Magnetism and electromagnetism Flashcards
Attraction
When two unlike poles attract each other
Repulsion
When two like poles repel each other
Permanent magnets
Produces its own magnetic field
Induced magnet
A material that becomes magnetised when it is placed in a magnetic field
When removed from the magnetic field an induced magnet loses its magnetism quickly
Magnetic field
The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel)
Weak magnetic field
The further the field lines are from each other the weaker the field
Strong magnetic field
The closer the field lines are from each other the stronger the field
Magnetic field with a plotting compass
Place the plotting compass near the magnet on a piece of paper
Mark the direction the compass needle points
Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
Join the points to show the field lines
Earth’s core
Contains liquid iron which is magnetic causing Earth to have a magnetic field
Magnetic field in a wire
When a current flows through a conducting wire a magnetic field is produced around the wire
The strength of the magnetic field depends on the current through the wire and the distance from the wire
Solenoid
A long coil of insulated wire, used in devices where a strong magnetic field is needs to be produced
Inside the solenoid
The magnetic field inside a solenoid is strong and uniform
The field lines are parallel to the axis of the solenoid
Outside the solenoid
The magnetic field around a solenoid has a similar shape to that of a bar magnet
Adding an iron core increases the strength of the magnetic field of a solenoid
Electromagnet
A solenoid with an iron core
Ways to increase the strength of an electromagnet
Increase the current flowing through the solenoid
Increase the number of coils (keeping the length the same)
Decrease the length (keeping the number of turns the same
Add an iron core
Motor effect
A current-carrying wire in a magnetic field experiences a force
Force
Force = magnetic flux density x current x length
Magnetic flux density - tesla (T)
Finding magnetic flux density practical
Place the magnet on a weighing scale
Take balance, measure mass when current is flowing through
Convert the mass into newtons
Measure the length of the wire in the field
Use an ammeter to measure the current flowing through the wire
Electric motors
Uses the motor effect
Both sides of the coil experience a force in the opposite direction, causing it to turn
A spilt-ring commutator is used to ensure that the current is reversed every half turn, keeping the coil spinning
Ways to increase the speed of an electric motor
Add more turns to the coil
Increase the voltage or current
Use a stronger magnet
Loudspeakers
Uses the motor effect
A current is sent from a source causing the coil and speaker cone to vibrate producing sound waves
Generator effect (induced potential)
Induces an alternating current in a spinning coil in a magnetic field
No split-ring commutator is needed as current is alternating, however slip rings are needed
Ways to increase output from a generator
Add more turns to a coil
Use a stronger magnet
Turn the coil faster, however, this requires energy
Transformers
Changes the voltage through the national gid to reduce energy lost as heat
Primary side of a transformer
An electromagnet
By passing a current through a coil of wire, we make a magnetic field (like a bar magnet)
Secondary side of a transformer
Not connected to a power supply
When there is an alternating current on the primary side, the magnetic field alternates, inducing a second alternating current on the secondary side
Process of a transformer
A changing AC is applied to the primary coil
This creates a changing magnetic field
The changing magnetic field is transferred around the
iron core
There is a changing magnetic field in the secondary coil
This causes a changing AC in the secondary coil
Step-up transformer
Increases the voltage before it enters the grid, the current decreases
Primary side voltage is lower + secondary side voltage is higher
Step-down transformer
Decreases the voltage before it enters the home, the current increases
Primary side voltage is higher + secondary side voltage is lower
Transformer equation
Voltage of the primary / voltage of the secondary =
turns on primary / turns on secondary
