CIE IGCSE Physics (0625) Force on a current-carrying conductor Study Notes - New Syllabus
CIE IGCSE Physics (0625) Force on a current-carrying conductor Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Force on a current-carrying conductor
Key Concepts:
- Experiment: Force on a Current-Carrying Conductor in a Magnetic Field
- Relationship Between Force, Magnetic Field, and Current
Experiment: Force on a Current-Carrying Conductor in a Magnetic Field
Experiment: Force on a Current-Carrying Conductor in a Magnetic Field
Objective: To demonstrate that a force acts on a wire carrying a current placed in a magnetic field, and to observe how changing the direction of the current or magnetic field affects the direction of the force.
Apparatus:
- Strong horseshoe magnet
- Straight conducting wire (e.g., copper)
- Variable power supply
- Switch
- Connecting wires
- Clamp stand to suspend the wire horizontally
Procedure:
- Set up the wire horizontally between the poles of the horseshoe magnet so that the magnetic field is perpendicular to the wire (i.e., into or out of the page).
- Connect the wire to a power supply with a switch so that you can control the direction and magnitude of the current.
- Close the switch to allow current to flow through the wire.
Observation:
When the current flows through the wire, the wire experiences a sideways push — this is the magnetic force acting on the current-carrying conductor.
Reversing the Current:
- Reverse the connections to the power supply.
- Now current flows in the opposite direction through the wire.
- Observation: The wire is pushed in the opposite direction compared to before.
Reversing the Magnetic Field:
- Flip the magnet so that the north and south poles are swapped.
- Keep the direction of the current the same.
- Observation: Again, the wire moves in the opposite direction compared to the original setup.
Conclusion:
A current-carrying conductor placed in a magnetic field experiences a force. The direction of this force depends on both:
- The direction of the current
- The direction of the magnetic field
This phenomenon is described by Fleming’s Left-Hand Rule:
Thumb = Force, First finger = Field, Second finger = Current
Relationship Between Force, Magnetic Field, and Current
Relationship Between Force, Magnetic Field, and Current
Fleming’s Left-Hand Rule:
Fleming’s Left-Hand Rule is used to determine the direction of the magnetic force on a current-carrying conductor or a moving charge in a magnetic field.
Hold your left hand with the:
- Thumb → Force (motion of conductor or particle)
- First Finger → Magnetic Field (from North to South)
- Second Finger → Current (conventional current: positive to negative)
For Beams of Charged Particles
Charged particles (like electrons or protons) moving through a magnetic field also experience a force.
The direction of this force depends on:
- Direction of the particle’s velocity
- Direction of the magnetic field
- Sign of the charge on the particle
Rule: Use Fleming’s Left-Hand Rule for positive charges (like protons). For negative charges (like electrons), the force is in the opposite direction to what the rule gives.
Example:
A proton is moving to the right. It enters a uniform magnetic field pointing into the page. What is the direction of the magnetic force on it?
▶️ Answer/Explanation
Use Fleming’s Left-Hand Rule:
First finger (field): into the page
Second finger (current/velocity): to the right
Thumb (force): points upward
Since it’s a proton (positive charge), the force direction given by the rule is correct.
Final Answer: \(\boxed{\text{Upward}}\)
Example:
An electron is moving upward into a region where the magnetic field is from left to right. In which direction does the electron experience a force?
▶️ Answer/Explanation
Use Fleming’s Left-Hand Rule for a positive charge first:
First finger (field): left to right
Second finger (velocity): up
Thumb (force): out of the page (towards you)
But the particle is an electron (negative charge), so the force is in the opposite direction.
Final Answer: \(\boxed{\text{Into the page}}\)
Lorentz Force
The Lorentz force is the total force experienced by a charged particle when it moves through both an electric field and a magnetic field.
Lorentz Force Equation
\(\vec{F} = q\vec{E} + q(\vec{v} \times \vec{B})\)
Where:
- \(\vec{F}\) = total force on the particle (N)
- \(q\) = charge of the particle (C)
- \(\vec{E}\) = electric field (V/m)
- \(\vec{v}\) = velocity of the particle (m/s)
- \(\vec{B}\) = magnetic field (T)
- \(\times\) = vector cross product
Special Cases
- If only electric field is present: \(\vec{F} = q\vec{E}\)
- If only magnetic field is present: \(\vec{F} = q(\vec{v} \times \vec{B})\)
Direction of Magnetic Lorentz Force
Use the Right-Hand Rule for the magnetic part:
- Point fingers in the direction of \(\vec{v}\) (velocity)
- Curl fingers in the direction of \(\vec{B}\) (magnetic field)
- Thumb gives the direction of \(\vec{F}\) (for positive charge)
For negative charges, the force is in the opposite direction.
Units
- Force \(\vec{F}\): newton (N)
- Electric field \(\vec{E}\): volts per metre (V/m)
- Magnetic field \(\vec{B}\): tesla (T)
Example:
An electron (\(q = -1.6 \times 10^{-19}\ \text{C}\)) is moving with velocity \(\vec{v} = 2.0 \times 10^6\ \text{m/s}\) perpendicular to a magnetic field of \(0.5\ \text{T}\). What is the magnitude of the magnetic force?
▶️ Answer/Explanation
Use: \(F = qvB\sin\theta\)
\(\theta = 90^\circ \Rightarrow \sin\theta = 1\)
\(F = (1.6 \times 10^{-19})(2.0 \times 10^6)(0.5)\)
\(F = 1.6 \times 10^{-13}\ \text{N}\)
Since it’s an electron, direction is opposite to right-hand rule prediction.
Final Answer: \(\boxed{1.6 \times 10^{-13}\ \text{N}}\)
Example:
A charged particle moves through perpendicular electric and magnetic fields such that it passes through undeviated. If \(E = 300\ \text{V/m}\) and \(B = 0.2\ \text{T}\), what is the velocity of the particle?
▶️ Answer/Explanation
For the particle to be undeviated: \(qE = qvB \Rightarrow E = vB\)
\(v = \frac{E}{B} = \frac{300}{0.2} = 1500\ \text{m/s}\)
Final Answer: \(\boxed{1500\ \text{m/s}}\)