Advice 1: What is the contact resistance

The contact connection part of the electrical circuit and are crucial elements. The state of the electrical contact connections depends trouble-free operation of electrical wiring and electrical equipment. An important value of the transitional contact resistance.
The transitional contact resistance


In the electric circuit, between two or more conductors creates an electric transition contact, or a conducting connection by which current flows from one part to another. With a simple overlay of the contacted surface of the connected conductors not give good contact. The real contact area is several times smaller than the whole contact surface , confirming what can be seen with a microscope.

Due to the small contact area between the contact connection offers a highly visible resistance to the passage of current from one surface to another and is called the transitional contact resistance. Itself the transitional contact resistance a priori more than the resistance of a solid conductor of the same shape and size.

Factors affecting the value of the transition resistance

The resistance of the contact zone does not depend on the size of the contact surfaces and is determined by the pressure or force of the contact pressing. Contact with the press is called the effort one contact surface acting on the other. In General, the magnitude of the pressing force and the strength of the contact material will depend on the total area of contact. The number of contacts in contact always grows when clicked.

At low pressures plastic deformation occurs in the contact, with the tops of the protrusions collapse and then, with increasing pressure, new points come into contact. As a result, the pressure should be large enough to ensure a small contact resistance, but should not result in plastic deformations in the metal contact, leading to its destruction.

The contact resistance largely depends on the degree of oxidation of the contact surfaces of the connected conductors. Regardless of the material of the conductor film of the oxide creates a greater electrical resistance.
The intensity of oxidation of the conductors depends on the contact temperature and the faster it is, the more the contact resistance.
Very much affected by the oxidation of aluminum conductors. For example, formed on the air their the oxide film has a resistivity in the 1012 Ohm*cm.

Since the properties of the contact connection can change. Only new, well-processed and stripped of the transitional contact may have the least probable transitional contact resistance with enough pressure.

During the formation of contact connections use different ways of bonding conductors. For example, a spike, welding, crimping, mechanical connection with the bolts, and put in contact with the elastic pressing of the springs.

In fact, if any method of wiring it is possible to achieve a consistently small transitional contact resistance. It is important to connect the wires strictly according to the technology and using each method of connection of wires of required tools and materials.
The contact connection of electrochemically incompatible conductors is the contact between two oxides, which will have a high value of the transition resistance.

In order to decrease the transition contact resistance to take into account all the above factors affecting its value and conduct the correspondence of types of the connecting contacts of the conductor materials and operating conditions.

Advice 2: How to find force of air resistance

To determine the force of resistance of air create the conditions in which the body begins under the action of gravity to move uniformly. Calculate the value of gravity, it is equal to the force of air resistance. If the body moves in the air, picking up speed, the force of his resistance is by using Newton's laws, the force of air resistance can be found from the law of conservation of mechanical energy and special aerodynamic formulas.
How to find force of air resistance
You will need
  • rangefinder, scale, speedometer or radar, ruler, stopwatch.
The definition of air resistance a falling body uniformly Measure body weight using a scale. Dropping it from some height, make sure it is moving evenly. Multiply body weight in kg by the acceleration due to gravity, (9.81 m/S2), the result is the force of gravity acting on the body. And since it moves uniformly and rectilinearly, gravity will be equal to the force of air resistance.
The definition of air resistance the body will skorostrel gaining body mass by using weights. After the body began to move, using a speedometer or radar measure its instantaneous initial velocity. In the end, measure its instantaneous final velocity. Speed is measure in metres per second. If you measure it in kilometers per hour, divide the value by 3.6. In parallel to using the stopwatch to measure the time in which occurred the change. Subtracting the final speed from the start and dividing the result by the time, find the acceleration with which the body moves. Then find the force that causes a body to change velocity. If the body falls, then it is gravity, if the body moves horizontally, the thrust of the engine. From this power subtract the weight of a body on its acceleration (Fc=F+m•a). This will be the force of air resistance. It is important that the body did not touch the ground, for example, moving on an air cushion or fallen down.
Determination of air resistance to a body falling from vicotiasecret body weight and drop it from a height, which is known in advance. Upon contact with the surface of the earth clamp the velocity of the body using a speedometer or radar. Then find the product of the gravitational acceleration 9.81 m/S2 on the height from which the falling body, subtract from this value the speed squared. The result, multiply by body weight and divide by the height from which it fell (Fc=m•(9,81•H-v2)/H). This will be the force of air resistance.
Useful advice
In the General case the resistance force can be found if the density of the air multiplied by the square of the velocity with which the body moves, divide this value by 2 and multiplied by the cross-sectional area of the body perpendicular to the direction of the velocity. The result must be multiplied by the aerodynamic coefficient, which is determined experimentally. Therefore, the application of this formula difficult.

Advice 3: How to measure the internal resistance of the battery

Any current source has a certain internal resistance. It is involved in limiting the current through the load along with the resistance of the load. In order to get it, will have to measure the voltage at the source under different loads, and then produce a simple calculation.
How to measure the internal resistance of the battery
Fully charge the battery.
Take two loads. Each of them must load the battery so current that it does not exceed the maximum permissible for him. One of the loads needs to consume current approximately 30 percent of the maximum allowable long-term (not short!) for the battery, and the other 70 percent of it. Very convenient to use low-voltage incandescent lamps. They should be designed for a voltage somewhat greater than the EMF of the battery (the voltage at its terminals at no load). If you use a powerful lamp, secured in such a manner that it cannot be contact with any body parts and any flammable objects.
Connect the battery to the first load through the ammeter and in parallel to the battery, attach the voltmeter. Both devices connect to the correct polarity. Wait for transients lasting several seconds. Measure the current through the load and voltage of the battery. Write them down.
Disassemble the chain, then similarly connect to the battery instead of the first second load. Also record the results. In both cases, the measurements were performed quickly (excluding the time required for the completion of transient processes) to battery turned.
If the measurements are not expressed in SI units (e.g., the battery is thin and the currents through the load is expressed in milliamperes), translate them into this system.
Subtract the first voltage from the second and the second current from the first. The result of the first subtraction divide by the result of the second subtraction. Get the internal resistance of the battery, expressed in ohms.
Note that the internal resistance of the battery increases as the discharge and wear. Specially to wear it, perhaps not. But one cycle of the discharge (to a voltage slightly higher than the minimum safe for him) pass. At several points in this cycle, momentarily disconnecting the battery from the main discharge circuit, measure its internal resistance using the above method. Make a curve of internal resistance to the degree of discharge, expressed in percentage.
Never exceed the maximum allowed discharge current of the battery, the more short it's short.
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