Topic 12 - Magnetism Flashcards
At which part of a magnet are the magnetic forces strongest?
The poles of the magnet.
What happens when two magnets are brought close to each other?
They exert a force on each other
What type of force is exerted if two like poles or magnet are brought near each other?
A repulsive, non-contact force
What type of force is exerted if two unlike poles of a magnet are brought near each other?
An attractive, non-contact force.
What is a magnetic field?
The region surrounding a magnet where another magnet or magnetic material experiences a non-contact force.
What is the difference between a permanent magnet and an induced magnet?
● A permanent magnet produces its own magnetic field
● An induced magnet becomes magnetic when placed in a magnetic field
Induced magnetism always causes what type of force?
A force of attraction.
What is the effect when an induced magnet is removed from a magnetic field?
The induced magnet loses most/all of its magnetism.
Give four examples of magnetic materials.
- Iron 2. Steel 3. Cobalt 4. Nickel
What can always be said about the force between a magnet and a magnetic material?
It is always attractive.
How does the strength of a magnetic field alter as you move further away from the magnet producing it?
The magnetic field strength decreases the further you move away.
In what direction does a magnetic field point?
In the direction of the force that a north pole would experience if placed in the field
● From the north seeking pole to the south seeking pole of a magnet
What does a magnetic compass contain?
A small bar magnet that points in the direction of the Earth’s magnetic field.
What is produced when current flows through a conducting wire?
A magnetic field is produced around the wire.
What determines the strength of the magnetic field around a current-carrying wire?
The magnitude of the current flowing through the wire.
Does a high concentration of field lines mean the field is strong or weak?
Strong
True or false: magnetic field lines never cross each other?
True; they never cross, touch or overlap.
How is a plotting compass used to map out a magnetic field?
● Place a compass (containing a needle magnet) on a piece of paper near the field
● Draw an arrow in the direction the compass points
● Repeat at different points on the paper
● Join the arrows to make a complete field pattern
How does a compass prove earth core is magnetic
- The core is magnetic, and creates a large magnetic field around the Earth
- We know this because a freely suspended magnetic compass will align itself with the earth’s
field lines and point North. - A compass is effectively a suspended Bar Magnet, with its own north pole lining up with Earth’s North Pole
- This cannot be right - like poles repel
- So in fact, Earth’s magnetic pole in the north is a magnetic South Pole and the geographic south pole is close to the magnetic North Pole
Right hand rule
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
Strength of magnetic field
Magnetic field strength depends on current size; Greater current, stronger magnetic field
- Strength also varies with distance from the conductor; Greater distance from wire, weaker
field
What is a solenoid
A coil of wire which when current passes through creates a strong magnetic field.
Describe the magnetic field found inside a solenoid
Strong and uniform.
Is the field on the outside of a solenoid strong or weak? Why?
Weak.
The fields from each coil cancel out, making the outside field weak.
Solenoids:summary
- Magnetic Field Shape is similar to a bar magnet
- Coiling the wire causes the field to align and form a giant single, almost uniform field along
the centre of the Solenoid. - Having an iron core in the centre increases its strength as it is easier for magnetic field lines
to pass through than air - The fields from individual coils cancel inside to produce a weaker field outside the solenoid
.
Factors that affect solenoid field strength
o Length
o Cross sectional area
o Number of turns (coils) o Using a soft iron core
Size of current
What is an electromagnet?
A solenoid with an added iron core
● Adding the iron core increases the
strength of the magnetic field
What is the motor effect
When a force is exerted between a magnetic field and a current-carrying conductor placed in that field.
What rule is used to determine the force produced by the motor effect?
Fleming’s Left-Hand Rule
Flemings left hand rule: summary
Each component is at 90° to the others
- Use this to work out the unknown factor out of the
three (usually the direction of the force felt) - Remember current is conventional current (motion of positive charge), which moves in opposite direction to electron flow.
When using Fleming’s left hand rule what does the forefinger represent?
The forefinger points in the direction of the magnetic field.
When using Fleming’s left hand rule what does the second finger represent?
The second finger points in the direction of current flow in the conduct
When using Fleming’s left hand rule what does the thumb represent?
The thumb points in the direction of the force produced by the motor effect
What factors affect the size of the force on a current-carrying wire in a magnetic field?
The magnitude of the current flowing through the conductor
● The strength of the magnetic field that the conductor is placed in
If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed, what happens to the force?
The direction of the force is reversed.
If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased, what happens to the force
The strength of the force is increased.
Give the equation linking force, magnetic flux density, current and length
force (N) = magnetic flux density (T) x current (A) x length (m)
F = BIL
Where Magnetic Flux Density is measured in Tesla [T] o The number of flux lines per metre squared
What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?
The conductor must be at right-angles to the magnetic field it is placed in
What is the unit used for magnetic flux density?
Tesla, T
If the current and the magnetic field are parallel to each other, which direction would the force act in?
No forcé will act
How does an electric motor work?
A coil of wire, carrying a current, is placed in a magnetic field ( between two permanent magnets)
● The forces on the two sides perpendicular to the field experience forces in opposite directions
● This causes a rotational effect
How are electric motors kept rotating?
A split ring commutator is used. This switches the current direction every half turn, which ensures that the coil keeps spinning.
