Question: 400 J of heat is produced is 4 s in a 4 Ω resistor. Find potential difference across the resistor.
Answer: H = V²t/R
V = √RH/t
= 20 V
Question: A wire of resistivity ‘ρ’ is pulled to double its length. What will be its new resistivity?
Answer: Resistivity remains the same, i.e., ‘ρ’.
Question: What is meant by potential difference between two points?
Answer: Potential difference between two points in an electric field is the amount of work done to move a unit charge from one point to other.
Question: What is meant by saying that the potential difference between the two points is ‘1 V’ ?
Answer: It means that 1 J of work has been done in moving 1 coulomb of between the two points.
Question: The amount of charge passing through a cell in four second is 12 C. Find the current supplied by cell.
Answer: Charge, Q = 12 C, time t=4 s
∴ Current, I = Q/t = 12/4 = 3 A
Question: Calculate the number of electrons that would flow per second through the cross-section of a wire when 1 A current flows in it.
Answer: Here, I = 1 A, t = 1 s
∴ Q = I×t = 1 A × 1 s = 1 C
Number of electrons = 1C/1.6 ×10-19C
= 0.625 ×119
= 6.25 × 1018
Question: Name the device / instrument used to measure potential difference. How is it connected in electric circuit?
Answer: Voltmeter is used to measure potential difference. It is connected in parallel.
Question: What is the resistance of on ideal voltmeter?
Answer: A voltmeter has a very high resistance and for an ideal voltmeter, its value is infinity.
Question: List in a tabular form two differences between a voltmeter and an ammeter.
- Used to measure potential difference.
- Connected in parallel in an electric circuit.
- Has high resistance.
- Used to measure current.
- Connected in series in the electric circuit.
- Has low resistance.
(a) What are the values of mA and µA?
(b) Draw the symbols of battery and rheostat.
Question: Given below is a circuit showing current flowing in it. Identify each component A, B, C, D of this circuit.
Answer: A = Electric bulb, B = Rheostat, C = Electric cell, D = Ammeter
Question: State the factors on which the heat produced in a current carrying conductor depends. Give one practical application of this effect.
- Heat produced in a current carrying conductor depends upon:
(i) Square of the current (I²).
(ii) Resistance of the given conductor (R).
(iii) Time for which the current flows (t).
- This effect is applicable in electric heating device like electric iron.
Question: A large number of free electrons are present in metals yet no current flows in the absence of electric potential across it. Explain the statement with reason.
Answer: A large number of free electrons are present in metals, yet no current flows. It is because the electrons move only if there is a difference of electric pressure – called as the potential difference along the conductor. The potential difference sets the charges in motion in the conductor and produces an electric current.
Question: How much current will an electric bulb of resistance 1100 Ω draw from a 220 V source? If a heater of resistance 100 Ω is connected to the same source instead of the bulb, calculate the current drawn by the heater.
Answer: I = V/R = 1100/220 = 5 A
In case of heater, I = 1100/100 = 11A
Question: Out of the wire X and Y shown below which one has greater resistance. Justify your answer.
Answer: Wire Y has greater resistance because resistance is directly proportional to the length of the conductor.
Resistivity of iron is 10 × 10-8 Ω m and of mercury is 94 × 10-8 Ω m. Which among iron and mercury is a better conductor? Give reason.
Resistivity of a material A is in the range of 1010 – 10-8 Ω m. Which of the two will behave as an insulator and why?
- Iron, because lower the resistivity, better is the conductor.
- Material A, because insulators have higher resistivity.
Question: Elements of electric toasters and electric iron are made of a alloy rather than a pure metal. Give two reasons to justify the statement.
- Alloys do not oxidize readily at high temperature, so they are more resistant to corrosion.
- Alloys have lower electrical conductivity than pure metals.
Question: A thick wire and a thin wire of the same material are successively connected to the same circuit to find their respective resistance. Which one will have lower resistance? Give reason.
Answer: Thick wire will have lower resistance because resistivity is inversely proportional to the area of cross-section of the conductor.
Question: Mention two special features of the material to be used as element of an electric iron.
Answer: The material should have: (1) high resistivity and (2) high melting point.
Question: Given reason for the following:
Tungsten used almost exclusively for filament of electric lamp.
Why do we use copper and aluminium wires for transmission of electric current?
- Tungsten is used in making the filament of an electric bulb because:
(i) Tungsten has high melting point.
(ii) Tungsten has high resistivity to retain much heat.
- Copper and aluminium have low resistivity and they are good conductors of electricity. So, they are used for transmission of electric current.