Topic 12 - Magnetism and the motor effect Flashcards
Unlike magnetic poles… and like magnetic poles…
Unlike magnetic poles attract and like magnetic poles repel
Explain permanent magnets
They are always magnets, produce their own magnetic fields, cannot be turned off and always have poles. They cause a force on other magnets or magnetic fields.
These are used in speakers, compasses and electric generators.
Describe the shape and direction of the magnetic field around bar magnets
Field lines point from north to south. Field strength decreases with distance from the magnet.
State some magnetic materials
-Iron
-Steel
-Nickel
-Cobalt
Explain induced magnets
It only becomes a magnet when placed in a magnetic field.
They are magnets but do not have fixed poles and are made temporary magnets by stroking them with a permanent magnet that aligns all domains in the material in the same direction.
Describe the use of plotting compasses
Plotting compasses are small compasses which show the direction and shape of a magnetic field at any given point.
Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic
A freely suspended magnetic compass will align itself with the earth’s field lines and point North. A compass’ North Pole lines up with the earth’s geographical North Pole because that is actually its magnetic South Pole.
What does the strength of the field depend on?
The strength of the field depends on the size of the current and the distance from the long straight conductor
Describe how to show that a current can create a magnetic effect around a long straight conductor
Current produces a magnetic field around a wire. The direction is dictated by the ‘right hand rule’ . Plotting compasses on a piece of paper through which a wire is pierced shows this. Current direction is perpendicular to the magnetic field direction.
What happens inside a solenoid?
Inside a solenoid (an example of an electromagnet) the fields from individual coils
add together to form a very strong almost uniform field along the centre of the solenoid
and cancel to give a weaker field outside the solenoid
What happens when a current carrying conductor is placed near a magnet?
A current carrying conductor placed near a magnet experiences a force and an equal and opposite force acts on the magnet.
What causes magnetic forces?
Magnetic forces are felt due to the interaction between any two magnetic fields.
Describe Fleming’s left hand rule.
Your thumb is the thrust of motion pointing up, your index finger pointing right represents the field and your middle finger pointing towards you represents the current. Each component is at 90 degrees to each other.
This represents the relative directions of the force, the current and the magnetic field for cases where they are mutually perpendicular
What is the equation for force on a conductor at right angles to a magnetic field carrying a current?
force on a conductor at right angles to a magnetic field carrying a current (newton, N) = magnetic flux density (tesla, T or newton per ampere metre, N/A m) × current (ampere, A) × length (metre, m)
F = B⋅ I ⋅l
Explain how the force on a conductor in a magnetic field is used to cause rotation in electric motors
If a coil of wire is in between two permanent magnets and current flows through the wire, the magnetic field it produces interacts with the magnets.
One side of the coil gets forced down, the other side gets forced up. This causes the coil to rotate and you can use the left hand rule to verify which side moves up or down.
