To solve the question, we will follow these steps:
### Step 1: Identify the Physical Quantity
The physical quantity whose unit is Tesla is the **magnetic field**.
### Step 2: Define a Tesla
A Tesla (T) is defined based on the relationship between force, charge, velocity, and the angle between the velocity and the magnetic field.
### Step 3: Write the Formula
The magnetic force \( F \) on a charged particle moving in a magnetic field can be expressed as:
\[
F = B \cdot q \cdot v \cdot \sin(\theta)
\]
where:
- \( F \) is the magnetic force in newtons (N),
- \( B \) is the magnetic field strength in teslas (T),
- \( q \) is the charge in coulombs (C),
- \( v \) is the velocity in meters per second (m/s),
- \( \theta \) is the angle between the velocity vector and the magnetic field vector.
### Step 4: Set Values for Definition
To define one Tesla, we consider the following conditions:
- Let \( F = 1 \, \text{N} \) (force),
- Let \( q = 1 \, \text{C} \) (charge),
- Let \( v = 1 \, \text{m/s} \) (velocity),
- Let \( \theta = 90^\circ \) (which means \( \sin(90^\circ) = 1 \)).
### Step 5: Substitute Values
Substituting these values into the formula gives:
\[
1 \, \text{N} = B \cdot 1 \, \text{C} \cdot 1 \, \text{m/s} \cdot 1
\]
This simplifies to:
\[
B = 1 \, \text{T}
\]
### Step 6: Final Definition
Thus, we can define one Tesla as:
**One Tesla is the magnetic field strength that exerts a force of one newton on a charge of one coulomb moving at a velocity of one meter per second, when the charge is moving perpendicular to the magnetic field.**
### Summary
- The physical quantity is the **magnetic field**.
- One Tesla is defined as the magnetic field that exerts a force of one newton on a charge of one coulomb moving at a velocity of one meter per second, perpendicular to the magnetic field.
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