A positively-charged particle (alpha particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is:
(a) towards south
(b) towards east
(c) downward
(d) upward

Options

A. towards south
B. towards east
C. downward
D. upward

Answer

To determine the direction of the magnetic field when a positively charged alpha particle is projected towards the west and deflected towards the north, we can use Fleming's Left-Hand Rule. Here’s a step-by-step solution:

### Step 1: Identify the Direction of Motion
The alpha particle is positively charged and is projected towards the west. This means the direction of the current (I) is also towards the west.

**Hint:** Remember that the direction of current is the same as the direction of positive charge movement.

### Step 2: Identify the Direction of Deflection
The alpha particle is deflected towards the north. According to Fleming's Left-Hand Rule, the direction of the force (F) acting on the charged particle is towards the north.

**Hint:** The force on a charged particle in a magnetic field is the direction in which it moves.

### Step 3: Apply Fleming's Left-Hand Rule
Fleming's Left-Hand Rule states:
- The thumb represents the direction of the force (F).
- The forefinger represents the direction of the magnetic field (B).
- The middle finger represents the direction of the current (I).

In this case:
- Point your thumb towards the north (direction of force).
- Point your middle finger towards the west (direction of current).

**Hint:** Ensure your thumb and middle finger are perpendicular to each other.

### Step 4: Determine the Direction of the Magnetic Field
With your thumb pointing north (force) and your middle finger pointing west (current), your forefinger will point upwards.

**Hint:** The forefinger indicates the direction of the magnetic field.

### Conclusion
Thus, the direction of the magnetic field is **upward**.

**Final Answer:** (d) upward