CBSE Electricity Subject Notes

CBSE Guess > eBooks > Class X > Electricity by Mr Sunil D Chauhan

Electricity

Electricity

Factors affecting resistivity

1. The resistivity of a substance depends only on the nature of material.
2. It also depends on the temperature and does not depend on length and thickness of conductor.

COMBINATION OF RESISTANCES

Resistances in series when the resistances are joined end to end, they are said to connected in series. If the resistances R₁, R₂, R₃,. are connected in series, their resultant resistance R = R₁ + R₂ + R₃+ . Derivation. Let the resistances R₁, R₂, and R₃ be connected in series. The current flowing through the circuit is I and the potential difference across the resistances is V , then according to Ohm's law, the potential difference across the resistors R₁, R₂ and R₃ is

V₁ = IR₁, V₂= IR₂ and V3 = IR₃.

Since, the total potential difference in the circuit remains same

V = V₁ + V₂ + V₃

=> IR = I R₁ + I R₂ + I R₃ =>R = R₁ + R₂ + R₃

Resistances in Parallel When one end of all the resistances is connected at one point and the other end is connected at another single point, then the resistances are said to be connected in parallel. If the resistances R₁, R₂ ,R₃. are connected in parallel, their resultant resistance

1/R = 1/R₁ + 1/R₂ + 1/R₃+ .

Derivation Let the resistances R_1, R₂, and R₃ be connected in parallel. The potential difference across the resistances is V and the current flowing through the circuit is I. Then according to Ohm's law, the current through the resistors

R₁, R₂ and R₃ is I₁= V/R₁, I₂ = V/R₂ and I₃ = V/R₃.

Since, the sum of current through each resistance in parallel is equal to the total current in the circuit.

I = I₁ + I₂ + I₃ = V/R = V/R₁ + V/R₂ + V/R₃ =1/R = 1/R₁ + 1/R₂ + 1/R₃

The sum of the potential differences across the ends of the resistances connected in series is equal to the total potential difference in the circuit.

The current passing through the resistances connected in series remains same. The sum of current through each resistance in parallel is equal to the total current in the circuit.

The potential difference across the resistances connected in parallel remains same.

Disadvantages of series circuit in domestic wiring.

1. In series circuit all the electrical objects have only one switch and cannot turned off separately.
2. If one gadget stops working due to some defect, then all the gadgets stop working.
3. All the gadgets do not work at the same potential difference (voltage).
4. The resultant resistance of the circuit becomes too high and the current becomes too low.

Advantages of parallel circuit.

1. Each gadget has its own switch and can be switched off separately without affecting others.
2. If one gadget stops working due to some defect, then all the gadgets keep working normally.
3. All the gadgets work at the same potential difference (voltage).
4. The resultant resistance of the circuit becomes less and the current becomes high.

Heating effect of electric current (Joule's Law of heating) The heat produced in a resistor is directly proportional to the -

1. Square of the current flowing through it.
2. Resistance in the circuit.
3. Time for which the current flows through the circuit. Heat produced in a resistor is H = I² Rt

Fuse wire The wire of short length which is used in series with an electrical appliance to make it safe is called electric fuse. They are made of metal like Al, Cu or alloy of lead and tin of suitable melting point.

Working of a fuse When current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. Due to this it melts and breaks the circuit.