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Physics Chapter 12 : Topic 3
Magnetic Force and Fleming’s Left-Hand Rule
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The equation used to calculate the force on a current-carrying conductor in a magnetic field is:
Reasoning: The magnetic force on a conductor is given by $\mathbf{F = BIL}\mathbf{\ sin\ }\mathbf{\theta}$, where $\mathbf{\theta}$ is the angle between current and magnetic field.
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In the equation $\mathbf{F = BIL}\mathbf{\ sin\ }\mathbf{\theta}$, the symbol $\mathbf{(}\mathbf{B}\mathbf{)}$ represents:
Reasoning: $\mathbf{(B)}$ represents magnetic field strength or magnetic flux density measured in tesla.
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The unit of magnetic flux density $\mathbf{(}\mathbf{B}\mathbf{)}$ is:
Reasoning: Magnetic flux density is measured in tesla $\mathbf{(T)}$.
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In $\mathbf{F = BIL}\mathbf{\ sin\ }\mathbf{\theta}$, the angle $\mathbf{\theta}$ is the angle between:
Reasoning: $\mathbf{\theta}$ represents the angle between the direction of current and magnetic field.
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A conductor carrying current is placed perpendicular to a magnetic field. The force is:
Reasoning: When $\mathbf{\theta}\mathbf{=}\mathbf{90}^{\mathbf{o}}$, $\mathbf{sin\ }\mathbf{90}^{\mathbf{o}}\mathbf{= 1}$, giving maximum force.
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A conductor carrying current is placed parallel to a magnetic field. The force is:
Reasoning: When $\mathbf{\theta}\mathbf{=}\mathbf{0}^{\mathbf{o}}$, $\mathbf{sin\ }\mathbf{0}^{\mathbf{o}}\mathbf{= 0}$, so no magnetic force acts.
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A wire of length $\mathbf{2\ m}$ carries a current of $\mathbf{5\ A}$ in a magnetic field of $\mathbf{0.4\ T}$. The wire is perpendicular to the field. The force is:
Reasoning: $\mathbf{F = BIL}\mathbf{\ sin\ }\mathbf{\theta}\mathbf{=}\mathbf{BIL}\mathbf{\ sin\ }\mathbf{90}^{\mathbf{o}}\mathbf{= 0.4 \times 5 \times 2 \times 1}\mathbf{= 4\ N}$
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A conductor carries $\mathbf{3\ A}$ current through a magnetic field of $\mathbf{0.5\ T}$. If its length is $\mathbf{4\ m}$ and it is at $\mathbf{30}^{\mathbf{o}}$ to the field, the force is:
Reasoning: $\mathbf{F = BIL}\mathbf{\ sin\ }\mathbf{\theta}\mathbf{= 0.5 \times 3 \times 4 \times sin\ }\mathbf{30}^{\mathbf{o}}\mathbf{= 6 \times 0.5 = 3\ N}$
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If the current in a conductor is doubled while $\mathbf{B}$, $\mathbf{L}$ and $\mathbf{\theta}$ remain constant, the force will:
Reasoning: Force is directly proportional to current. $\mathbf{\Longrightarrow}$ $\mathbf{F}\mathbf{\propto}\mathbf{I}$
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If the magnetic field strength is tripled, the force on the conductor will:
Reasoning: Force is directly proportional to magnetic field strength. $\mathbf{\Longrightarrow}$ $\mathbf{F}\mathbf{\propto}\mathbf{B}$
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A conductor of length $\mathbf{5\ m}$ experiences force in a magnetic field. If its length is reduced to half, the force becomes:
Reasoning: Force is directly proportional to the length of conductor in the field.
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The value of $\mathbf{\sin}\mathbf{\ \theta}$ becomes maximum when $\mathbf{\theta}$ is:
Reasoning: $\mathbf{sin\ }\mathbf{90}^{\mathbf{o}}\mathbf{= 1}$, giving maximum force.
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A wire carries current in a magnetic field. If the angle between current and field changes from $\mathbf{90}^{\mathbf{o}}$ to $\mathbf{30}^{\mathbf{o}}$, the force:
Reasoning: Since $\mathbf{sin\ }\mathbf{30}^{\mathbf{o}}$ is less than $\mathbf{sin\ }\mathbf{90}^{\mathbf{o}}$, the force decreases.
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If a conductor carries current of $\mathbf{10\ A}$ and experiences a force of $\mathbf{20\ N}$ in a magnetic field of $\mathbf{0.5\ T}$ at $\mathbf{90}^{\mathbf{o}}$, its length is:
Reasoning: $\mathbf{F = BIL\ }\mathbf{sin\ }\mathbf{90}^{\mathbf{o}}$ $\mathbf{\Longrightarrow}$ $\mathbf{20 = 0.5 \times 10 \times L \times 1}$ $\mathbf{\Longrightarrow}$ $\mathbf{L = 4\ m}$
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The direction of force on a current-carrying conductor is determined by:
Reasoning: Fleming’s left-hand rule gives the direction of force.
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In Fleming’s left-hand rule, the thumb represents:
Reasoning: The thumb points in the direction of force or motion.
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In Fleming’s left-hand rule, the first finger points in the direction of:
Reasoning: The first finger represents the magnetic field direction.
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A current-carrying conductor experiences force upward. If the current direction is reversed, the force will:
Reasoning: Reversing current reverses the direction of magnetic force.
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If the magnetic field direction is reversed while current remains unchanged, the force direction:
Reasoning: Force direction depends on both current and magnetic field directions.
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A conductor carries current perpendicular to a magnetic field. If current becomes zero, the force becomes:
Reasoning: No current means no moving charges, so no magnetic force.
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A force of $\mathbf{10\ N}$ acts on a conductor. If the angle changes from $\mathbf{90}^{\mathbf{o}}$ to $\mathbf{0}^{\mathbf{o}}$, the force becomes:
Reasoning: At $\mathbf{0}^{\mathbf{o}}$, $\mathbf{sin\ }\mathbf{0}^{\mathbf{o}}\mathbf{= 0\ }$, so force becomes zero.
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In Fleming’s left-hand rule, the second finger represents:
Reasoning: The second finger indicates conventional current direction.
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A wire carrying current is placed at $\mathbf{45}^{\mathbf{o}}$ to a magnetic field. Compared with the maximum force, the force is:
Reasoning: $\mathbf{sin\ }\mathbf{45}^{\mathbf{o}}\mathbf{= 0.707}$, so force is about $\mathbf{71\%\ }$of maximum.
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Which change will NOT affect the magnetic force on a conductor?
Reasoning: Force depends on $\mathbf{B}$, $\mathbf{I}$, $\mathbf{L}$ and $\mathbf{\theta}$ not colour.
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A conductor placed in a magnetic field experiences maximum force because:
Reasoning: Maximum interaction occurs when current is perpendicular to the magnetic field.
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If $\mathbf{B}\mathbf{= 2\ T}$, $\mathbf{I}\mathbf{= 4\ A}$, $\mathbf{L}\mathbf{= 3\ m}$ and $\mathbf{\theta}\mathbf{=}\mathbf{90}^{\mathbf{o}}$, the force is:
Reasoning: $\mathbf{F = BIL\ }\mathbf{sin\ }\mathbf{90}^{\mathbf{o}}\mathbf{= 2 \times 4 \times 3 \times 1 = 24\ N}$
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A current-carrying conductor in a magnetic field is the basic principle of:
Reasoning: Electric motors use magnetic force on current-carrying conductors.
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If the force on a conductor is zero, possible reasons include:
Reasoning: Force becomes zero when $\mathbf{I}\mathbf{= 0}$ or $\mathbf{\theta}\mathbf{=}\mathbf{0}^{\mathbf{o}}$.
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A conductor carrying current in a magnetic field experiences force because:
Reasoning: The magnetic field exerts force on moving charges in the conductor.
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The direction of force on a current-carrying conductor can be changed by:
Reasoning: Force direction depends on current and magnetic field directions.
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