Physics Unit 14.2+3: A.C. Generator, Transformer

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Unit 14.2: A.C. Generator

1. Describe a rotating-coil generator and the use of slip rings

A little terminology to learn first:
A.C. = alternating current. This means instead of the current flowing in one direction, it flows back and forth.

Here's an example of an A.C. generator

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Between the magnet is a magnetic field. The wire loop is made of insulated copper wire and is turned. As it turns, it cuts those magnetic field lines, inducing current and generating electricity that can power a bulb if it is attached to the generator. 

The slip-rings are connected to the coil of wire, while the carbon brushes are connected to an external circuit. The brushes are constantly in contact with the slip-rings so that the current can flow from the coil to the external circuit. 

You can see that the magnetic field lines run horizontally. If the coil is also laid horizontally, it lays in line with the field lines and don't cut them, so no current is induced. When the coil is rotated, it begins to cut the field lines at angles. At 45 degrees, a small current produced; at 90 degrees, a larger current is produced. 

2. Sketch a graph of voltage output against time for a simple a.c. generator.

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This diagram further elaborates on the 45° and 90° explanation from above.

Unit 14.3: The Magnetic Effects of a Current

1. Describe the construction of a basic iron-cored transformer as used for voltage transformations.

A transformer is an electrical device that changes the voltage of an A.C. current supply. For some devices, only a small voltage is needed to charge it. A transformer helps decrease the high voltage from the electrical source so when charged, the device will not break. This transformer is called a step-down transformer as it is stepping down the voltage. This can be reversed and we will have a step-up transformer.

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An electric current is passed through the primary coil of wire. The magnetic field from the primary wire grows, collapses, turns (because it's an a.c. current), grows, collapses; those field lines cut the secondary wires, inducing electricity. The diagram above shows a step-down transformer as the secondary wire has less coils. Therefore, the output voltage will be smaller. The more coils present, the stronger the current will be. 

2. Recall and use the equation (VP / VS) = (NP / NS). 

V stands for the voltage while N stands for the number of turns. The voltage and number of turns are proportional, so the factor affecting one side of the equation will also affect the other. Below is an example:

3. Describe the use of the transformer in high-voltage transmission of electricity.

The National Grid is the nation's power supply. It transfers electricity from the Grid to homes for use. When a current is passed through those the wires of the Grid, heat is lost. Since it needs to supply electricity for many different places, a high current is needed; however this results in a lot of energy being lost as thermal energy. Instead, the Grid transmits electricity at a low current to reduce heat loss. A high voltage is required in order for this to work. Since a high voltage is dangerous to use in homes, transformers are used to step down this power supply, making it safe to use.

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4. Recall and use the equation Vp / Ip = Vs / Is (for 100% efficiency).

We know that voltage divided by current gives power, so this equation tells us the primary power is the same as the secondary power. With this equation you can determine how much power goes through one end of the transformer and how much goes out, since it is assumed that the transformer is 100% efficient and does not waste any energy.

5. Explain why energy losses in cables are lower when the voltage is high. 

As said above, energy is lost as heat when a current is run through a wire. From the power equation we know that voltage and current are proportional to make the final power product. If the voltage is high, this means the current is low, resulting in less power loss.