P7 - Magnetism And Electromagnetism Flashcards
Where are the magnetic forces the strongest in a bar magnet?
The magnetic forces are the strongest at the poles of the magnet
Describe the attraction and repulsion between unlike and like pole for permanent magnets
Two magnets are brought together and they exert a force on each other
- Two LIKE poles repel each other
- Two UNLIKE poles attract each other
Attraction and repulsion between two magnetic poles are examples of what type of force?
Non-contact force
What is the difference between permanent and induced magnets?
A permanent magnet (like a bar magnet) produces its own magnetic field, whereas an induced magnet is a material that becomes a magnet when it is placed in a magnetic field
Explain when an induced magnet loses most of its magnetism
Induced magnetism always causes a force of attraction
When removed from the M.F. an induced magnet loses most/all of its magnetism quickly because it is no longer placed in a M.F.
-> This means it can be used for picking up and dropping metallic materials
What are the magnetic materials?
Iron, steel, cobalt and nickel
What is the magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material
The force between a magnet and a magnetic material is always one of …
Attraction
What does the strength of a magnetic field depend on?
Distance from the magnet (strongest at the poles)
Describe how to plot the magnetic field pattern of a magnet using a compass
Place the compass near the North Pole of the bar magnet
Mark a cross at the North Pole of the compass
Move the compass so the South Pole of the compass is on the cross
Again, mark another cross at the North Pole of the compass
Continue until a complete magnetic field line is plotted
Connect all the dots with a circular line and the direction of the field line is shown with an arrow
Repeat this starting at different points around the North Pole of the bar magnet
What is the direction of the magnetic field at any point given by?
Direction of the force that would act on another North Pole placed at that point
Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic
The compass contains a small bar magnet
if we hold the compass away from any magnets, the compass needle points in the direction of the Earth’s M.F.
This means the earth has a magnetic field which is due to the earth’s core
When a current flows through a _____ wire, a ________ _______ is produced around the wire
Conducting
Magnetic field
What does the strength of the magnetic field through a conducting wire depend on?
Current through the wire
Distance from the wire
Explain how a solenoid arrangement can increase the magnetic effect of the current
Shaping a wire to form a solenoid increases the strength of the MF created by a current through a conducting wire
The MF inside a solenoid is strong and uniform
Can be increased by:
- increase current as increases strength of MF
- increase turns of the coil
- add an iron core as increases strength of MF
Describe how to use Fleming’s left-hand rule
What is an electromagnet?
A solenoid containing an iron core
Describe the motor effect
When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor interact and exert a force on each other
What are the factors that affect the size of the force on the conductor?
Increase current
Use stronger magnet
Greatest when the wire is perpendicular to the magnetic field; zero when the wire is parallel to the MF lines
What are the SI units for the equation: F = BIL
F (force) in newtons, N
B (magnetic flux density) in tesla, T -> measure of strength of MF
I (current) in amperes, A
L (length) in metres, m
Explain why the coil of a simple electric motor rotates continuously when the motor is connected to a battery
A force acts on each side of the coil due to the motor effect
The force on one side is in the opposite direction to the force on the other side
The split-ring commutator reverses the current around the coil every half turn of the coil. As the sides swap over, the coil is pushed in the same direction every half-turn
How do loudspeakers and headphones use the motor effect?
Convert variations in current in electrical circuits to the pressure variations in sound waves
Explain how a moving-coil loudspeaker/headphones works
Current passes through the coil and generates a MF
MF from coil interacts with the MF from permanent magnet
This produces a resultant force that causes the cone to move
When the AC current switches direction, the direction of the force on the cone reverses
Causing the cone to move in and out, generating sound waves
How can the frequency and amplitude of a loudspeaker/microphone be changed?
Changing frequency of AC supply will change frequency of cone vibration. Higher frequency = higher pitch sound
Increasing size of current increases amplitude of the vibration. This increases the volume of the sound
Describe the generator effect
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If it is part of a complete circuit, a current is induced in the conductor
[Generator effect is the effect of inducing a pd using a magnetic field]
What are the factors that affect the size of the induced pd/induced current
Use a stronger magnetic field from a stronger magnet
Move the conducting wire more quickly
Shape the wire into a coil for greater turns
What are the factors that affect the direction of the induced potential/induced current?
An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in the magnetic field
What is a use of the generator effect?
Alternator to generate an ac
Dynamo to generate a dc
