Current Electricity
- The solids which have the negative temperature coefficient of resistance are : Answer: 4. insulators and semiconductors Metals have a positive temperature coefficient of resistance: resistance rises with temperature as... 1
- Six similar bulbs are connected as shown in the figure with a DC source of emf E, and zero internal resistance.The ratio of power consumption by the bulbs when (i) all are glowing and (ii) in the situation when two from section A and one from section… Answer: 2. 9 : 4 From the figure, the three bulbs of section A are in parallel with each other, as are the three bulbs of section B. The two... 1
- Which of the following acts as a circuit protection device ? Answer: 4. fuse A fuse contains a thin wire that melts when the current exceeds a safe limit, breaking the circuit and protecting the applia... 1
- In the circuits shown below, the readings of the voltmeters and the ammeters will be : Answer: 3. V₁ = V₂ and i₁ = i₂ Treat the voltmeters as ideal (infinite resistance), so they draw no current. Circuit 1: the 10 Ω resistor is... 1
- A battery consists of a variable number 'n' of identical cells (having internal resistance 'r' each) which are connected in series. The terminals of the battery are short-circuited and the current I is measured. Which of the graphs shows the correct… Answer: 1. (graph: I constant, horizontal line vs n) For n identical cells in series, total emf is nE and total internal resistance is nr .... 1
- A set of 'n' equal resistors, of value 'R' each, are connected in series to a battery of emf 'E' and internal resistance 'R'. The current drawn is I. Now, the 'n' resistors are connected in parallel to the same battery. Then the current drawn from ba… Answer: 1. 10 Series: total resistance nR+R , so I=E/(n+1)R . Parallel: total resistance R/n+R=(n+1)R/n , so I'=nE/(n+1)R . I'/I=n=10⇒ n=10 1
- A carbon resistor of (47±4.7) kΩ is to be marked with rings of different colours for its identification. The colour code sequence will be Answer: 3. Yellow - Violet - Orange - Silver Colour code digits: Yellow 4, Violet 7, Orange 3 (multiplier 10³ ), Silver ±10% , Gold ±5% . 47... 1
- A potentiometer is an accurate and versatile device to make electrical measurements of E.M.F. because the method involves : Answer: 2. a condition of no current flow through the galvanometer Key idea: at the balance point no current flows through the galvanometer,... 1
- The resistance of a wire is 'R' ohm. If it is melted and stretched to 'n' times its original length, its new resistance will be : Answer: 2. n²R Key idea: melting does not change the volume, so Al stays constant. If length becomes nl , area becomes A/n . Resistance R=ρl... 1
- A filament bulb (500 W, 100 V) is to be used in a 230 V main supply. When a resistance R is connected in series, it works perfectly and the bulb consumes 500 W. The value of R is Answer: 2. 26 Ω The bulb rated 500 W at 100 V must carry its rated current for full power. Rated current I=P/V=500/100=5 A. Supply 230 V; bu... 1
- The potential difference (V_A-V_B) between the points A and B in the given figure is Answer: 3. +9 V Move from A to B and add the potential change across each element. Resistor 2 Ω in the direction of current: drop of 2×2=4 V... 1
- A potentiometer wire is 100 cm long and a constant potential difference is maintained across it. Two cells are connected in series first to support one another and then in opposite direction. The balance points are obtained at 50 cm and 10 cm from th… Answer: 4. 3 : 2 Key idea: balance length is proportional to the emf in the circuit. Supporting: ε₁+ε₂∝50 ; opposing: ε₁-ε₂∝10 . So ε₁∝30 an... 1
- The charge flowing through a resistance R varies with time t as Q = at - bt^2, where a and b are positive constants. The total heat produced in R is : Answer: 1. a³R/6b Key idea: current I=dQ/dt ; heat H=∫ I²R dt . I=a-2bt , which becomes zero at t=a/2b . H=R∫₀^(a/2b)(a-2bt)²dt=R[-(a-2bt)³/... 1