Question: Find the resistance of bulb rated as 100 W – 250 V.
Answer: P = V²/R or R = V²/p = 250×250/100 = 625 Ω
Question: Explain the terms potential and potential difference. Give their SI units.
- Potential: Potential at a point is defined as the amount of work done in moving a unit charge form infinity to that point.
- Potential difference: Potential difference between two points is defined as the amount of work done in moving a unit charge between the two point.
- Both potential and potential difference are measured in volt.
Question: Define 1 Volt.
Answer: The potential across a conductor is said to be one volt, if 1 joule of work is done in moving charge of 1 coulomb between the two ends of the conductor.
Question: What are the conditions under which Ohm’s law is not obeyed?
Answer: Ohm’s law is not obeyed under the following conditions:
- Potential difference depends upon current non-linearly and
- For the same value of V, I changes sign with V.
- V and I have non-unique relation.
Question: Name a few practical applications of heating effect of current.
Answer: A few examples of the practical applications of heating effect are the various heating devices. Some of these are safety fuse, Electric lamp, Electric iron, Electric oven, Electric geyser, Electric Kettle etc.
Question: Why alloy are used to make standard resistors?
Answer: This because there is a negligible change in the resistance of an alloy with the change in temperature.
Question: The V-I graph for series and parallel combination of two metallic resistors is as shown in the figure. Which graph represents parallel. Which graph represents parallel combination? Justify your answer.
Answer: Slope of V-I graph is equal to the resistance of the combination. It is less for graph A, which means less resistance. In parallel combination, resistance becomes less.
Question: Out of the following bulbs rated 40 W, 220 V, 60 W, 220 V and 100 W, 220 V which one will glow the brightest when connected in series to a supply of 220 V?
Answer: The resistance of a bulb is given by the expression, R = V²/p. Since all have the same voltage rating, therefore, the 40 W bulbs has maximum resistance. When these three bulbs are connected in series, same current passes trough each. As heat produced is directly proportional to the resistance when the current is constant, therefore, the 40 W bulb will produced maximum heat hence will be the brightest.
Question: What is a voltmeter? How is it connected in a circuit?
Answer: A voltmeter is a device used to measure potential different across the two ends of a conductor. It is always connected in parallel in a circuit.
Define Current, given its mathematical expression. What is its SI unit?
Answer: Electric current is defined as the rate of flow of electrons. Mathematically, it is expressed as I = Q/t. In SI, it is measured in ampere.
Question: If 2×10-3 J of work is done in moving a particle carrying a charge of 10×10-6 coulomb from infinity to a point P. What will be the potential at the point P?
Answer: We know that V = W/q = 2×10-3/10×10-6 = 200 V.
Question: Define one ampere.
Answer: The current through a conductor is said to be 1 ampere, if a change of 1 coulomb flows through it in 1 second.
Question: Explain the term resistance. Give its unit of measurement.
Answer: The word resistance means resist or opposition that a conductor offers to the flow of charges through it. When a potential difference is applied across the conductor, the free electrons get accelerated. These moving electrons collide with other electrons and atoms due to which the electrons are slowed down. In other words, the motion of the electrons is opposed. This opposition is called resistance of the conductor Resistance is measured in ohm (Ω).
Question: Explain how do we classify materials as conductor and insulators on the basis of their resistance.
Answer: Conductors are the materials, which have a large number of free electrons (≅ 1028 m-3) and small resistivity (≅ 10-8 Ω m). The resistivity of an ideal conductor is zero and it increases with the rise in temperature in metals.
Insulators are the substances, which have no practically free electrons and have very high resistivity (≅ 1016 Ω m). The resistivity of an ideal insulator and decreases with the rise in temperature. Mica, rubber, glass porcelain are some examples of insulator.
Question: Define electric power. Give its SI unit and write its mathematical equation.
Answer: The rate of work, or the rate of dissipation of energy, is called electric power.
It is denoted by P, i.e., P = W/t = V²/R.
In SI, it is measured in watt.
Question: Name the commercial unit of electrical energy. Convert it into joule. What is the other name the commercial unit?
Answer: The commercial unit of electrical energy is kilowatt – hour (kW h).
1 kW h = 1 kW × 1 hour
= 1000 W × 3600 s
= 1000 J s-1 × 3600 s
1 kW h = 3.6 × 106 J
It is also called Board of Trad Unit (BOTU).
Question: State Joule’s law of heating and write its mathematical form.
Answer: It states that when current is passed through a conductor the heat produced across it is
(i) directly proportional to the square of current through the conductor i.e.,
H ∝ I² …. (1)
(ii) directly proportional to the resistance of the conductor i.e.,
H ∝ R …. (2)
(iii)directly proportional to the time for which the current is passed i.e.,
H ∝ t …. (3)
Combining the above three equations, we have
H ∝ I²Rt
or H = I²Rt/J calories …. (4)
It is expressed as H = I² Rt Joules.
Question: An electric bulb draws a current of 0.2 A when the voltage is 220 volts. Calculate the amount of charge flowing through it in one hour.
Answer: Q = I × t
= 0.2 A × 1 hour
= 0.2 A × 3600 s = 720 C
Question: An electric iron draws a current of 0.5 A when the voltage is 200 volt. Calculate the amount of electric charge flowing through it one hour.
Answer: Q = I × t = 0.5 × 3600 = 1800 C
Question: An electric appliance draws a current of O.4 A when the voltage is 200 volt. Calculate the amount of charge following through it in one hour.
Answer: Q = I × t
= 0.4 × 1 hour = 0.4 A × 3600 s = 1440 C
Question: A bulb is rated at 5.0 V, 100 mA. Calculate its (i) power and (ii) resistance.
Answer: Give = 5.0 V,
I = 100 mA = 100 × 10-3 A = 0.1 A
P = VI = 5.0 × 0.1 = 0.5 W
Also, R = V/I = 5.0/0.1 = 50 Ω
Question: If you connect three resistors having values 2 ohm, 3 ohm and 5 ohm in parallel, will the value of total resistance be less than 2 ohm or greater than 5 ohm or lie between 2 ohm and 5 ohm? Explain.
Answer: The value of the equivalent resistance will be smaller than the least value of resistance in the combination. Thus, the total resistance will be less than 2 ohm. This can be found as follows:
1/R = 1/2 + 1/3 +1/5
= 15+10+6/30 = 31/30
or R = 30/31 = 0.967 ohm
Question: Three bulbs of 40 W, 60 W and 100 W are connected n series across the mains. Across which the potential difference is highest? Across which is lowest?
Answer: In series, the current is same in all the three bulb and so the potential difference across a bulb is proportional to its resistance. The resistance of lowest wattage bulb is highest in accordance with the relation R ∝ 1/P. Therefore, potential difference is highest across the 40 W bulb and is lowest across the 100 W bulb.
Question: State the factors on which the resistance of a cylindrical conductor depends. How will resistance of a conductor change if it is stretched so that its length is doubled?
Answer: The resistance of a cylindrical conductor depends upon:
(i) its length, and
(ii) its area of cross-section
When a conductor is stretched to twice its original length, then the new resistance becomes RN = n²R, where n is the times the conductor is stretched. Since n = 2, therefore, RN = 4R
Question: (1). What material is used in making the filament of an electric bulb?
(2). Name the characteristics which make it suitable for this.
- High melting point and high resistivity to retain heat
Question: How are ammeters and voltmeters connected in a circuit? What do they help us measure?
Answer: An ammeter is always connected in series. It measures the electric the electric current flowing in the circuit.
A voltmeter is always connected in parallel. It measures the potential difference across a conductor.