Electromagnetism Flashcards
What are hard magnets
Retain their magnetic properties for some time
Eg. Steel
What are soft materials
Gain/lose their magnetic properties immediately (used in electromagnets)
Eg. Iron
What is magnetic field
Magnets exert forces without touching
Can magnetism be induced in some material
Yes when they are placed in an external magnetic field
Practical- investigate the magnetic field pattern for a bar magnet
- Place magnet under paper and sprinkle with iron fillings, tap paper so fillings settle into field lines
- Place magnet on paper and place test compass nearby - draw points and line for where compass points, and repeat in many places around the magnet (this shows field direction)
Electric current in a conductor
Produces a magnetic field around it
What is a uniform magnetic field
When two opposite poles are put next to each other (but still separated)
Parallel field lines, evenly spaced field lines
What is an electromagnet
- a coil of wire carrying a current
- iron
- produces a magnetic field like a bar magnet
How does a scrap yard crane work
- current flows through coil of wire
- wrapped around soft iron core
- magnetic field
- when current is turned off it loses magnetisation
When is there a force on a charged particle
When it moves in a magnetic field as long as it’s motion is not parallel to the field
How does a Loudspeaker work?
- current runs through coil and current alternates
- current produces magnetic field
- magnetic field constantly changes because current is changing
- interacts with second external permanent magnetic field
- applies force on magnet/washers
- cup vibrates air making longitudinal wave
What is the motor effect
Original magnetic field + magnetic field from wire = the motor effect = force felt by current in magnetic field
How does force on wire increase
- if strength of magnetic field increases
- current increases
- number of coils increases
How does force on wire reverse
- if magnetic field reverses
- if current reverses
What is a motor?
Current flowing around the coil feels oppositely directed forces on each side from a second external magnetic field
These forces cause the coil to move
Every half turn the split ring commutator causes the current to reverse in the coil
Keeps the coil spinning in one direction
How do split ring commutators work
Brushes lose contact with the split ring commutator
Current no longer flows through the motor coil
The coil will continue to rotate clockwise due to its momentum
Brushes regain contact with split ring commutator
Current flows through the motor coil but in the opposite direction, forces exert a clockwise turning effect on the coil
When do you use the left hand rule
to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field
When is a voltage induced in a conductor or coil
Moving a magnetic field across a wire or moving a wire through a magnetic field
How to increase the induced voltage
Stronger magnetic field Faster movement More turns on a coil Wider coils Put iron core in the coil
How does a generator work?
A coils rotates through a magnetic field so the field is constantly changing direction through the loop
Structure of a transformer
Two unconnected coils wrapped around an iron core
How does a transformer work
Current produces a magnetic field
AC current in 1st coil goes to alternating magnetic field
Changing magnetic field induces AC current in 2nd
Induced voltage affected by number of turns in coil
Step up transformers
2nd coil voltage and coil turns > 1st coil voltage and coil turns
Why do national grid transmit at high voltages
Resistance transforms electric energy to heat so they lose energy
High current produces heat, but as v goes up current goes down
Why is voltage stepped down
Voltage is stepped down for different consumers, lowest level is 230V for domestic use
(Final step down transformer will be at a sub station within a few hundred metres of each group of houses)
Relationship between input and output voltages and turn ratio is
Input voltage / output voltage = primary turns / secondary turns
If a transformer is 100 percent efficient what is the power input
P1 = P2
So VI1 = VI2
