Deflection of Alpha Particles and Electrons | Class 10

Quick answer: A moving charged particle in a magnetic field feels a force, just like a current-carrying wire. To find its direction with Fleming's left-hand rule, first find the direction of current: for a positive particle (alpha particle, proton), the current is in the same direction as its motion; for a negative particle (electron, beta particle), it is in the opposite direction. For example, an alpha particle moving west and deflected north must be in a magnetic field pointing vertically upwards.

When a current-carrying wire is placed in a magnetic field, the wire experiences a force and can move. A current is simply a flow of charges, so a beam of charged particles moving through a magnetic field also experiences a force and is deflected.

Both cases follow the same rule: Fleming's left-hand rule.

In CBSE Class 10 Science, Chapter 12 (Magnetic Effects of Electric Current), questions on the force on a current-carrying conductor and the deflection of alpha particles and electrons are frequently asked in board exams.

What You Will Learn

• How a current-carrying conductor moves in a magnetic field
• How to find the direction of current for positive and negative particles
• How to use Fleming's left-hand rule
• When a moving charge feels no magnetic force
• Two solved examples: an alpha particle and an electron beam

1. Force on a Current-Carrying Conductor

A simple demonstration (similar to the NCERT activity) shows this force:

1. A light metal rod or wire is hung so that it can swing freely between the poles of a horseshoe magnet.
2. When current is passed through it, the rod is pushed sideways, out of the gap between the poles.
3. Reversing the current reverses the direction of the push.
4. Reversing the magnet's poles also reverses the direction of the push.

Key point: A current-carrying conductor placed in a magnetic field experiences a force. The force is largest when the current is perpendicular to the magnetic field, and zero when the current is parallel to the field.

Extra fact (beyond the Class 10 textbook): In higher classes, the force on a single charge $q$ moving with speed $v$ at an angle $\theta$ to a magnetic field $B$ is written as $F = qvB\sin\theta$. It is zero when $\theta = 0^\circ$ or $180^\circ$ (motion along the field) and largest when $\theta = 90^\circ$.

2. Direction of Current for Moving Particles

To use Fleming's left-hand rule for a moving particle, first find the direction of conventional current:

Rule: For a positive particle, current is in the direction of motion. For a negative particle, current is opposite to the direction of motion.

3. Fleming's Left-Hand Rule

Stretch the thumb, forefinger and middle finger of your left hand so that they are at right angles to each other.

4. Solved Example 1: Alpha Particle (NCERT)

Question

An alpha particle (positively charged) projected towards the west is deflected towards the north by a magnetic field. What is the direction of the magnetic field?

Solution

Step 1: Direction of current. The alpha particle is positive, so the current is in the direction of its motion: towards the west.

Step 2: Direction of force. It is deflected towards the north, so the force is towards the north.

Step 3: Apply Fleming's left-hand rule. Point the middle finger west (current) and the thumb north (force). The forefinger then points vertically upwards.

Answer: The magnetic field is directed vertically upwards.

5. Solved Example 2: Electron Beam

Question

A horizontal beam of electrons moving from south to north enters a uniform magnetic field directed vertically downwards. In which direction is the beam deflected?

Solution

Step 1: Direction of current. Electrons are negative, so the current is opposite to their motion: from north to south, i.e. towards the south.

Step 2: Direction of field. The field is vertically downwards.

Step 3: Apply Fleming's left-hand rule. Point the forefinger downwards (field) and the middle finger towards the south (current). The thumb then points towards the east.

Answer: The electron beam is deflected towards the east.

Check: If the beam were made of positive particles moving the same way, the current would be towards the north and the force would be towards the west. Electrons are deflected the opposite way, towards the east. ✓

6. Summary Table

7. Exam Tips and Common Mistakes

Exam Tip: Always write the direction of current first, then apply Fleming's left-hand rule.

Exam Tip: In the rule, the forefinger is the field, the middle finger is the current and the thumb is the force.

Common Mistake: Using the direction of electron motion as the current. For electrons, the current is in the opposite direction.

Common Mistake: Using the right hand. Fleming's left-hand rule gives the direction of force on a current-carrying conductor (as in a motor); the right-hand rule is for induced current (as in a generator).

Frequently Asked Questions

What is Fleming's left-hand rule?

Stretch the thumb, forefinger and middle finger of the left hand at right angles to each other. If the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of the force.

An alpha particle moving west is deflected north. What is the direction of the magnetic field?

The magnetic field is vertically upwards.

Which way is an electron beam moving north deflected in a downward magnetic field?

The current is towards the south, so by Fleming's left-hand rule the beam is deflected towards the east.

Why is the current opposite to the motion of electrons?

Conventional current is taken as the direction in which positive charge flows. Electrons are negative, so their motion is opposite to the conventional current.

When does a moving charge experience no magnetic force?

When it moves parallel to the magnetic field, or when it has no charge, like a neutron.