Class 10 Science Chapter 11 Case Based Questions - Electricity

Case Study - 1

When electric current flows through the circuit this electrical energy is used in two ways, some part is used for doing work and remaining may be expended in the form of heat. We can see, in mixers after using it for long time it become more hot, fans also become hot after continuous use. This type of effect of electric current is called as heating effect of electric current. If I is the current flowing through the circuit then the amount of heat dissipated in that resistor will be H = VIt
This effect was discovered by Joule, hence it is called as Joule’s law of heating.
Also, we can write, H = I²Rt
Thus, heat produced is directly proportional to the square of the electric current, directly proportional to the resistance of the resistor and the time for which electric current flows through the circuit. This heating effect is used in many applications. The heating effect is also used for producing light. In case of electric bulb, the filament produces more heat energy which is emitted in the form of light. And hence filament are made from tungsten which is having high melting point.
In case of electric circuit, this heating effect is used to protect the electric circuit from damage.
The rate of doing work  or rate of consumption of energy is called as power. Here, the rate at which electric energy dissipated or consumed in an electric circuit is called as electric power. And it is given by P= VI
The SI unit of electric power is watt.

Q1: What is the SI unit of electric energy?
Ans: The SI unit of electric energy is watt hour. And the commercial unit of electric energy is kW h.

Q2: How heating effect works to protect electric circuit?
Ans: In case of electric circuit fuse is connected in series with the circuit which protects the electric devices by stopping the extra current flowing through them. When a large amount of current is flowing through the circuit the temperature of the fuse wire increases and because of that fuse wire melts which breaks the circuit.

Q3: 1KW h = ?
Ans: 1kW h = 3.6*10⁶ joule 

Q4: If a bulb is working at a voltage of 200V and the current is 1A then what is the power of the bulb?
Ans: Given that,
V = 200V, I = 1A
Then, P = VI = 200*1 = 200 J/s = 200 W

Case Study - 2

Resistance is the opposition offered by the conductor to the flow of electric current. When two or more resistors are connected in series then electric current through each resistor is same but the electric potential across each resistor will be different. If R1, R2 and R3 are the resistance connected in series then current through each resistor will be I but potential difference across each resistor is V1, V2 and V3 respectively.
Thus, the total potential difference is equal to the sum of potential difference across each resistor. Hence, V= V1 + V2 + V3
Again, IR = IR1 + IR2 + IR3
Thus, R = R1 + R2 + R3
Hence in case of series combination of resistors, the total resistance is the sum of resistance of each resistor in a circuit.
Now, in case of parallel combination of resistors electric current through each resistor is different but the potential difference across each resistor is same. If resistors R1, R2 and R3 are connected in parallel combination then potential difference across each resistor will be V but current through each resistor is I1, I2 and I3 respectively.
Thus, total current through the circuit is the sum of current flowing through each resistor.
I = I1 + I2 + I3
Again, V/R= V/R1 + V/R2 + V/R3
Thus, 1/R = 1/R1 + 1/R2 + 1/R3
Hence, in case of parallel combination of resistors, the reciprocal of total resistance is the sum of reciprocal of each resistance connected in parallel.

Q1: In which case the equivalent resistance is more and why?
Ans: In case of parallel combination of resistors the equivalent resistance is less than the individual resistance connected in parallel.
Since, 1/R = 1/R1 + 1/R2 + 1/R3 +….

Q2: In our home, which type of combination of electric devices is preferred? Why?
Ans: At our home, we are connecting electrical devices in parallel combination because in parallel combination equivalent resistance is less and also we can draw an electric current according to the need of electric devices.

Q3: If n resistors of resistance R are connected in parallel then what is the equivalent resistance?
Ans: If n resistors of resistance R are connected in parallel then equivalent resistance is given by,
1/Re = 1/R + 1/R + 1/R +….n times 1/R
Thus, 1/Re = n/R
Hence, Re= R/n is the required equivalent resistance of the given combination.

Case study - 3

We can see that, as the applied voltage is increased the current through the wire also increases. It means that, the potential difference across the terminals of the wire is directly proportional to the electric current passing through it at a given temperature.
Thus, V= IR
Where R is the proportionality constant called as resistance of the wire. Thus, we can say that the resistance of the wire is inversely proportional to the electric current. As the resistance increases current through the wire decreases. The resistance of the conductor is directly proportional to length of the conductor, inversely proportional to the area of cross section of the conductor and also depends on the nature of the material from which conductor is made. Thus R= qL/A, where q is the resistivity of the material of conductor. According resistivity of the material they are classified as conductors, insulators and semiconductors. It is observed that the resistance and resistivity of the material varies with temperature. And hence there are vast applications of these materials based on their resistivity.
The SI unit of resistance is ohm while the SI unit of electric current is ampere. The potential difference is measured in volt. Conductors are the materials which are having less resistivity or more conductivity and hence they are used for transmission of electricity. Alloys are having more resistivity than conductors and hence they are used in electric heating devices. While insulators are bad conductors of electricity.

Q1: What is SI unit of resistivity?
Ans: The SI unit of resistivity is ohm meter.

Q2: What is variable resistance?
Ans: The electric component which is used to regulate the electric current without changing voltage source is called as variable resistance.

Q3: Why tungsten is used in electric bulbs?
Ans: Tungsten filament are used in electric bulbs because the resistivity of Tungsten is more and it’s melting point is also high.

Q4: 1M ohm = ?
Ans: 1M ohm = 10⁶ ohm