Unit Test (Solutions): Electricity and Its Effects | Science Class 7 (Old NCERT) PDF Download

Time: 1 hour
Maximum Marks: 30
Attempt all questions.

• Question numbers 1 to 5 carry 1 mark each.
• Question numbers 6 to 8 carry 2 marks each.
• Question numbers 9 to 11 carry 3 marks each.
• Question numbers 12 & 13 carry 5 marks each.

Q1: Which of the following is an electrical component used in a circuit?  (1 Mark)
(a) Bulb
(b) Wire
(c) Battery
(d) All of the above
Ans: (d) All of the above
These components help in the functioning of an electric circuit.

Q2: What does an electric fuse do in a circuit?  (1 Mark)
Ans: An electric fuse protects the circuit by breaking the connection when excessive current flows through it.

Q3: Which of the following devices does not work based on the heating effect of electric current?  (1 Mark)
(a) Electric iron
(b) Refrigerator
(c) Immersion heater
(d) Toaster
Ans: (b) Refrigerator
A refrigerator works on cooling mechanisms, while the others generate heat using electric current.

Q4: What is an electromagnet?  (1 Mark)
Ans: An electromagnet is a temporary magnet created when electric current flows through a coil of wire wound around an iron core.

Q5: Name one safety device other than a fuse that prevents excessive current from damaging electrical appliances.  (1 Mark)
Ans: Miniature Circuit Breaker (MCB)
MCBs automatically switch off the circuit when the current exceeds a safe limit.

Q6: Differentiate between open circuit and closed circuit.  (2 Marks)
Ans:

Q7: Explain how overloading can cause damage to electrical circuits.  (2 Marks)
Ans:

• Overloading happens when too many electrical appliances are connected to a single socket.
• The excessive flow of current generates excess heat, which can damage appliances and cause fires.

Q8: Why do CFLs consume less electricity than ordinary bulbs?  (2 Marks)
Ans:

• Ordinary bulbs use the heating effect of current to produce light, wasting energy as heat.
• CFLs use a fluorescent coating and electrodes to generate light efficiently, consuming less energy.

Q9. Explain how an electric fuse works.  (3 Marks)

Ans:

• A fuse is a thin wire that melts when excessive current flows through it.
• This breaks the circuit, preventing damage to electrical appliances.
• Example: If a sudden voltage surge occurs, the fuse blows, stopping the current flow and preventing fires.

Q10: Describe how electromagnets are used in daily life.  (3 Marks)
Ans:

1. In Cranes: Used to lift heavy metal objects in scrapyards.
2. In Electric Bells: Creates magnetic attraction to produce sound.
3. In MRI Machines: Used in hospitals for medical imaging.
4. In Maglev Trains: Uses magnetic levitation to reduce friction.

Q11: How does a Miniature Circuit Breaker (MCB) work?  (3 Marks)
Ans:

• MCBs automatically turn off when excessive current flows in a circuit.
• Unlike fuses, MCBs do not burn out and can be reset manually.
• They help prevent electric shocks and fires in households and industries.

Q12: Explain the construction and working of an electric bell with a diagram. (5 Marks)
Ans:

Construction:

1. Electromagnet: A coil wound around an iron core.
2. Armature: A piece of iron that moves when attracted by the magnet.
3. Hammer: Attached to the armature, strikes the gong.
4. Spring: Returns the armature to its original position.
5. Gong: Produces sound when struck by the hammer.

Working:

1. When the switch is pressed, current flows through the electromagnet, creating a magnetic field.
2. The electromagnet attracts the armature, making the hammer strike the gong.
3. This breaks the circuit, turning off the electromagnet.
4. The spring pulls the armature back, reconnecting the circuit.
5. The process repeats rapidly, producing a ringing sound.

Q13: How does the magnetic effect of electric current help in making an electromagnet? Explain its applications. (5 Marks)
Ans:

How an Electromagnet is Made:

1. A coil of insulated copper wire is wound around an iron core.
2. When electric current flows, the iron core behaves like a magnet.
3. The magnetism lasts only as long as the current flows.
4. When the current is switched off, the iron core loses its magnetism.

Applications of Electromagnets:

1. Lifting Heavy Metals: Used in cranes for lifting scrap iron.
2. MRI Scanners: Used in hospitals for medical imaging.
3. Loudspeakers & Microphones: Convert electrical signals into sound waves.
4. Electric Motors: Convert electricity into mechanical motion in fans and washing machines.
5. Maglev Trains: Use powerful electromagnets for fast, frictionless movement.