A current-carrying wire can feel a real mechanical force when it is placed in a magnetic field. This effect happens because moving charges inside the wire interact with the magnetic field around them. It explains how electrical energy can be converted into motion.
The idea is essential for understanding motors, speakers, meters, and many electromagnetic devices.
The size of the force depends on the current, the length of wire in the field, the magnetic field strength, and the angle between the wire and the field. The direction of the force is found with a right-hand rule using current, magnetic field, and force as three perpendicular directions. When many wires are shaped into a loop, the magnetic forces on opposite sides can produce a turning effect called torque.
This is the basic motor effect used to make rotating machines.
Understanding Physics: Magnetic Force on a Current
Inside a metal wire, the mobile charges drift very slowly, even though the electrical signal through a circuit is established much faster. A magnetic field acts on each moving charge sideways. The charges then press into the fixed atoms of the metal.
Those countless tiny pushes are passed to the whole wire, so the wire can move. The magnetic field changes the direction of a charge's motion but does not directly give that charge extra kinetic energy.
When a motor speeds up or lifts a load, the energy comes from the battery or power supply. The field provides the route by which electrical energy becomes mechanical motion.
Direction is often the hardest part of this topic. Physics uses conventional current, defined as the direction positive charge would move. In a metal, electrons actually move in the opposite direction.
A right hand rule based on conventional current still gives the force on the wire correctly. If a student uses electron motion by mistake, the predicted direction is reversed. It helps to draw three clear arrows first for current, field, and force.
The field arrow points from a magnet's north pole toward its south pole outside the magnet. The force must point at right angles to both other arrows. It never points along the wire or directly along the magnetic field.
A motor uses a loop because forces on separated parts of the loop can form a pair. One side is pushed one way while the opposite side is pushed the other way. The total sideways push may be zero, yet the two forces act at different places.
This creates a turning effect. A larger loop, more turns of wire, a stronger current, or a stronger magnetic field can increase the turning effect. As the loop rotates, its position changes until the turning effect would reverse.
A simple direct current motor uses a split ring called a commutator to reverse the current every half turn. This keeps the turning effect in the same rotation direction. In many modern motors, electronic circuits perform this switching.
When solving problems, pay attention to the part of the wire that actually lies in the magnetic field. A long wire does not gain force from sections outside the field region. The angle is the angle between the current direction and the field direction, not the angle between the wire and the page.
Force is measured in newtons, field strength in teslas, current in amperes, and wire length in metres. Real devices have extra effects too.
Wire resistance causes heating, friction resists rotation, and a moving motor coil can generate a voltage that opposes the supply. This opposing voltage is why a motor usually draws a large current when it first starts, then less current once it is spinning freely.
Key Facts
- Magnetic force on a straight current-carrying wire: F = BIL sin(theta)
- F is maximum when the wire is perpendicular to the magnetic field, so theta = 90 degrees and F = BIL
- F is zero when the current is parallel to the magnetic field, so theta = 0 degrees or 180 degrees
- The direction of force is perpendicular to both the current direction and the magnetic field direction
- For positive conventional current, use the right-hand rule: fingers point with I, curl toward B, thumb points in the direction of F
- The motor effect occurs when magnetic forces on current-carrying conductors produce motion or rotation
Vocabulary
- Current
- Current is the rate of flow of electric charge through a conductor, measured in amperes.
- Magnetic field
- A magnetic field is a region where magnetic forces act on moving charges, currents, or magnetic materials.
- Magnetic force
- Magnetic force is the push or pull exerted by a magnetic field on moving charge or on a current-carrying wire.
- Right-hand rule
- The right-hand rule is a method for finding the direction of magnetic force using the relative directions of current and magnetic field.
- Motor effect
- The motor effect is the production of force or rotation when a current-carrying conductor is placed in a magnetic field.
Common Mistakes to Avoid
- Using F = BIL without checking the angle is wrong because the full formula is F = BIL sin(theta). Only use F = BIL when the wire is perpendicular to the magnetic field.
- Pointing the force in the same direction as the magnetic field is wrong because magnetic force on a current is perpendicular to both I and B. Use the right-hand rule to find the third direction.
- Confusing electron flow with conventional current is wrong because most right-hand rule diagrams use conventional current from positive to negative. If using electron flow, the force direction is opposite.
- Forgetting that only the wire length inside the magnetic field counts is wrong because L in F = BIL sin(theta) is the portion of conductor actually exposed to the field.
Practice Questions
- 1 A straight wire carries a current of 4.0 A through a 0.30 m region of uniform magnetic field. The field strength is 0.50 T and the wire is perpendicular to the field. What is the magnetic force on the wire?
- 2 A 0.80 m wire carries 2.5 A in a 0.60 T magnetic field. The angle between the current and the field is 30 degrees. Calculate the magnetic force on the wire.
- 3 A horizontal wire carries current to the right while a magnetic field points into the page. Use the right-hand rule to determine the direction of the magnetic force, and explain how reversing the current would change the result.