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Resistance

The resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor. Resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm (Ω).

An object of uniform cross section has a resistance proportional to its resistivity and length and inversely proportional to its cross-sectional area. All materials, aside from super conductors, show some resistance.  The resistance of an object can be defined as

R = V/I   

Or

G = I/V

Where

R is the resistance

V is the voltage across

I is the current through

G is the conductance

For most materials and conditions, V and I are directly proportional to each other, and therefore R and G are constant. This proportionality is called Ohm’s law.

There are some cases where V and I are not directly proportional, such as a diode or battery. The I-V curve is not a straight line through the origin and Ohm’s law does not hold. In this instances, resistance and conductance are less useful concepts and more difficult to define. The ration V/I is however sometimes still useful. It refers to a chordal resistance or static resistance as it corresponds to the inverse slope of a chord between the origin and an I-V curve. Sometimes the derivative dV/dI is the most useful. This derivative is called the differential resistance. 

Categories within Resistance

Ohm's Law

Postings: 111

Ohm's law states that the current through a conductor between two points is proportional to the potential difference across the points.

The current in a diode is given as a function of voltage

Q- The current I as a function of voltage V for a certain diode is given by I(V)=I_0 (e^(eV/kT) - 1) Here k is Boltzmann's constant, e is the charge on an electron (magnitude), and T is the absolute temperature. Let at room temperature (t = 20 degree C), I_0 = 1.5 × 10^(-9)A. (A) What is the resistance of the diode for

Force of the Box, Neglecting Friction and Air Resistance

Problem 1: A box is sliding down a plane inclined at an angle of 20 degrees from the horizontal. Find the acceleration of the box, neglecting friction and air resistance. Possible answers: (1) 0.342 m^2g (2) 0.342 m (3) 0.342 g (4) 0.342 mg

Kinematics Question: Boy kicks a ball horizontally: Speed, Height

A boy kicks a ball horizontally near the edge of a boardwalk, with an initial speed of 9.0 m/s. A blowing wind gives the ball a constant horizontal acceleration of 12 m/s^2. The ball falls into the water directly under the boy. Ignore the effect of air resistance on the vertical motion of the ball. a. Determine the height

Direction of the resultant force of a ball on a track

** Please see the attached file for the complete problem description ** 1. After an initial impulse, a ball moves along a track as shown and then moves through the air, following the path shown in the diagram. (please see the attached file for the diagram) Copy the diagram and draw arrows to indicate the direction of the

Initial Speed and Height

Calculate the initial speed or height in this instance. An egg is thrown nearly vertically upward from a point near the cornice of a tall building. It just misses the cornice on the way down and passes a point a distance 50.0 m below its starting point at a time 5.00 s after it leaves the thrower's hand. Air resistance may b

Football go above the ground when kicked vertically upward

9. A football is kicked vertically upward from the ground and a student gazing out of the window sees it moving upward past her at 5.00 m/s. The window is 12.0 m above the ground. You may ignore air resistance. a) How high does the football go above the ground? b) How much time does it take to go from the ground to its hi

Current Electricity: Resistance and Resistivity Problem

Please solve and explain problem. Part 1 A 42.9g sample of a conducting material is all that is available. The resistivity of the material is measured to be 1.07E-7 ohm* m and the density is 7.76g/ cm3. The material is to be shaped into a solid cylindrical wire that has a total resistance of 1.87 ohm. What length is requir

Laws of Motion: Determining Height and Distance

A baseball is hit off the edge of a cliff horizontally at a speed of 30 m/s. It takes the ball 3 seconds to reach the ground, with no air resistance. a) How far from the cliff wall does the ball land? b) How high is the cliff wall?

Current Electricity: Resistance and Resistivity

Stretch That Wire A wire of initial length L0 and radius r0 has a measured resistance of 1.56 ohm. The wire is drawn under tensile stress to a new uniform radius of r = 0.420r0. What is the new resistance of the wire?

Resistance in Copper and Tungsten Wires

Can a copper wire and a tungsten wire of the same length have the same resistance? (a) yes, but only if the tungsten wire has a greater cross-sectional area (b) yes, but only if the copper wire has a greater cross-sectional area (c) yes, under any circumstances

Air pressure in a closed tank & speed of water at nozzle

See attached .gif file. In a very large closed tank, the absolute pressure of the air above the water is 5.46 x 105 Pa. The water leaves the bottom of the tank through a nozzle that is directed straight upward. The opening of the nozzle is 9.91 m below the surface of the water. (a) Find the speed at which the water leaves

Rank each object's speed when they hit bottom of their fall or ramp.

Consider 3 situations. 1.) An object, with mass m is held over a vertical drop of height, h. It is then dropped. 2.) An object, with mass m is at the top of a ramp with ramp length, 2h and vertical height, h. It is then allowed to slide down the slope. 3.) An object, with mass 2m is at the top of a ramp with ramp length,

Missing Kinetic Energy

In the absence of air resistance, a falling rock gains kinetic energy and loses potential energy, with the total energy of the rock remaining constant. In the presence of air resistance, however, the rock eventually reaches terminal velocity. Now the kinetic energy is constant, but the potential energy continues to decrease as t

Distance of a ski jumper

A skier leaves the ramp of a ski jump with a velocity of 11.9 m/s, 17.5?above the horizontal. The slope is inclined at 48.1? and air resistance is negligible. Calculate the distance from the ramp to where the jumper lands.

Fabricate a Uniform Wire Out of Copper

Suppose that you wish to fabricate a uniform wire out of 0.850 g of copper. The wire is to have a resistance of R = 0.700, and all of the copper is to be used. (a) What is the length of this wire? m (b) What is the diameter of this wire? µm

Mechanics: Vertical projection with air resistance

A particle is projected vertically upward in a constant gravitational field with an initial speed of v_0. Show that if there is a retarding force proportional to the square of the instantaneous speed, the speed of the particle when it returns to the initial position is: (v_0*v_t)/[(v_0)^2 + (v_t)^2]^1/2 where v

Physics: Projectile motion problem

In this question, use g = 10m*s^-2. In cricket, a fast bowler projects a ball at 40m*s^-1 from a point h m above the ground, which is horizontal, and at an angle alpha above the horizontal. The trajectory is such that the ball will strike the stumps at ground level a horizontal distance of 20 m from the point of projection.

Net Force acting on a ball

1) A ball is thrown up into the air. At the highest point in its trajectory, the net force acting on it is equal to its weight. greater than its weight. less than its weight, but not zero N. zero N. 2) A ball is thrown up into the air. Ignore air resistance. When it is rising and reaches half of it

The Thrust and the Lift Reduced by the Pilot

A pilot suddenly jettisons 2800 N of fuel. If the plane is to continue moving with the same velocity under the influence of the same air resistance R, by how much does the pilot have to reduce: a) the thrust, and b) the lift?

Lift Force and Resistance on Helicopter at Constant Velocity

A helicopter is moving horizontally to the right at a constant velocity. The weight of the helicopter is W= 53800 N. The lift force L generated by the rotating blade makes an angle of 21.0 degrees with respect to the vertical. (a.) what is the magnitude of the lift force? (b) What is the magnitude of the air resistance R that op

Projectile Motion and Ball Motion

A student throws a ball from the roof of his house at an angle of 20 degrees below the horizontal with an initial speed of 10.0 m/s. If the roof is 8.0m above the ground, how far horizontally from the point where it was thrown will the ball hit the ground? (Ignore air resistance).

Projectile Motion with Air Drag

I need help with these two questions: 2.12. A gun is fired straight up. Assuming that the air drag on the bullet varies quadratically with speed, show that the speed varies with height according to the equations: v^2 = Ae^-2kx - g/k (upward motion) v^2 = g/k - Be^2kx (downward motion) in which A and B are constants of integ

Differential Equation for Mechanical System

An electrophysiological recording table is subject to floor vibrations. Often each leg of such a table is placed on a "damper" consisting of a dashpot and spring in parallel. Assume that a dashpot and a spring are placed under each leg of the recording table, which has total mass M, so that the combined damping resistance is R a

Straight Motion: watermelon dropped from roof sound splash

A physics student with too much free time drops a watermelon from the roof of a building. He hears the sound of the watermelon going "splat" after a time interval of delta (change) t. You may ignore air resistance. How high is the building? The speed of sound is vs . Take the free fall acceleration to be g.

Motion Along Straight-Line

In the vertical jump, an athlete starts from a crouch and jumps upward to reach as high as possible. Even the best athletes spend little more than 1.00 s in the air (their "hang time"). Treat the athlete as a particle and let ymax be his maximum height above the floor. To explain why he seems to hang in the air, calculate th

Distance/velocity physics problems

Take g=9.8m s(-2) for all these questions: A tile slides off a roof pitched at an angle of 30 degrees to the horizontal. The distance from the roof to the ground (flat) is 12m. If the speed of the tile is 8m s(-1) when it leaves the roof find: Please answer all the following questions with full explanations: 1) The hori

The maximum height a projectile will reach.

A projectile is launched with an initial velocity of 2.8 m/s at an angle of 30 degrees above the ground. Assume there is no air resistance and that the acceleration due to gravity is g = 9.81 m/s^2. ??What is the maximum height the projectile will reach???

Oscillatory motion

A 10.6 kg object oscillates at the end of a vertical spring that has a spring constant of 2.05 * 10^4 N/m. The effect of air resistance is represented by the damping coefficient b= 3.00 Ns/m. (a) Calculate the frequency of the damped oscillation. (b) By what percentage does the amplitude of the oscillation decrease in each cycl

Movement of a Canoe - Problem Involving Center of Mass

A 45.0-kg woman stands up in a 60.0-kg canoe of length 5.00 m. She walks from a point 1.00 m from one end to a point 1.00 m from the other end. If you ignore resistance to motion of the canoe in the water, how far does the canoe move during this process?