Current
is proportional to drift velocity
, as expressed in the relationship
. Here,
is the current through a wire of cross-sectional area
. The wire’s material has a free-charge density
, and each carrier has charge
and a drift velocity
.
Electrical signals travel at speeds about
times greater than the drift velocity of free electrons.
Conceptual questions
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 (
). 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
, by considering how the density of charge carriers
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.
Problems&Exercises
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?
0.278 mA
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
moves from your finger to a metal doorknob in
?
0.250 A
Find the current when 2.00 nC jumps between your comb and hair over a
time interval.
A large lightning bolt had a 20,000-A current and moved 30.0 C of charge. What was its duration?
1.50ms
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:
.)
The capacitor in a defibrillation unit drives a current through the heart of a patient.
(a)
(b) If a 50 times larger resistance existed, keeping the current about the same, the power would be increased by a factor of about 50 (based on the equation
), causing much more energy to be transferred to the skin, which could cause serious burns. The gel used reduces the resistance, and therefore reduces the power transferred to the skin.
During open-heart surgery, a defibrillator can be used to bring a patient out of cardiac arrest. The resistance of the path is
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) 0.120 C
(b)
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 (
) of electrons at this rate?
96.3 s
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
nuclei onto a target with a beam current of 0.250 mA. (a) How many
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
nuclei strike the target?
(a)
(b)
(c)
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?
Electrons circulating in the storage ring called SPEAR constitute a 20.0-A current. Because they travel close to the speed of light, each electron completes many orbits in each second.
Questions & Answers
A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
