SP12 Magnetism and the Motor Effect Flashcards
Describe the law of magnetism
- Two like poles (e.g.: S,S or N,N) will repel each other
- Two unlike poles will attract each other
What type of force is the attraction and repulsion of different magnetic poles?
Non contact force
What elements are magnetic?
- Iron
- Nickel
- Cobalt
How do we test if an unknown material is a magnetic metal or a magnet?
- If it can be repelled by the magnet, the material itself is a magnetic
- If it can only be attracted by the magnet, the material is a magnetic metal
What are the two types of magnets?
- Permanent
- Induced
Define permanent magnet
A material that will produce its own magnetic field. It will not lose its magnetism
Define induced magnet
When a magnetic material is placed into a magnetic field, it can temporarily turn into a magnet
How can we induce a magnet?
By stroking it with a permanent magnet.
Why does stroking the magnetic material with a magnet induce its magnetism?
It aligns all the domains of the material in the same direction, creating a temporary magnet
Define magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material
What are magnetic field lines used for?
Used to represent the strength and direction of a magnetic field
How do magnetic field lines travel?
Point from North to South
How is the strength of the magnetic field shown?
Spacing of the magnetic field lines:
- close together = strong magnetic field
- far apart = weaker magnetic field
What is a uniform magnetic field?
One that has the same strength and direction at all points
How is a uniform magnetic field shown?
- equal spacing between all the magnetic field lines
- arrow going from north to south
- produced between the gaps of two opposite poles
- the field will not be uniform outside that point
Where is the magnetic field of a bar magnet the strongest?
At the poles
Why does the magnetic field become weaker as we move away from the bar magnet?
The magnetic field lines are getting further apart, so it cannot attract or repel as much
How do we know that the Earth’s core is magnetic?
A free suspended magnetic compass will align itself with Earth’s field lines and always point North
Why is Earth’s magnetic north actually its geographic south pole?
- a compass is basically a suspended bar magnet
- its north pole lines up with that of the Earth’s geographic north pole
- which by the law of magnetic attraction cannot be right
- therefore, the north of the compass must be lining up with the Earth’s magnetic south
What happens when a current flows through a wire?
A magnetic field is produced
Describe how to find the direction of the magnetic field using the right hand rule
- Thumb: points along the direction of the current
- Magnetic field has a direction similar to the way your fingers are pointing
How do we change the direction of the magnetic field using current?
reversing current = reversing direction of the magnetic field
How do we increase the strength of a magnetic field produced due to a current?
- Increase the size of the current
- wrapping the wire in coils also increases the size of the current
What does the strength of the magnetic field depend on?
- size of the current
- distance from the long straight conductor
What is a solenoid?
A series of coils that has a similar magnetic field as a bar magnet
Describe the magnetic field of a solenoid
- uniform
- similar to that of a bar magnet
Why is the magnetic field of a solenoid uniform?
Coiling the wire causes the field to align
How can we work out the poles of a solenoid?
- needs to be viewed from the end
- if the current is travelling in a clockwise direction, then it is the south pole
- if the current is travelling in an anticlockwise direction, then it is the north pole
How do we increase the magnetic field of a solenoid?
- increase the current
- increase the number of coils
- adding an iron core
When does the motor effect occur?
When a wire with current flowing through it is placed in a magnetic field and experiences a force
What is the effect of a motor effect?
The interaction of two magnetic fields:
- one is produced around the wire due to current flowing through it
- second is the magnetic field where the wire is placed between
This produces a force
How must the wire be placed in relation to the magnetic field for a motor effect to be produced?
Perpendicular to each other
What does the size of the force of a motor effect depend on?
- strength of magnetic field
- strength of current in wire
- length of wire
How can we increase the size of the force in a motor effect?
- increasing the amount of current that flows through a wire
- using stronger magnets
Describe the magnetic field around a wire
Circular
Describe Flemmings Left hand rule
- thumb: represents force
- first finger: represents magnetic field
- middle finger: represents current
What is Flemming’s left hand rule used to find out?
The direction of the force/current/magnetic field in a motor effect
What is the equation used to calculate force in a motor effect?
Force = Magnetic flux density (field strength) x current x length of wire
What can the motor effect be used for?
Force produced by the magnetic field and current is used to cause rotation, therefore producing a D.C. motor
How does motor effect cause movement?
- The current will flow in opposite directions on each side of the coil
- force produced from the magnetic field will push one side of the coil up and the other side down
- causing the coil to rotate
What is the split ring commutator used for?
Reverses the direction of the current flow
