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.

The kinetic friction: 1.2N ± 0.05 N

Coefficient of static friction = static friction / normal reaction force

                       = 1.4 / (0.194*10)

                       = 0.722 (Cor. to 3 sig. Fig.)

The uncertainty = 0.05/1.4 + 0.0005/0.194 = 0.0383 (Cor. to 3 sig. Fig.)

The percentage error = 0.0383 * 100% = 3.83%

So, the coefficient of static friction = 0.722 ± 0.0383

Coefficient of kinetic friction = kinetic friction / normal reaction force

                       = 1.2 / (0.194*10)

                       = 0.619 (Cor. to 3 sig. Fig.)

The uncertainty = 0.05/1.2 + 0.0005/0.194 = 0.0442 (Cor. to 3 sig. Fig.)

The percentage error = 0.0442 * 100% = 4.42%

So, the coefficient of kinetic friction = 0.619 ± 0.0442

Part B: Coefficient of friction for various masses

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 until the block was moving.
7. The reading of the static friction was recorded.
8. The block was pulled to move in constant velocity.
9. The reading of the kinetic friction also was recorded.
10. Step 6 and 7 were repeated except increasing the numbers of the blocks that were held by the rubber band.

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.

   In part A, we can find out the relationship between the friction and the applied force. When the applied force increases, the static friction also increases and it is direct proportional to the applied force which can be seen from the graph. So it can balance the applied force to make the block remain at rest. However, when the applied force further increases and overcome the maximum limiting friction which is the peak of the graph. After reaching the limiting friction, the graph drops and levels off. It shows that the block obeys the Newton’s first law. The applied force is same as the kinetic friction but in opposite direction. From graph, it shows that the static friction is smaller than the kinetic friction.

   In part B, we can study the effect of normal reaction force on the static and kinetic friction and estimate the coefficient of static and kinetic friction from the graph, From the graph plotted, we can conclude that the normal reaction force is direct proportional to the static and kinetic friction from the graph. We also can estimate the coefficient of the static and kinetic friction by finding out the slope of the graphs. So the coefficients of static friction and the kinetic friction are 0.727 ± 0.033 and 0.574 ± 0.050 respectively.

    In part C, we study the effect of contact surface on the friction. From the experimental result, we can conclude that the size of the contact surface does not cause significant change to the kinetic friction. The difference between the ways of placing the blocks is caused by the experimental errors discussed above. However, we can further investigate how the nature of the contact surface affects the kinetic friction such the metal surface.

References

Wikipedia (Friction)

http://en.wikipedia.org/wiki/Friction

New Way Physics for advanced level Mechanics