Embedded Files
x70tjw / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)
THE IB SAYS
THE IB SAYS
Essential idea: The effect scientists call magnetism arises when one charge moves in the vicinity of another moving charge.
Understandings: Magnetic fields; Magnetic force
Applications and skills: Determining the direction of force on a charge moving in a magnetic field; Determining the direction of force on a current-carrying conductor in a magnetic field; Determining the direction of the magnetic field based on current direction; Solving problems involving magnetic forces, fields, current and charges; Sketching and interpreting magnetic field patterns
Guidance: Magnetic field patterns will be restricted to long straight conductors, solenoids, and bar magnets
Data booklet equations: F=qvB sinθ; F = BIL sinθ
My vodcast
My vodcast
The magnetic field of a conductor
The magnetic field of a conductor
All moving charges affect the electric and magnetic fields they pass through. For charges constrained in a wire or other conductor the magnetic field created looks like this:
The side on view shows what is happening, but sometimes we need to view the field when the current (or a field) is pointing in or out of the page. For this we use a convention of X for fields or current into the page or screen, and a dot for out of the screen.
Thus viewing the field of a current in or out of the screen shows you a field pattern like this:
The motor effect - the force on a current carrying conductor in a magnetic field
The motor effect - the force on a current carrying conductor in a magnetic field
If a current carrying conductor is placed is a magnetic field then it will experience a force (unless the flow of charge is parallel to the field lines). Like all forces, it can be described using a magnitude and a direction.
Direction
Direction
The direction of the force is described using Fleming's Left Hand Rule (LHR). The left hand is held up with the thumb, first finger and second finger all held at right-angles (90˚) to each other. If the first finger is held in the direction of the field lines, and second finger in the direction of the current, then the thumb will give the direction of the thrust (force).
Magnitude
Magnitude
The magnitude of the force is directly proportional to magnetic field strength (B), the current in the conductor (I) and the length of the wire in the field (L). Additionally the effect is strongest when the field and the current are orthogonal (at right angles), and zero when they are parallel. This is accounted for using sin θ, where θ is the angle between the field lines and the direction of current flow.
F = BIL sin θ
F = BIL sin θ
F: Force [N]
B: Magnetic Field Strength / Flux Density [T]
I: Current [A]
L: Length of conductor in the field [m]
θ: Angle between field and current [˚ or radians]
Force on a free charge
Force on a free charge
The current in a conductor is made from a flow of individual charges. These charges each experience a force. The same equation applies in principle, but it is convenient to reframe the equation in terms of the charge carried and the relative velocity of the particle.
Direction
Direction
The direction can be described by Fleming's LHR, with the direction of movement of the charge in place of the current, as long as the charge is positive. In the case that the charge is negative, you can either using the LHR and remember to point the current / velocity finger backwards, or use Fleming's Right Hand Rule (RHR). Choose what works for you, but be consistent!
Magnitude
Magnitude
F = qvB sin θ
F = qvB sin θ
F: Force [N]
B: Magnetic Field Strength / Flux Density [T]
q: Charge on the charge-carrier [C]
v: velocity of the charge-carrier [m/s]
θ: Angle between field and current [˚ or radians]
Force between two parallel conductors
Force between two parallel conductors
If two conductors carry current while parallel to each other then they will exert a force on each other.
SAME DIRECTION: if the currents are flowing in the same direction the conductors will attract each other
OPPOSITE DIRECTION: if the currents are flowing in opposite directions the will repel each other.
Current flows in same direction
Current flows in opposite direction
Electromagnetic induction
Electromagnetic induction
If a conducting material is moved in a magnetic field then the free charge carriers will experience a force as described above. If that motion is continuous then an imbalance in the charges will result, creating a potential difference. If an electrical connection is made between the two ends of the conductor then this induced EMF will result in a flow of charge.
It is this principle behind many forms of electricity generation.
Additional videos
Additional videos
Resources from textbooks
Resources from textbooks
Oxford Physics: pages 233 - 239
Hamper HL (2014): pages 244 - 249
Hamper SL (2014): pages 219 - 222
Relevant questions from Dot Point Physics (6 Fields and forces)
Relevant questions from Dot Point Physics (6 Fields and forces)
pages 305 - 330: Lots of questions and excellent preparation
Other resources
IOP Spark Lesson - A lesson plan with notes
Page updated
Google Sites
Report abuse