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Electric doorbells found in many homes require \(10.0 \mathrm{~V}\) to operate. To obtain this voltage from the standard 120-V supply, a transformer is used. Is a step-up or a stepdown transformer needed, and what is its turns ratio \(N_{\mathrm{s}} / N_{\mathrm{p}}\) ?

Short Answer

Expert verified
A step-down transformer with a turns ratio of 1/12 is needed.

Step by step solution

01

Identify Transformer Type

Determine whether a step-up or step-down transformer is needed based on the voltages given. A step-down transformer is used when the output voltage is lower than the input voltage. Since the doorbell requires 10.0 V from a 120 V supply, a step-down transformer is needed.
02

Understand the Transformer Formula

Transformers relate the primary voltage (V_{p}) and secondary voltage (V_{s}) using the turns ratio (N_{s} / N_{p}). The formula is: \[\frac{V_{s}}{V_{p}} = \frac{N_{s}}{N_{p}}\]where V_{s}is the secondary voltage, and V_{p}is the primary voltage.
03

Substitute Known Values

Insert the known voltage values into the transformer equation. Given that V_{p}=120 \text{V} and V_{s}=10.0 \text{V}, the equation becomes:\[\frac{10.0}{120} = \frac{N_{s}}{N_{p}}\]
04

Solve for Turns Ratio

Simplify the equation to find the ratio of the number of turns in the secondary coil (N_{s}) to the number of turns in the primary coil (N_{p}). Solving \[\frac{10}{120}=\frac{1}{12}\]gives the turns ratio as:\[N_{s} / N_{p} = 1 / 12\]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Step-Down Transformer
A step-down transformer is an essential component used when you need to reduce voltage from a higher value to a lower one. It's typically used in scenarios where devices require less power than the supply voltage provides. In our example with the doorbell needing 10 volts from a 120-volt source, a step-down transformer is necessary. It takes the higher primary voltage and scales it down to the desired secondary voltage. This transformation ensures that your device can operate safely and efficiently without risking damage from too much voltage. Stepping down voltage is crucial for anything from doorbells to chargers for electronics, ensuring that the supplied voltage matches the device’s requirement.
Turns Ratio
The turns ratio is a fundamental concept in understanding how transformers work. This ratio, expressed as \(N_{s}/N_{p}\), refers to the number of wire coils or "turns" in the secondary coil (\(N_s\)) compared to the primary coil (\(N_p\)). The turns ratio directly affects how the voltages transform between the input and output, following the equation \(\frac{N_s}{N_p} = \frac{V_s}{V_p}\). For a step-down transformer, this ratio is less than one, such as the 1:12 ratio in our example. This means that for every 12 turns in the primary coil, there is just 1 turn in the secondary coil, which decreases the voltage by a factor of 12. Knowing the turns ratio allows you to manage and design the transformer’s effectiveness in delivering the correct voltage.
Voltage Transformation
Voltage transformation is at the heart of what transformers do. It involves changing the voltage from one level to another while maintaining the same power, assuming ideal conditions. The relationship between the input voltage (\(V_p\)) and the output voltage (\(V_s\)) is determined using the aforementioned turns ratio. The principle of voltage transformation is crucial in electricity distribution, allowing for the safe and efficient delivery of power across various distances. In the case of the doorbell example, this principle enables the downscaling of 120 volts to a manageable 10 volts, making the operation of the device possible without exceeding its electrical limits.
Electric Circuits
Electric circuits form the backbone of all powered devices, allowing electricity to flow through components and perform work. In circuits containing transformers, the primary circuit is connected to the incoming current while the secondary circuit connects to the device that needs the adjusted voltage. Each circuit has its voltage requirements managed by using transformers to either step up or step down the voltage as needed. These circuits transform and control electricity in homes and electric devices. Properly functioning electric circuits ensure that your devices receive the appropriate level of power, preventing damage and optimizing performance. Understanding how electric circuits, in conjunction with transformers, manage voltage is essential in both designing and troubleshooting electrical systems.

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Most popular questions from this chapter

The armature of an electric drill motor has a resistance of \(15.0 \Omega\). When connected to a 120.0-V outlet, the motor rotates at its normal speed and develops a back emf of \(108 \mathrm{~V}\). (a) What is the current through the motor? (b) If the armature freezes up due to a lack of lubrication in the bearings and can no longer rotate, what is the current in the stationary armature? (c) What is the current when the motor runs at only half speed?

Concept Questions The rechargeable batteries for a laptop computer need a much smaller voltage than what a wall socket provides. Therefore, a transformer is plugged into the wall socket and produces the necessary voltage for charging the batteries. (a) Is the transformer a step-up or a step-down transformer? (b) Is the current that goes through the batteries greater than, equal to, or smaller than the current coming from the wall socket? (c) If the transformer has a negligible resistance, is the electric power delivered to the batteries greater than, equal to, or less than the power coming from the wall socket? In all cases, provide a reason for your answer. Problem The batteries of a laptop computer are rated at \(9.0 \mathrm{~V},\) and a current of \(225 \mathrm{~mA}\) is used to charge them. The wall socket provides a voltage of \(120 \mathrm{~V}\). (a) Determine the turns ratio of the transformer, (b) What is the current coming from the wall socket? (c) Find the power delivered by the wall socket and the power sent to the batteries. Be sure your answers are consistent with your answers to the Concept Questions.

Mutual induction can be used as the basis for a metal detector. A typical setup uses two large coils that are parallel to each other and have a common axis. Because of mutual induction, the ac generator connected to the primary coil causes an emf of \(0.46 \mathrm{~V}\) to be induced in the secondary coil. When someone without metal objects walks through the coils, the mutual inductance and, thus, the induced emf do not change much. But when a person carrying a hand gun walks through, the mutual inductance increases. The change in emf can be used to trigger an alarm. If the mutual inductance increases by a factor of three, find the new value of the induced emf.

A step-down transformer (turns ratio \(=1: 8\) ) is used with an electric train to reduce the voltage from the wall receptacle to a value needed to operate the train. When the train is running, the current in the secondary coil is \(1.6 \mathrm{~A}\). What is the current in the primary coil?

In 1996 , NASA performed an experiment called the Tethered Satellite experiment. In this experiment a \(2.0 \times 10^{4}-\mathrm{m}\) length of wire was let out by the space shuttle Atlantis to generate a motional emf. The shuttle had an orbital speed of \(7.6 \times 10^{3} \mathrm{~m} / \mathrm{s},\) and the magnitude of the earth's magnetic field at the location of the wire was \(5.1 \times 10^{-5} \mathrm{~T}\). If the wire had moved perpendicular to the earth's magnetic field, what would have been the motional emf generated between the ends of the wire?

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