Physics: Which Hand Rule Applies to Motors and Generators Practice
Choosing the correct rule for motor effect and generator effect problems
Physics: Which Hand Rule Applies to Motors and Generators Practice
Choosing the correct rule for motor effect and generator effect problems
Physics - Grade 9-12
- 1
A current-carrying wire is placed between the north and south poles of a magnet. The wire experiences a force and begins to move. Which hand rule should you use to predict the direction of the force on the wire?
Look for the situation where electrical energy is being changed into motion.
You should use Fleming's left-hand rule. This is a motor effect situation because a current in a magnetic field produces a force and motion. - 2
A coil is rotated inside a magnetic field, and an electric current is induced in the coil. Which hand rule should you use to find the direction of the induced current?
You should use Fleming's right-hand rule. This is a generator effect situation because motion in a magnetic field is producing an induced current. - 3
A student says, "I see current and a magnetic field, so I should always use the left-hand rule." Explain why this is not always correct.
Decide which quantity is the result: force or induced current.
The left-hand rule is used for the motor effect when current in a magnetic field produces force or motion. If motion in a magnetic field produces an induced current, the right-hand rule is used instead. - 4
In a simple electric motor, a battery sends current through a coil inside a magnetic field. The coil turns because forces act on its sides. Which hand rule applies, and why?
Fleming's left-hand rule applies because the motor uses current and a magnetic field to produce a force that makes the coil rotate. - 5
In a bicycle dynamo, a magnet rotates near a coil and produces an electric current for the light. Which hand rule applies, and why?
A dynamo is a small generator.
Fleming's right-hand rule applies because the dynamo is a generator. Motion in a magnetic field induces an electric current. - 6
A wire is moved downward through a magnetic field directed from left to right. An emf is induced in the wire. Which hand rule should be used to determine the current direction?
Fleming's right-hand rule should be used because the wire is being moved through a magnetic field, causing an induced emf and current. - 7
A straight wire carries conventional current upward. You want to find the direction of the magnetic field that circles around the wire. Which rule should you use?
This problem asks for the magnetic field around a current, not motor force or generator current.
You should use the right-hand grip rule, not Fleming's left-hand or right-hand rule. Point your right thumb in the direction of conventional current, and your curled fingers show the magnetic field direction. - 8
A student is given a diagram of a generator and uses Fleming's left-hand rule to find the induced current. Identify the mistake and correct it.
The mistake is using the motor rule for a generator. The correct rule is Fleming's right-hand rule because the generator uses motion in a magnetic field to induce current. - 9
A current in a wire points east, and the magnetic field points north. The question asks for the direction of the force on the wire. Which rule applies?
Current plus magnetic field gives force in a motor effect problem.
Fleming's left-hand rule applies because the problem gives current and magnetic field and asks for the force, which is the motor effect. - 10
A metal rod slides to the right across two conducting rails in a magnetic field. The motion creates a current in the circuit. Which hand rule applies?
Fleming's right-hand rule applies because the rod's motion through the magnetic field induces a current. This is a generator effect. - 11
Complete the comparison: Fleming's left-hand rule is mainly for motors because it finds the direction of blank. Fleming's right-hand rule is mainly for generators because it finds the direction of blank.
Motors produce motion, while generators produce current.
Fleming's left-hand rule is mainly for motors because it finds the direction of force or motion. Fleming's right-hand rule is mainly for generators because it finds the direction of induced current. - 12
A problem describes a loudspeaker coil carrying an alternating current in a magnetic field. The coil moves back and forth to vibrate the speaker cone. Which hand rule is used to predict the force direction at an instant?
Fleming's left-hand rule is used because the loudspeaker coil is acting like a motor. Current in the coil and the magnetic field produce a force and motion.