DC Current and Circuits
- The rate of flow of charge (electrons) through a closed circuit is called electric current.
- Electric current I is the ratio of charge to time (Q/t). The unit of charge is coulomb.
- The SI unit of electric current is It is denoted by capital letter I.
- Electric current is a scalar quantity.
- The instrument is used to measure electric is called Ammeter.
- Ammeter always connected in series.
- The resistance of ammeter is very low.
- Electric pressure is also known as voltage.
- The SI unit voltage is It is denoted by capital letter V.
- The instrument is used to measure voltage is called Voltmeter.
- Voltmeter always connected in parallel.
- The resistance of the voltmeter is high.
- EMF is the force that causes a current of electric current to flow.
- The unit of EMF is Volt.
- The opposition to the flow of electric current is known as resistance.
- The SI unit of resistance is ohm. It is denoted by symbol Ω.
- The instrument used to measure resistance is known as Ohm meter.
Factor Affecting resistance
- The resistance of a conductor is directly proportional to the length of the conductor.
- The resistance of a conductor is inversely proportional to the area of cross section of the conductor.
- It depends on the nature of material.
- It also depends on the temperature.
Effect of temperature on resistance
- With increase in temperature the resistance of a conductor is increase. Conductor have positive temperature coefficient of resistance.
- With increase in temperature the resistance of an insulator is decrease. Insulator have negative temperature coefficient of resistance.
- With increase in temperature the resistance of a semiconductor is decrease. Semiconductor have negative temperature coefficient of resistance.
Resistance in series and parallel
- In series circuit’s current remains same but voltage is divided.
- In parallel circuit’s voltage remains same current is divided.
- The resistance between the opposite faces of a metre cube of that material
- The unit of resistivity ohm-metre (Ω-m).
- Conductance is the reciprocal of resistance.
- It is denoted by capital letter (G).
- Conductivity offers the inducement to the flow of electric current.
- The unit of conductivity is mho or Siemens.
High conductivity and low resistivity materials
- The loss of electrical potential energy in maintaining current in a circuit is called electrical energy.
- The unit of electrical energy is
- Electrical energy consumed per unit time is called electric power.
- The unit of power is Watt. It is denoted by P.
- Ohm’s law states that current flowing through a conductor is directly proportional to the applied voltage. Conditions apply: – Temperature and physical conditions do not change.
Limitations of ohm’s Law
- This law is not applicable to semiconductor devices (like diode, transistors, FET, SCR etc.).
- Heating element of heater is made of Nichrome.
- Element of incandescent lamp (Bulb) is made of Tungsten.
- It allows the passage of electric current through them.
- Conductors are formed by metallic bond.
- Conductors have postive temperature coefficient of resistance
- Conductors have number of free electrons available for conduction.
- The conductors are used to make of connecting wires.
- It does not allow the passage of electric current through them.
- Insulators are predominantly covalent bond.
- Insulators have negative temperature coefficient of resistance.
- Insulators have not free electrons for conduction.
- The resistivity of semiconductor materials is lies in between conductor and insulator.
- Semiconductors are formed by covalent bond.
- Semiconductors have negative temperature coefficient of resistance.
- Semiconductor material has four valence electrons.