An electric circuit is a collection of electrical devices, called circuit elements connected by conductors in a closed path (i.e., in a complete loop).

Circuit elements include, source of electrical energy (e.g. battery), sink of electrical energy (e.g. light bulb), and switch to complete or break the circuit. Conductors (e.g., copper wires) are then used to connect them together.

 

Electrical Symbols

Circuit diagrams are essential part of the study of current electricity and are often drawn symbolically. In circuit diagrams, various circuit elements are represented by standard electric symbols.

Electrical symbols

Circuit Diagrams

A simple circuit, with a cell (if a series of cells is used, it is called a battery) and a resistor or bulb.

simple-circuit-diagram-300x274

Circuit diagram showing the measurement of current in the circuit and potential difference across a circuit element (e.g. the resistor).

  • An ammeter is used to measure the current flowing in the circuit and must be inserted in series with the circuit element as shown.
  • A voltmeter measures the potential difference between two points in a circuit and must be connected in parallel to the circuit element as shown.

 

Note that:

  • An ideal ammeter has zero resistance so that when inserted into a circuit, it does not reduce the current that was previously flowing.
  • An ideal voltmeter has infinite resistance so that it takes no current. A finite resistance causes it to take current from the circuit, and to lower the potential difference between the points to which it is connected.

 

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  • Variable resistor can be used as a rheostat to control current or as a potential divider to control voltages.
  • The fuse is a short piece of thin wire which overheats and melts to break the circuit if current of more than its rated value flows through it. A fuse is connected in series to protect devices in the circuit.
  • Resistance of a thermistor (thermally sensistive resistor) decreases rapidly as its surrounding temperature rises.
  • Resistance of light dependent resistor (LDR) decreases with increasing surrounding light intensity.
  • A light emitting diode (LED) allows current to flow in only one direction. When current is allowed to pass, it shines brightly with only a small voltage across it. LED is used as on/off indicator in many electronic devices such as televisions, computers.
  • A earth connector is a conductor that connects directly to earth.

 

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