DC Current and Circuits

Current

1. The rate of flow of charge (electrons) through a closed circuit is called electric current.
2. Electric current I is the ratio of charge to time (Q/t). The unit of charge is coulomb.
3. The SI unit of electric current is It is denoted by capital letter I.
4. Electric current is a scalar quantity.
5. The instrument is used to measure electric is called Ammeter.
6. Ammeter always connected in series.
7. The resistance of ammeter is very low.

Voltage

1. Electric pressure is also known as voltage.
2. The SI unit voltage is It is denoted by capital letter V.
3. The instrument is used to measure voltage is called Voltmeter.
4. Voltmeter always connected in parallel.
5. The resistance of the voltmeter is high.

EMF

1. EMF is the force that causes a current of electric current to flow.
2. The unit of EMF is Volt.

Resistance

1. The opposition to the flow of electric current is known as resistance.
2. The SI unit of resistance is ohm. It is denoted by symbol Ω.
3. The instrument used to measure resistance is known as Ohm meter.

Factor Affecting resistance

1. The resistance of a conductor is directly proportional to the length of the conductor.
2. The resistance of a conductor is inversely proportional to the area of cross section of the conductor.
3. It depends on the nature of material.
4. It also depends on the temperature.

Effect of temperature on resistance

1. With increase in temperature the resistance of a conductor is increase. Conductor have positive temperature coefficient of resistance.
2. With increase in temperature the resistance of an insulator is decrease. Insulator have negative temperature coefficient of resistance.
3. With increase in temperature the resistance of a semiconductor is decrease. Semiconductor have negative temperature coefficient of resistance.

Resistance in series and parallel

1. In series circuit’s current remains same but voltage is divided.
2. In parallel circuit’s voltage remains same current is divided.

Resistivity

1. The resistance between the opposite faces of a metre cube of that material
2. The unit of resistivity ohm-metre (Ω-m).

Conductance

1. Conductance is the reciprocal of resistance.
2. It is denoted by capital letter (G).

Conductivity

1. Conductivity offers the inducement to the flow of electric current.
2. The unit of conductivity is mho or Siemens.

High conductivity and low resistivity materials

Gold

Silver

Copper

Aluminium

Iron

Electrical energy

1. The loss of electrical potential energy in maintaining current in a circuit is called electrical energy.
2. The unit of electrical energy is

Electric Power

1. Electrical energy consumed per unit time is called electric power.
2. The unit of power is Watt. It is denoted by P.

Ohm’s Law

1. 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

1. This law is not applicable to semiconductor devices (like diode, transistors, FET, SCR etc.).

Materials

1. Heating element of heater is made of Nichrome.
2. Element of incandescent lamp (Bulb) is made of Tungsten.

Conductor

1. It allows the passage of electric current through them.
2. Conductors are formed by metallic bond.
3. Conductors have postive temperature coefficient of resistance
4. Conductors have number of free electrons available for conduction.
5. The conductors are used to make of connecting wires.

Insulators

1. It does not allow the passage of electric current through them.
2. Insulators are predominantly covalent bond.
3. Insulators have negative temperature coefficient of resistance.
4. Insulators have not free electrons for conduction.

Semiconductors

1. The resistivity of semiconductor materials is lies in between conductor and insulator.
2. Semiconductors are formed by covalent bond.
3. Semiconductors have negative temperature coefficient of resistance.
4. Semiconductor material has four valence electrons.