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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   


G = I/V


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

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.

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

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

Sled traveling down the slope

A sled is being held at rest on a slope that makes an angle theta with the horizontal. After the sled is released, it slides a distance d1 down the slope and then covers the distance d2 along the horizontal terrain before stopping. Which assumptions are reasonable to make in this problem? There can be more than one answer. A)

BUNGEE JUMPING,Problem is SEEMS long but it is not

Kate, a bungee jumper, wants to jump off the edge of a bridge that spans a river below. Kate has a mass m, and the surface of the bridge is a height h above the water. The bungee cord will first straighten and then stretch as Kate falls. Assume that the bungee cord behaves as an ideal spring once it begins to stretch. Neglect th

Capacitance of a Capacitor

In a heart pacemaker, a pulse is delivered to the heart 81 times per minute. The capacitor that controls this pulsing rate discharges through a resistance of 1.8e6 ohms. One pulse is delivered every time the fully charged capacitor loses 62.9 % of its original charge. What is the capacitance of the capacitor? Answer in farads.

Vectors, velocity & mass: Speed of six model rockets

Six model rockets (ABCDEF) have just had their engines turned off. All of the rockets are aimed at the same angle, but their speeds differ. All of the rockets are the same size and shape, but they carry different loads, so their masses differ. (Their respective masses are listed below.) At the instant when the engines are turned

Vectors/time in motion

"An artillery shell is fired with an initial speed of 1.80x10^3 m/s at a angle of 54.3 degrees above the horizontal and returns to its original level before impact. Neglecting air resistance, calculate the time it is in motion. And calculate the horizontal distance traveled." Thank you!

Find the maximum height of a cannonball fired at an angle from the ground.

A battalion of soldiers aims their cannon at an angle of 30° up from horizontal and fires it as shown below in a large, level field. The initial speed of the cannon ball is 196 m/s. Neglect air resistance and assume that the cannon ball starts at ground level for simplicity. Use g = 9.8 m/s2. What is the maximum height reac