Seneca P7 Flashcards
What is Magnetism
Magnetism describes the ability of magnets to attract (pull towards) and repel (push away) other magnets without touching them.
Attraction
Magnets have a north pole (N) and south pole (S).
If two magnets are close enough to each other, then the south pole of one magnet will attract the north pole of the other magnet.
This is an example of a non-contact force.
Repulsion
If one magnet is turned so that both south poles (or both north poles) are close together, then the magnets will repel each other.
Opposite poles on a magnet attract and like poles on a magnet repel. This is an example of a non-contact force.
Poles of a magnet
A magnet’s poles are where the magnetic forces are strongest.
What are the only pure metals that can be turned into magnets
Iron, nickel and cobalt are the only pure metals that can be turned into a magnet. These metals are “magnetic materials”.
Properties of magnetic materials
Magnetic materials, like iron, don’t always act as magnets.
For example, a normal piece of iron won’t attract (pull) or repel (push) another piece of iron.
But magnetic materials, like iron, are attracted to a magnet if they are close enough to the magnet. Magnetic materials can be attracted by a magnet, but can’t be repelled by a magnet.
Only two magnets can repel each other.
Non-magnetic materials will never be attracted to magnets.
Permanent magnets are always magnetic, even when they are not near other magnets.
Induced magnetism
Induced magnets are magnetic materials that become magnets when they are in a magnetic field.
When moved away from the permanent magnet, the magnetic material will stop being a magnet.
This is called induced magnetism.
Induced Magnetism poles
The permanent magnet induces (creates) temporary poles in the magnetic material and these align so that the magnetic material is attracted to the permanent magnet.
If the north pole of the magnet is next to the magnetic material, then a south pole will be induced in the part of the material closest to the magnet.
How can you tell if an object is a magnet?
It will attract magnetic materials and will repel one pole of another magnet.
What type of magnetism is it when a magnetic material temporarily becomes a magnet when it’s close to a permanent magnet?
Induced magnetism
What are magnetic forces caused by
Magnetic forces (the forces between magnets) are caused by invisible magnetic fields.
Magnetic fields
Every magnet creates a magnetic field around itself.
A magnetic field is the area around a magnet where another magnet or magnetic material (iron, nickel, cobalt and steel) feels a force.
Strength of a magnetic field
The strength of a magnetic field depends on the distance from the magnet. The magnetic field is strongest at the magnet’s poles.
What direction do the arrows on magnets point to
The arrows always point from the north pole of the magnet to the south pole.
When do magnets attract or repel each other?
When their magnetic fields overlap
Where does the needle of compass point to
The needle of the compass points in the direction of the Earth’s magnetic field.
Strength of magnetic field
The strength of the field depends on the size of the current and the distance away from the wire.
The strength of any given magnetic field depends upon:
. Size of the current
. The distance from the wire
What is a solenoid
A solenoid is a loop of wire coiled into a cylindrical (cylinder like) shape
Outside the solenoid
Outside of the coil of wire, the field lines look like the field lines around a bar magnet, with a clear north and south pole
Inside the solenoid
Inside the coil of wire, the field lines are evenly spaced, parallel, straight lines.
Solenoids magnetic field
The magnetic field is strong and uniform. This means that the magnetic field is of the same strength and in the same direction at every point.
Solenoid magnetic field strength
For a solenoid (coil of wire), the magnetic field inside the solenoid is strong and uniform.
Outside of the solenoid the magnetic field decreases in strength the further away it is from the solenoid.
Wire magnetic field strength
For a straight wire carrying a current, the magnetic field is strongest at the points closest to the wire, and weakest further away from the wire.
Increasing the size of the ——- increases the strength of the magnetic field produced by a ——–.
Current , solenoid
What is an electromagnet
An electromagnet is formed when a current carrying wire is wrapped around an iron core. The iron core increases the magnetic field strength of the solenoid (coil of wire)
Where are electromagnets used
. Scrap yards
. Washing machines
What are the parts of an electromagnet
. Electric current
. Iron core
. Coil of wire
What is the motor effect
When a wire with a current flowing through it (a current carrying wire) is placed in a magnetic field, the magnet and the conductor will experience a force
Cause of the motor effect
The field created by the electric current interacts with the magnetic field that the wire is placed inside. This causes the force on the wire.
The direction of motor effect forces on a wire depends on:
. Direction of magnetic field
. Direction of current flow
Flemings Left Hand Rule
Hold your left hand so your thumb, first finger and second finger are all at right-angles to one another.
Point your first finger in the direction of the magnetic field ((F)ield = (F)irst finger).
Point your second finger in the direction of the current ((C)urrent = se(C)ond finger).
Your thumb will be pointing in the direction of the force, which is the direction the conductor would move ((M)otion = thu(M)b).
Formula for force and magnetic flux density
force=magnetic flux density×current×length
What are the factors that affect the force on a current carrying wire in a magnetic field
. Magnetic flux density
. Current
. Length
Why does a coil carrying current rotate in a magnetic field
A coil carrying an electrical current will rotate in a magnetic field.
This is because the current going up one side of the coil is flowing in the opposite direction to the current coming back down the other side.
How do we increase the force acting on a coil
. Higher current
. Increase the magnetic field strength
. Increase number of turns on the coil
Loudspeakers
. As the direction of current changes, the direction of the magnetic induced field in the electromagnet changes
. A stationary magnet repeatedly attracts and repels the electromagnet
. This causes the speakers to move forwards and backwards creating waves
. What are the two ways a potential difference can be induced across the ends of an electrical conductor
. Conductor moving in a magnetic field
. Change in a magnetic field around a conductor
Conductor moving in a magnetic field
A potential difference is induced if a conductor moves relative to a magnetic field.
Change in magnetic field around a conductor
A potential difference is induced if there is a change in the magnetic field around a conductor.
How can we induce a bigger potential difference in an electrical conductor
. Faster Movement
. More coils
Faster Movement
Making the magnetic field change more rapidly by moving the wire or the magnet more quickly.
More coils
Increasing the number of turns of coils of wire.
For both solenoids and straight wires, reversing the direction of the magnetic field’s movement will _______ the direction of any induced current.
reverse
Inducing current opposing change
An induced current generates a magnetic field that opposes the original change
(the movement of the conductor or the change in the magnetic field).
If the magnetic field pointing upwards is increased, in which direction will the induced magnetic field of the conductor be pointing?
Downwards
When a potential difference (p.d) is induced in a wire, it creates a force acting in the opposite direction to the action that induced the p.d. This is due to the motor effect.
Moving downwards
If a wire moves downwards into a horseshoe magnet:
A p.d is induced in the wire and an electric current flows through the wire.
A force will push the wire upwards, opposing the downwards movement of the wire that induced the p.d.
Moving upwards
If the wire was moving upwards to create the p.d, then the force on the wire due to the induced p.d would push it downwards.
What are the two types of current
. Direct current (d.c)
. Alternating current (a.c)
Direct current
The electric current flows in only one direction.
Batteries and cells are sources of direct current.
Alternating current
The electric current repeatedly changes direction.
In the UK, the electricity from the plug sockets is a source of alternating current.
Alternating Current Generators
An alternator generates (produces) alternating current (a.c.).
In an alternator, there is a rotating magnet and a fixed coil of wire.
When the magnet rotates, the direction of the field (that the coil passes through) alternates.
This produces alternating current.
