4.5 Electromagnetic effects
4.5.4 Force on a current-carrying conductor
Core Content
1. The Motor Effect
- When a current flows through a wire, it creates a magnetic field around it.
- If this wire is placed inside another magnetic field (e.g., between magnets), the two magnetic fields interact.
- This interaction produces a force — causing movement.
- This is called the motor effect.
Important:
To get movement (force), you need:
- A magnetic field.
- A current flowing across the magnetic field lines.
Figure 1: Basic Motor Effect Diagram
Supplement Content
2. Fleming's Left-Hand Rule
- There are 3 things to remember:
- First finger = Magnetic Field (North to South)
- Second finger = Current (positive to negative)
- Thumb = Force (direction of movement)
Figure 2: Fleming's Left-Hand Rule
How to use it:
- Point your first finger (field), second finger (current), and thumb (force) at 90° to each other.
- Move your whole hand if you need to change directions — don't bend fingers separately!
Note:
If you reverse the current or the magnetic field, the force will also reverse.
3. Force on a Beam of Charged Particles
- A magnetic field can also push a beam of charged particles like electrons.
- This happens because electrons have electric charge, just like current in a wire.
Example:
- In a vacuum tube, a beam of electrons travels from left to right.
- This is the same as a conventional current from right to left.
- If a magnet's north pole is placed behind the tube (magnetic field from north to south):
- The field points out of the page.
- Using Fleming's Left-Hand Rule ➔ the electron beam is pushed upwards.
Figure 3: Electron Beam Deflection Upward
- If the magnetic field is reversed, the beam will deflect downward.
Figure 4: Electron Beam Deflection Downward
Practice Questions
Core Questions
Q1. List two ways to reverse the force on a current-carrying conductor in a magnetic field.
You can reverse the direction of the force by:
- Reversing the current (swap the connections).
- Reversing the magnetic field (flip the magnets).
Supplement Questions
Q2.
a. Determine the direction of the force on a beam of electrons passing from right to left in a magnetic field into the page. [1]
Figure 5: Magnetic field into paper
b. What would happen if the electron beam was replaced by a beam of positively charged ions? [2]
c. Explain why the path of the beam curves as it passes through the field. [1]
Q2
a. up; [1]
b. The beam would deflect in the opposite direction (down) [1] but would not deflect as much; [1]
c. The force will always be at right angles to the direction in which the beam is travelling; [1]
IGCSE Physics | Magnetic Effects of Electric Current