Which of the following operates based on magnetic effect of electric current?

1. Analyze the question

The question asks us to identify which of the listed devices functions primarily through the magnetic effect of electric current (also known as electromagnetism).

2. Explain the concept

The magnetic effect of electric current states that whenever an electric current flows through a conductor, a magnetic field is produced around it. Devices that utilize this principle include:

• Electromagnets: Used in electric bells, relays, and circuit breakers.
• Electric Motors: Use the force exerted on a current-carrying conductor in a magnetic field.
• Generators/Dynamos: While they primarily use electromagnetic induction (Faraday's Law), they rely on magnetic fields to convert mechanical energy into electrical energy.

Let's look at the options:

• Electric heater: Operates on the heating effect of electric current (Joule's Law), where electrical energy is converted to heat.
• Photocell: Operates on the photoelectric effect, where light energy is converted into electrical energy.
• Dynamo: Operates on the principle of electromagnetic induction (a magnetic field is essential for the generation of current).
• Cold cathode: Operates on the principle of thermionic or field emission of electrons.

3. Show the solution

• Option A (Electric heater): Incorrect. It uses the heating effect ($I^2Rt$).
• Option B (Photocell): Incorrect. It uses the photoelectric effect.
• Option C (Dynamo): Correct. A dynamo (or generator) works by rotating a coil within a magnetic field. The interaction between the magnetic field and the motion of the conductor is the core of its operation. In the context of JAMB physics, the dynamo is the standard device associated with magnetic fields among these options.
• Option D (Cold cathode): Incorrect. It relates to electron emission.

4. Identify the answer

The correct option is C.

5. Tip

When you see questions about the "effects of electric current," remember the "Big Three":

1. Heating Effect: Toasters, irons, heaters (Resistors).
2. Chemical Effect: Electrolysis, electroplating (Electrolytes).
3. Magnetic Effect: Motors, dynamos, bells, transformers (Electromagnets).