I Electricity is distributed from electrical substations to neighborhoods at 15,000 V. This is a oscillating voltage. Neighborhood transformers, seen on utility poles, step this voltage down to the that is delivered to your house.
a. How many turns does the primary coil on the transformer have if the secondary coil has 100 turns?
b. No energy is lost in an ideal transformer, so the output power from the secondary coil equals the input power to the primary coil. Suppose a neighborhood transformer delivers at. What is the current in the line from the substation?
(a)there are turns does the primary coil
(b)The current in the 15000V line is .
(a) In order to step down the voltage, the primary coil must have more coils than the secondary coil. Faraday's law states that the number of turns N is directly proportional to the induced emf. We combine the number of turns and the induced emf for the two coils using equation (30.30)in the form
Where and are the emf and the number of turns for the primary coil while and are the emf and the number of turns for the secondary coil. We rearrange equation (l) for to be in the form
Now, we plug the values for and into equation (2) to get
(b) When a current flows through a resistor, it dissipates energy; the rate of energy dissipation is the power. The process by which energy is transferred from the current to the resistor is
The output power is at the secondary coil where and the voltage is . So, we get the output power by
Let us use equation
(3) to get
FIGURE shows a bar magnet, a coil of wire, and a current meter. Is the current through the meter right to left, left to right, or zero for the following circumstances? Explain.
a. The magnet is inserted into the coil.
b. The magnet is held at rest inside the coil.
c. The magnet is withdrawn from the left side of the coil.
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