Motion in an magnetic field IB DP Physics Study Notes - 2025 Syllabus
Motion in an magnetic field IB DP Physics Study Notes
Motion in an magnetic field IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand
The force per unit length between parallel wires as given by \(\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi r}\) where \( r \) is the separation between the two wires.
Standard level and higher level: 6 hours
Additional higher level: There is no additional higher level content.
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Physics 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Physics 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Force on a current-carrying conductor in a B-field
∙A conductor carrying current experiences a magnetic force when placed in an external magnetic field.
∙This is because moving charges (i.e., the current) interact with the magnetic field lines.
∙The direction of the force is given by the Right Hand Rule:
- Thumb: Direction of current \( I \)
- Fingers: Direction of magnetic field \( B \)
- Palm: Direction of force \( F \) (for positive charge)
Mathematically, the magnetic force is given by \( F = I L B \sin\theta \),
where:
- \( I \): Current in the conductor (A)
- \( L \): Length of conductor in field (m)
- \( B \): Magnetic field strength (T)
- \( \theta \): Angle between current and magnetic field
Example:
Explain the colors of the Aurora Borealis, or northern lights.
▶️ Answer/Explanation
∙The aurora borealis is caused by the interaction of charged particles from space with the earth’s magnetic field.
∙When these particles collide with nitrogen and oxygen molecules in the upper atmosphere, energy is released as light.
- Nitrogen produces violet light
- Oxygen produces green light
This process is part of the de-ionization and recombination of atmospheric molecules.
Example:
Explain the source of the charged particles causing the Aurora Borealis.
▶️ Answer/Explanation
∙The charged particles originate from the sun in the form of solar wind—streams of protons and electrons.
∙These particles are guided by Earth’s magnetic field lines toward the polar regions where they interact with the atmosphere.
Example:
Explain why the Aurora Borealis occurs near the north pole.
▶️ Answer/Explanation
∙Earth’s magnetic field lines are vertical near the poles, which directs the incoming charged particles into the upper atmosphere there.
∙This is why auroras (both Borealis in the north and Australis in the south) are seen near the poles and not at the equator.
We can use this special case of parallel current carrying wires and Ampère’s Force Law:
Where:
- \( L \) is the length of the wires
- \( r \) is the distance between the wires
- \( \mu_0 = 4\pi \times 10^{-7} \, \text{T·m/A} \) is the permeability of free space
- \( I_1 \), \( I_2 \) are the currents in each wire
Practice :
Two parallel 10 cm wires are shown. The given current in wire 1 is 0.50 A. What is the force acting on the wires if they are 5.0 cm apart and current in wire 2 is 1.5 A?
▶️ Answer/Explanation
Use: \( \frac{F}{L} = \frac{\mu_0 I_1 I_2}{2 \pi r} \)
Substitute: \( \mu_0 = 4\pi \times 10^{-7} \), \( I_1 = 0.50 \, \text{A} \), \( I_2 = 1.5 \, \text{A} \), \( r = 0.050 \, \text{m} \)
\( \frac{F}{L} = \frac{(4\pi \times 10^{-7})(0.50)(1.5)}{2\pi \times 0.050} \)
\( \frac{F}{L} = \frac{3.0 \times 10^{-7}}{0.10} = 3.0 \times 10^{-6} \, \text{N/m} \)
So the force on a 1-meter segment is:
\( \boxed{F = 3.0 \times 10^{-6} \, \text{N}} \)
This is a very small force, which is why we don’t notice wires attracting or repelling under normal conditions.