Force on a Current-Carrying Conductor (Cambridge (CIE) O Level Physics): Revision Note
Force on a Current-Carrying Conductor
A current-carrying conductor produces its own magnetic field
When interacting with an external magnetic field, it therefore will experience a force
A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines
A simple situation would be a copper rod placed within a uniform magnetic field
When current is passed through the copper rod, it experiences a force which makes it move
Force on a Current-Carrying Copper Rod in a Magnetic Field
A copper rod moves within a magnetic field when current is passed through it
Two ways to reverse the direction of the force (and therefore, the copper rod) are by reversing:
The direction of the current
The direction of the magnetic field
Examiner Tips and Tricks
This phenomenon is sometimes referred to as 'the motor effect'. The direction of the force is determined by Fleming's left-hand rule.
Left Hand Rule
The direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field
All three will be perpendicular to each other
This means that sometimes the force could appear to be acting either into or out of the page
The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:
Fleming's Left-Hand Rule
Fleming's left-hand rule can be used to determine directions of the force (thumb), magnetic field (first finger) and current (second finger)
Worked Example
Use Fleming’s left-hand rule to show that if the current-carrying wire is placed into the magnetic field between the poles of the magnet, as shown below, there will be a downwards force acting on the wire.
Answer:
Step 1: Determine the direction of the magnetic field
Start by pointing your First Finger in the direction of the (magnetic) Field.
Step 2: Determine the direction of the current
Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current
Step 3: Determine the direction of the force
The THumb will now be pointing in the direction of the THrust (the force)
Therefore, this will be the direction in which the wire will move
Examiner Tips and Tricks
Remember that the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.
Feel free to use Fleming's left hand rule in your exam, just don't make it too distracting for other students!
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