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Direct Current Circuits

Beauty of DC Circuits Beauty of DC circuits LEDs(Light Emitting Diodes) are the light sources which require a direct current to glow. These are energy efficient and low cost. The lights in the above image must be connected in a particular fashion. Suppose one of the light fails to glow, then what happens to the rest of the lights, will they continue to glow? Why are the resistors used before the LEDs? What is the maximum current that the device can bear? Lets learn more about these concepts in this topic.

Learning objectives

After completing the topic, the student will be able to:

  • Understand the direction of flow of electrons in a circuit and explore the advantages and disadvantages of series circuits in comparison with parallel circuits.
  • Determine the voltage and current of any given circuit at any point based on the Kirchhoff's laws.
  • Distinguish between meter bridge and Wheatstone bridge based on their properties and determine the value of unknown resistance.
  • Discuss and understand the working of a potentiometer and combinations of capacitors along with its advantages and applications in electrical circuits.
  • Discover the working of RC circuits and its relevance in bio-medical applications.
  • Understand the importance of grounding of electrical circuits and explore how fuses protect the expensive electronic devices of the household.
Series Combination Series circuit A series circuit has more than one element (anything that uses electricity to do work) and gets its name from having only one path for the charges to move along. Charges must move in series first going to one element, then the next. If one of the elements in the circuit is broken, then no charge will move through the circuit because there is only one path. There is no alternative route.
Series circuit

Consider three bulbs connected one after the other. If you look closely inside of the bulb, you will notice that it has a tungsten wire coiled on two metal connectors. This tungsten coil provides the resistance when current is passed through the bulb. Due to the resistance, the tungsten coil heats up and emits light. This is an example of a simple series circuit. The 6 V battery provides 2 V across each lamp. Three bulbs are connected in series with a battery. The same current exists almost immediately in all three lamps when the switch is closed. The charge does not "pile up" in any lamp but flows through each lamp.

Electrons in all parts of the circuit begin to move at once. Some electrons move away from the negative terminal of the battery, some move toward the positive terminal, some move through the filament of each lamp. Eventually the electrons move all the way around the circuit (the same amount of current passes through the battery). This is the only path of the electrons through the circuit. A break anywhere in the path results in an open circuit, and the flow of electrons ceases. Burning out one of the lamp filaments or simply opening the switch could cause such a break.

Series Combination Series circuit A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take. The current is the same through each resistor. The total resistance is found by adding the resistance values of each resistor.
Characteristics of series circuit

The circuit shown illustrates the following important characteristics of series connections:

  • Electric current has but a single pathway through the circuit. This means that the current passing through each electrical device is the same.
  • This current is resisted by the resistance of the first device, the resistance of the second, and that of the third also, so that the total resistance to current in the circuit is the sum of the individual resistances along the circuit path.
  • The current in the circuit is numerically equal to the voltage supplied by the source divided by the total resistance of the circuit. This is in accord with Ohm’s law.
  • Ohm’s law also applies separately to each device. The voltage drop, or potential difference, across each device is proportional to its resistance. This follows from the fact that more energy is used to move a unit of charge through a large resistance than through a small resistance.
  • The total voltage impressed across a series circuit divides among the individual electrical devices in the circuit so that the sum of the voltage drops across each individual device is equal to the total voltage supplied by the source. This follows from the fact that the amount of energy used to move each unit of charge through the entire circuit equals the sum of the energies used to move that unit of charge through each electrical device in turn.

Disadvantage of a series circuit: If one device fails, current in the whole circuit ceases. Most circuits are wired so that it is possible to operate several electrical devices, each independently of the other. For example, a lamp can be turned on or off without affecting the operation of other lamps or electrical devices. This is because these devices are connected not in series, but in parallel with one another.

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