Quick Review Flashcards - Click to flip and test your knowledge!
Question
At what voltage is electric power typically generated at power stations to avoid insulation difficulties?
Answer
Electric power is generated at $11\text{ kV}$.
Question
Why is power generation at voltages lower than $11\text{ kV}$ avoided?
Answer
It involves a very high current, leading to significant energy loss in the form of heat.
Question
Which mathematical relationship explains why raising voltage reduces energy loss in transmission lines?
Answer
The energy loss due to heating is given by $H = I^2Rt$.
Question
The current $I$ for a given power $P$ and voltage $V$ is expressed by the relation _____.
Answer
$I = \frac{P}{V}$
Question
What type of transformer is used at the grid sub-station to raise voltage from $11\text{ kV}$ to $132\text{ kV}$?
Answer
A step-up transformer.
Question
At the main sub-station, the voltage is stepped down from $132\text{ kV}$ to which level?
Answer
The voltage is stepped down to $33\text{ kV}$.
Question
Which sub-station is responsible for stepping down voltage from $11\text{ kV}$ to $220\text{ V}$ for domestic consumers?
Answer
The city sub-station.
Question
Why can alternating voltage (a.c.) be easily transmitted over long distances while direct voltage (d.c.) cannot?
Answer
Alternating voltage can be stepped up or stepped down using transformers, which is not possible with d.c. voltage.
Question
What is the standard frequency of the alternating voltage generated in India?
Answer
The frequency is $50\text{ Hz}$.
Question
Name the three wires that typically make up the cable supplied to a house.
Answer
The live (or phase) wire, the neutral wire, and the earth wire.
Question
At what potential are the neutral and earth wires maintained at the local sub-station?
Answer
They are maintained at a potential of $0\text{ V}$.
Question
What is the purpose of the 'company fuse' (or pole fuse) connected before the kWh meter?
Answer
It protects the cable and meter from excessive current from the mains supply.
Question
Which device in a household circuit records the consumption of electrical energy in $\text{kWh}$?
Answer
The $\text{kWh}$ meter (or energy meter).
Question
What is the primary function of a 'main switch' in a distribution board?
Answer
It breaks the connections of the live and neutral wires simultaneously from the main supply.
Question
Concept: Double pole switch
Answer
Definition: A switch, such as the main switch, that disconnects both the live and neutral wires at the same time.
Question
In a house wiring ring system, where do the wires for each portion of the house return after running through the rooms?
Answer
They return back to the distribution box.
Question
State one advantage of the ring system regarding the thickness of the wires used.
Answer
Current can reach an appliance through two paths, allowing the use of thinner wires for the main ring.
Question
How are individual appliances connected relative to each other in a standard household circuit?
Answer
All electrical appliances are connected in parallel with the mains.
Question
Why is it advantageous for household appliances to be connected in parallel regarding voltage?
Answer
Each appliance receives the full $220\text{ V}$ supply required for its normal working.
Question
What happens to other appliances in a series circuit if one appliance is switched off?
Answer
All other appliances in the circuit stop operating.
Question
Term: Electric Fuse
Answer
Definition: A safety device used to limit the current in an electric circuit by melting and breaking the circuit when current exceeds a safe limit.
Question
On what physical principle does an electric fuse operate?
Answer
The heating effect of electric current.
Question
The rise in temperature $\Delta T$ of a fuse wire is directly proportional to the square of current $I$ and inversely proportional to the _____.
Answer
Cube of its radius $r^3$.
Question
Why is an alloy of lead and tin typically used as the material for a fuse wire?
Answer
It has a low melting point (approx. $250^{\circ}\text{C}$) and high specific resistance.
Question
Why are copper or aluminium wires unsuitable for use as fuse wires?
Answer
They have very high melting points (approx. $1080^{\circ}\text{C}$), meaning they will not melt even if the current exceeds safe limits.
Question
In which specific wire of a circuit must a fuse always be connected?
Answer
The fuse must always be connected in the live wire.
Question
Why is it dangerous to connect a fuse in the neutral wire?
Answer
If the fuse blows, the appliance remains connected to the high potential live wire, posing an electric shock risk.
Question
How is the current rating of a fuse in a circuit calculated based on appliance power?
Answer
$\text{Current rating} = \frac{\text{total power of appliances}}{\text{voltage of the supply}}$
Question
Term: Miniature Circuit Breaker (MCB)
Answer
Definition: A device that automatically switches off (trips) the circuit in a very short time if the current exceeds a safe limit due to a fault.
Question
State one reason why an MCB is more convenient than a conventional fuse.
Answer
It can be reset quickly by hand without the inconvenience of replacing a fuse wire.
Question
In which wire of an appliance circuit should a single pole switch be connected?
Answer
It should be connected in the live wire.
Question
Why is a switch connected in the neutral wire considered deceptive and dangerous?
Answer
Even when the switch is 'off', the appliance remains connected to the live wire at high potential.
Question
What safety precaution should be observed regarding the condition of hands when operating a switch?
Answer
A switch should never be touched with wet hands to avoid electric shock through the water.
Question
Which type of switch is used to control a single light from two different places, such as the top and bottom of a staircase?
Answer
A dual control switch (or two-way switch).
Question
What is the primary purpose of 'local earthing' near the kWh meter?
Answer
To provide a low-resistance path for leakage current to flow into the ground, protecting the house from fire.
Question
Why is the metallic case of appliances like refrigerators or electric irons earthed?
Answer
To ensure that if the live wire touches the case, the current flows to earth and blows the fuse, preventing electric shock.
Question
In a three-pin plug, which pin is made longer and thicker than the others?
Answer
The earth pin.
Question
Why is the earth pin in a three-pin plug made longer than the live and neutral pins?
Answer
To ensure the earth connection is made first and broken last for safety.
Question
According to the new international convention, what are the insulation colours for the live, neutral, and earth wires?
Answer
Live is Brown, Neutral is Light Blue, and Earth is Green or Yellow.
Question
What were the traditional (old) insulation colours for live, neutral, and earth wires?
Answer
Live was Red, Neutral was Black, and Earth was Green.
Question
Term: High Tension Wire
Answer
Definition: A special type of wire with low resistance and a large surface area, designed to carry heavy currents at high voltages.
Question
Why are high tension wires made by twisting many thin wires together instead of using one thick wire?
Answer
To provide a larger surface area for better heat radiation while maintaining low resistance.
Question
List two major dangers associated with using electricity in a household.
Answer
The two major dangers are fire and electric shock.
Question
What is the standard voltage and frequency supplied to houses in India?
Answer
The supply is $220\text{ V}$ at $50\text{ Hz}$.
Question
Calculate the current drawn by a $3\text{ kW}$ electric motor connected to a $220\text{ V}$ main supply.
Answer
The current $I = \frac{3000\text{ W}}{220\text{ V}} \approx 13.6\text{ A}$.
Question
In a distribution box, what is the typical current rating of the MCB for a ring circuit?
Answer
The rating is typically $30\text{ A}$.
Question
What is the equivalent energy value of $1\text{ kWh}$ in Joules?
Answer
$1\text{ kWh} = 3.6 \times 10^6\text{ J}$.
Question
If two identical bulbs are connected in series across $220\text{ V}$, what is the voltage across each bulb?
Answer
The voltage across each bulb is $110\text{ V}$.
Question
Why does a car's headlamps become dim when the engine is started?
Answer
The starter draws a heavy current, causing the battery's terminal voltage to fall due to voltage drop.
Question
What is the function of the salt and charcoal mixture used in local earthing?
Answer
It keeps the ground damp to maintain a low-resistance conducting layer between the copper plate and the earth.