The aim of the experiment is to verify the maximum power theorem and investigate the efficiency with which energy is transferred from a source of e.m.f to a load resistance.

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Experiment 8: Energy Transfer in a D.C. Circuit

Objectives:

    The aim of the experiment is to verify the maximum power theorem and investigate the efficiency with which energy is transferred from a source of e.m.f to a load resistance.

Experimental Design

Apparatus:

Setup:

Description of design:

    In this experiment, we will verify the maximum power theorem by using the above circuit. When we vary the resistance of the resistance box provided, the potential differences across the resistance box and internal resistor, hence current drawn from the dry cell also is varied, so the ammeter can measure the current flowing in the circuit at different equivalent resistance of the circuit.

Theory:

    Friction is a very common and important force in our daily life. Although friction may disturb our motion, many movements of our human also need the help of the friction. For example, friction between our shoes and the ground helps us to walk and the friction between the wheels and the ground also helps the car to move. So friction is essential for the motions in our daily life.

    Friction always opposes the motion performed by any object. It forms when two surfaces are in contact. It increases as the other forces tending to produce the motion increase, however, it has its maximum value. When an object is in contact with a rough surface, friction is formed between the two contact surfaces. As the applied increases, the static friction also increases. When the applied force is equal to the maximum magnitude of the static friction, it will move. If an object can move over the rough surface, kinetic friction will exist between the surfaces, which is slightly smaller than the static friction.

    In the above cases, we know that friction can be mainly divided into two types, which are static friction and kinetic friction. Static friction occurs when two surfaces are in contact but without any relative motion. It always acts in a direction along the surface and tends to oppose the relative motion. The magnitude of static friction increases with the applied force to oppose the star of motion, until reaches a certain value called limiting friction. Therefore, the applied force must be same as or larger than the limiting friction in order to make the object to move. To find out the static friction, we can make use of the following formula:

fL = µs R   where µs = coefficient of static friction.

As the coefficient of static friction is constant, by the formula of the static friction, the magnitude of the static friction is direct proportional to the normal reaction force. The magnitude also depends on the roughness of the surfaces and the normal force between the two contact surfaces.

    Kinetic friction is another type of the friction. It occurs when two surfaces are in contact with relative motion such sliding and moving. The kinetic friction is always opposite to the velocity of the body. However, it is independent of the magnitude of the velocity of the object. Therefore, the magnitude of the kinetic friction is constant. The kinetic friction also cannot be represented as below:

fk = μk R   where µk = coefficient of kinetic friction.

From the formula, we can know that the magnitude of the kinetic friction is direct proportional to the normal reaction force as the coefficient of kinetic friction is constant. The factors that can affect the kinetic friction are same as that of the static friction.

    Moreover, the magnitude of the static friction usually is slight larger than that of the kinetic friction. So the relationship between the magnitudes of friction against that of the applied force has shown below:

Part A: Frictional force and applied force

Procedures

  1. All wooden blocks were weighed by the beam balance and labeled with numbers.
  2. The scale of the spring balance was set properly to zero.
  3. The sand paper was placed on the table.
  4. A wooden block was placed on the table.
  5. The wooden block was connected to the spring balance in series.
  6. The block was pulled slowly.
  7. Five readings had been recorded before the block moved.
  8. The block was pulled in a constant velocity after the block moved.
  9. Another five readings had been recorded from the spring balance.

Precautions

    In this experiment, we should keep the spring balance in horizontal position in order to ensure that only the applied force support the motion of the wooden block. If it is not in horizontal position, horizontal component of the applied force will support motion. Secondly, we should keep the wooden block that it is moving in constant velocity as much as possible; otherwise, the magnitudes of the applied force will be different from time to time. Moreover, the sand paper may make the wooden surface become smoother, and hence the static and kinetic friction may be different from the original one. So the numbers of pulling process should be minimized. On top of the above precautions, the spring balance is not connected to the wooden block firmly; it may be disconnected during the pulling process. Therefore, we should pay attention to the connection point between the spring balance and the wooden block during the experiment.

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In addition, beam balance is necessary in the experiment for measuring the masses, which have to be hung at the hanger. Except we have to know how to use the beam balance correctly, we should know how to minimize depletion of the balance. When we do not use the beam balance, we have to move one of masses to the right-hand sides.

Results & Calculations

Reading taken before the block begins to move:

Reading taken after the block is moving:

In the following calculations, we have made several assumptions:

  1. Air resistance is neglect
  2. ...

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