Current Electricity — Part 1 MCQs with Explanations

A closed switch allows current to flow freely through the circuit, meaning its resistance is zero or very close to zero. This lack of resistance ensures that there is no significant opposition to the flow of electric current, allowing the circuit to be completed. In contrast, an open switch would have infinite resistance, preventing any current from flowing through the circuit.

A rheostat is designed to handle higher power levels compared to a potentiometer. While both devices are used to vary resistance, rheostats are typically used in applications where high current is involved and are built to dissipate more heat, thus having a higher wattage rating . Potentiometers , on the other hand, are generally used in low-power applications and have a lower wattage rating. The key difference lies in their power handling capacity: Rheostat = High wattage rating Potentiometer = Low wattage rating

Positive temperature coefficient : This is the correct answer. A thermistor with a positive temperature coefficient (PTC) exhibits an increase in resistance with an increase in temperature. The resistance-temperature relationship is positive for PTC thermistors. Negative temperature coefficient : This is not the correct answer. A thermistor with a negative temperature coefficient (NTC) exhibits a decrease in resistance with an increase in temperature. The resistance-temperature relationship is negative for NTC thermistors. Zero temperature coefficient : This is not the correct answer. A zero temperature coefficient would mean that the resistance does not change with temperature, which is not a characteristic of thermistors. Variable temperature coefficient : This is not the correct answer. Thermistors typically have a well-defined temperature coefficient, either positive or negative, and are not variable in this aspect. Therefore, in the context of the question, a thermistor has a Positive temperature coefficient .

The correct answer is D: 20 joules Here's why: Energy used by a battery is related to the charge it moves and the voltage across it. The formula we need to use is: Energy (E) = Charge (Q) x Voltage (V) We are given the voltage (V) as 20 V. We need to find the charge (Q). We know the number of electrons (6.28 x 10 18 ). The charge of a single electron is approximately 1.6 x 10 -19 Coulombs. To find the total charge (Q), we multiply the number of electrons by the charge of a single electron: Q = (6.28 x 10 18 ) x (1.6 x 10 -19 ) Coulombs Q ≈ 1 Coulomb Now we can calculate the energy: E = Q x V E = 1 Coulomb x 20 V E = 20 Joules Therefore, the energy used by the battery is 20 joules.

Insulating materials are used in electrical and electronic systems to prevent the flow of electric current between conducting wires or components. They serve as barriers that hinder the movement of electrons and the flow of electricity. The primary function of insulating materials is to prevent unwanted electrical connections, such as short circuits, that can lead to electrical hazards and malfunctions. Among the options provided, the correct answer is Option A: Preventing a short circuit between conducting wires . Insulating materials help maintain the separation and isolation of conducting elements to ensure that current flows only where intended and to prevent accidental connections that could lead to short circuits and electrical failures. Therefore, the correct answer is Option A: Preventing a short circuit between conducting wires .

To find the length of the nichrome wire required, you can use the formula: $$P = \frac{{V^2}}{{R \cdot L}}$$ Where: - $$P$$ is the power (1 kW = 1000 W) - $$V$$ is the voltage (200 V) - $$R$$ is the resistance per meter (2 Ω/m) - $$L$$ is the length of the wire (which we want to find) Let's rearrange the formula to solve for $$L$$: $$L = \frac{{V^2}}{{P \cdot R}}$$ Now, substitute the values: $$L = \frac{{(200\, \text{V})^2}}{{1000\, \text{W} \cdot (2\, Ω/\text{m})}}$$ Calculate this to find $$L$$: $$L = \frac{{40000\, \text{W}}}{(2000\, Ω/\text{m})} = 20\, \text{m}$$

Absolute measurement of resistance involves determining the resistance value without reference to any standard resistor. The Wheatstone bridge method is a precise technique for absolute measurement of resistance. It balances two legs of a bridge circuit and directly relates the unknown resistance to known resistances and measurements of voltage or current. Other methods: Lorentz method and Raleigh method are not used for absolute resistance measurements but are applied in other contexts. Ohm's law method involves relative measurements and requires reference standards for voltage and current, so it cannot be used for absolute measurements. Hence, Option D is the correct answer.

Power dissipated in a heating element is given by: P = V² / R, where V is the supply voltage (constant from mains) and R is the resistance of the heating element. Heat produced is proportional to power × time. To boil the same quantity of water in more time (15 minutes instead of 10 minutes) , the heating element must supply less power . Since V is constant, to reduce power, the resistance R of the element must be increased . Resistance of a wire is proportional to its length (R ∝ L). Therefore, to increase resistance, the length of the heating element should be increased . Final Answer: Length of heating element should be increased (Option B)

Voltage dependent resistors (VDRs) , also known as varistors, are semiconductor devices designed to protect electronic circuits from excessive voltage spikes and surges. Silicon carbide is commonly used as the material for VDRs due to its properties: 1. Non-linear resistance: Silicon carbide exhibits a non-linear relationship between voltage and current, which allows it to act as a variable resistor that decreases its resistance as voltage increases beyond a certain threshold. 2. High breakdown voltage: Silicon carbide can withstand high voltages, making it effective for protecting circuits against transient overvoltage conditions. Option A: Charcoal is incorrect because charcoal is not suitable for making VDRs due to its poor electrical properties. Option C: Nichrome is incorrect because nichrome is an alloy used in heating elements due to its high electrical resistance and heat resistance, but it is not used in VDRs. Option D: Graphite is incorrect because graphite is a good conductor of electricity and is used in electrodes and other applications requiring high electrical conductivity, not in VDRs. Therefore, the correct answer is Silicon carbide .

Resistance (R) of a wire is directly proportional to its length (L) and inversely proportional to its cross-sectional area (A). This relationship is given by: R = ρL/A, where ρ (rho) is the resistivity of the material. When the length of the wire is doubled , the new length becomes 2L. However, if the wire's volume remains the same (which is generally assumed when we stretch a wire), doubling the length reduces the cross-sectional area to half (A/2). Think of it like stretching dough - it gets longer but thinner. So, the new resistance (R') will be: R' = ρ(2L) / (A/2) = 4ρL/A. Since the original resistance R = ρL/A, we can say that R' = 4R. Therefore, if the original resistance was 0.2 ohm, the new resistance becomes: R' = 4 × 0.2 = 0.8 ohm. Final Answer: 0.8 ohm

Explanation: The power dissipated by a resistor is given by the formula: \( P = I^2 \times R \) , where \( P \) = Power in watts, \( I \) = Current in amperes, \( R \) = Resistance in ohms. Given: Current, \( I = 2 \, A \) Resistance, \( R = 3 \, \Omega \) Calculating power: \( P = (2)^2 \times 3 = 4 \times 3 = 12 \, \text{watts} \) Hence, the power dissipated is 12 watts . Therefore, the correct answer is Option E: 12 watts .

When electric current passes through a circuit, it produces several effects. Here are the effects associated with each option: Option A: Magnetic effect - This is true . Electric current passing through a conductor generates a magnetic field around the conductor, leading to the magnetic effect. This phenomenon is the basis for the operation of electromagnets and transformers. Option B: Luminous effect - This is false . Electric current passing through a conductor does not typically produce a luminous effect. Luminous effects are associated with phenomena like incandescence or the emission of light due to high temperatures, not with the flow of current itself. Option C: Thermal effect - This is true . Electric current flowing through a conductor results in the generation of heat, known as the thermal effect. This effect is used in applications like electric heaters and toasters. Option D: Chemical effect - This is true . Electric current can cause chemical changes in certain materials, leading to chemical effects. For example, it plays a crucial role in electrochemical processes like electrolysis and in batteries. Therefore, the correct statements are Option A (Magnetic effect), Option C (Thermal effect), and Option D (Chemical effect) . These effects are observed when electric current passes through a circuit.

Number of free electrons available becomes more is the correct answer to “Resistance of a material always decreases if”. This question is standard in competitive examination syllabi for Current Electricity.

Amperes is the correct answer to “The rating of a fuse wire is always expressed in”. This question is standard in competitive examination syllabi for Current Electricity.

Equal to the charge of an electron is the correct answer to “The minimum charge on an ion is”. This question is standard in competitive examination syllabi for Current Electricity.

of the order of a few micro m/s is the correct answer to “Current velocity through a copper conductor is”. This question is standard in competitive examination syllabi for Current Electricity.

Charge is the correct answer to “For which of the following 'ampere second' could be the unit”. This question is standard in competitive examination syllabi for Current Electricity.

Galvanometer is the correct answer to “An instrument which detects electric current is known as”. This question is standard in competitive examination syllabi for Current Electricity.

Third electron from the orbit is the correct answer to “It becomes more difficult to remove”. This question is standard in competitive examination syllabi for Current Electricity.

Rl is the correct answer to “Two resistances Rl and Ri are connected in series across the voltage source where Rl > Ri. The largest drop will be across”. This question is standard in competitive examination syllabi for Current Electricity.

Increase is the correct answer to “A 40 W bulb is connected in series with a room heater. If now 40 W bulb is replaced by 100 W bulb, the heater output will:”. This question is standard in competitive examination syllabi for Current Electricity.

Copper is the correct answer to “Which of the following materials has the least resistivity”. This question is standard in competitive examination syllabi for Current Electricity.

The other two bulbs will light is the correct answer to “Three 60 W bulbs are in parallel across the 60 V power line. If one bulb burns open”. This question is standard in competitive examination syllabi for Current Electricity.

2 : 5 is the correct answer to “Two bulbs of 500 W and 200 W rated at 250 V will have resistance ratio as”. This question is standard in competitive examination syllabi for Current Electricity.

High is the correct answer to “For testing appliances, the wattage of test lamp should be”. This question is standard in competitive examination syllabi for Current Electricity.

Semiconductors is the correct answer to “Are the materials having electrical conductivity much less than most of the metals but much greater than that of typical insulators.”. This question is standard in competitive examination syllabi for Current Electricity.

The highest resistance has the highest voltage drop is the correct answer to “In a series circuit with unequal resistances”. This question is standard in competitive examination syllabi for Current Electricity.

Area of cross-section is the correct answer to “The resistance of a conductor varies inversely as”. This question is standard in competitive examination syllabi for Current Electricity.

Wire wound resistor is the correct answer to “A resistance having rating 10 ohms, 10 W is likely to be a”. This question is standard in competitive examination syllabi for Current Electricity.

Losses is the correct answer to “If the efficiency of a machine is to be high, what should be low”. This question is standard in competitive examination syllabi for Current Electricity.

Nickel and chromium is the correct answer to “Nichrome wire is an alloy of”. This question is standard in competitive examination syllabi for Current Electricity.

484 Ω is the correct answer to “Resistance of 220 V, 100 W lamp will be”. This question is standard in competitive examination syllabi for Current Electricity.

Watt is the correct answer to “The S.I. unit of power is”. This question is standard in competitive examination syllabi for Current Electricity.

Ampere/volt is the correct answer to “All of the following are equivalent to watt except”. This question is standard in competitive examination syllabi for Current Electricity.

9 times is the correct answer to “Copper wire of certain length and resistance is drawn out to three times its length without change in volume, the new resistance of wire becomes”. This question is standard in competitive examination syllabi for Current Electricity.

Parallel is the correct answer to “Bulbs in street lighting are all connected in”. This question is standard in competitive examination syllabi for Current Electricity.

All above is the correct answer to “An electric filament bulb can be worked from”. This question is standard in competitive examination syllabi for Current Electricity.

Decreases is the correct answer to “With rise in temperature the resistance of semiconductors”. This question is standard in competitive examination syllabi for Current Electricity.

1 n-mho is the correct answer to “When a voltage of one volt is applied, a circuit allows one micro ampere current to flow through it. The conductance of the circuit is”. This question is standard in competitive examination syllabi for Current Electricity.

Two in parallel and one in series is the correct answer to “You have to replace 1500 ω resistor in radio. You have no 1500 ω resistor but have several 1000 ω ones which you would connect”. This question is standard in competitive examination syllabi for Current Electricity.