To solve the question, "When bulk pieces of conductors are subjected to changing magnetic flux, ...A... are produced in them," we need to identify what "A" refers to in the context of electromagnetic induction.
### Step-by-Step Solution:
1. **Understanding Magnetic Flux**:
- Magnetic flux (Φ) is defined as the product of the magnetic field (B) and the area (A) through which it passes, considering the angle (θ) between the field lines and the normal to the surface:
\[
\Phi = B \cdot A \cdot \cos(\theta)
\]
- When the magnetic flux changes, it implies that either the magnetic field strength, the area, or the angle is varying.
**Hint**: Recall the definition of magnetic flux and how it can change.
2. **Induced EMF**:
- According to Faraday's law of electromagnetic induction, a change in magnetic flux through a conductor induces an electromotive force (EMF) in the conductor. The induced EMF (E) is given by:
\[
E = -\frac{d\Phi}{dt}
\]
- Here, \( \frac{d\Phi}{dt} \) is the rate of change of magnetic flux.
**Hint**: Remember Faraday's law and how it relates to changing magnetic flux.
3. **Induced Current**:
- When EMF is induced in the conductor, it causes a current (I) to flow through the conductor. This current is a result of the induced EMF.
- The relationship between induced EMF and current can be expressed as:
\[
I = \frac{E}{R}
\]
- where R is the resistance of the conductor.
**Hint**: Think about how EMF leads to current flow in a circuit.
4. **Heat Generation**:
- As current flows through the conductor, it encounters resistance, which leads to the generation of heat due to Joule heating. The power (P) dissipated as heat can be expressed as:
\[
P = I^2 R
\]
- Thus, when current flows due to induced EMF, heat is produced in the conductor.
**Hint**: Consider the relationship between current, resistance, and heat generation.
5. **Conclusion**:
- Therefore, when bulk pieces of conductors are subjected to changing magnetic flux, the following are produced:
- Induced EMF
- Induced Current
- Heat
- The correct answer for "A" in the question is "induced EMF," which leads to induced current and subsequently heat generation.
### Final Answer:
A refers to **induced EMF**.
