Magnetism and Electromagnetism Flashcards
What do magnets produce?
Magnetic Fields
What is a magnetic field?
A region where other magnets or magnetic materials experience a force
Name 4 magnetic materials
iron, steel, nickel, cobalt
What is a magnetic field strongest (and why)?
At poles of magnet (magnetic force is strongest)
Magnetic field lines go from…
north to south
What happens when 2 magnets are brought close together?
They exert a force on each other
Further away from magnet = …
weaker field
Closer together the lines (of magnetic field) = …
stronger magnetic field
The strength and direction of a magnetic field change …
from one point to another
What are the lines of the magnetic field like in a uniform magnetic field?
Are parallel
What can be used to show the direction of magnetic fields?
Compasses
Why can compasses show the direction of magnetic fields?
- Inside compass there’s tiny bar magnet (the needle)
- North of pole of magnet is attracted to south pole of any other magnet near it
- Compass needle points in direction of the magnetic field it is in (will point towards south pole of magnet)
When not near magnet, why does a compass needles point north?
Earth generates its own magnetic field - core of earth is magnetic
Magnets can be _____ or _____
Permanent or Induced
What does a permanent magnet do?
Produce their own magnetic field
What is an induced magnet?
Magnetic materials that turn into magnet when put into magnetic field
What is the force between a permanent magnet and induced magnet and why?
Always attractive ∵ when you take away magnetic field = induced magnets quickly lose most or all of their magnetism
A moving charge creates a _____ ___
Magnetic field
When current flows through a wire, what happens?
Magnetic field is created around wire
When current flows through a wire, what is the magnetic field (created around wire) made up of?
Concentric circles perpendicular to wire (with wire in centre)
How can you see that a moving charge creates a magnetic field?
- By placing a compass near wire that’s carrying current
- As you move compass = it’ll trace direction of magnetic field
What will changing the direction of current do to the magnetic field?
Will change the direction of magnetic field
What two things does the strength of magnetic field produced (from a moving charge) change with?
current and distance from wire
Larger current through wire or closer to wire you are = …
stronger field
Describe how you can use the right-hand thumb rule to work
out the direction of magnetic field
What is a solenoid?
A coil of wire
How can you increase the strength of the magnetic field created by current through a wire?
By wrapping wire into coil (solenoid)
Why does a solenoid increase the strength of a magnetic field?
- Happens ∵ field lines around each loop of wire line up with each other
- = lots of field lines pointing in same direction that are very close to each other
- Closer together field lines = stronger field is
What is magnetic field around a solenoid like?
Like one round bar magnet
How can you increase the field strength of solenoid even more and why does this work?
- By putting block of iron in centre
- Iron core becomes an induced magnetic whenever current is flowing
What does an electromagnet consist of?
Solenoid with an iron core
How can the magnetic field of electromagnet be turned on and off?
With an electric current
Name 2 uses of electromagnets
- Used in cranes attract & pick up things made from magnetic materials e.g. scarps in yard
- Used within circuits to act as switches
Explain how electromagnets are used within circuits to act as switches
- When switch in circuit 1 is closed = turns on electromagnet = attracts iron contact on rocker
- Rocker pivots and closes the contacts = completing circuit 2 & turning on motor
Current in magnetic field experiences a ___
Force
Describe the motor effect
- Current-carrying wire (or any conductor) is put between magnetic poles
- Magnetic field around wire interests with magnetic field between magnets
- Causes magnet and wire to exert force on each other, causing wire to move
What angle does the current-carrying wire/conductor have to be to experience full force?
Wire has to be 90° to magnetic field (wire runs parallel to magnetic field ≠ force)
Why does the current-carrying wire/conductor have to be at a 90° experience full force?
Forces always acts at right angles to magnetic field of magnets & to direction of current in wire
Motor effect: How can the size of the force be increased? (name 3 ways)
- Increasing strength of magnetic field
- Increasing size of current
- Increasing length of conductor in magnetic field
Motor effect: How can you reverse the direction of the force? (name 2 ways)
- Reversing the direction of the current
- Reversing the direction of the magnetic field
Describe Fleming’s Left-hand Rule (i.e. what finger means what)
Q:
Bigger magnet…
Stronger magnetic field
What is the generator effect?
Induction of p.d. (& current if there’s complete circuit) in a wire which is experiencing a change in magnetic field
Cutting magnetic field lines ____ a potential difference/current
Induces
Q:
When is p.d. induced across the ends of a conductor wire? (name 2 times)
- Conductor moves relative to a magnetic field
- i.e. When you move wire up and down through a magnetic field
- Conductor is in a changing magnetic field
- i.e. Keep wire still but move magnetic field up and down
When do we only see the generator effect?
If the wire passes through the magnetic field
Generator effect: If the wire moves along the magnetic field, what happens?
We don’t get induced p.d. or current
When is p.d. induced across the ends of a coil of wire? (name 2 times)
- Permanent magnet is moved into or out of the coil
- Coil is moved relative to magnet
Generator effect: How can you increase the size of the induced p.d./current? (name 4 ways)
- Increases speed of the movement (i.e. cutting more magnetic field lines in a given time)
- Stronger magnetic field (i.e. more field lines that can be cut)
- Increase no. of of turns on coil
- Increase area of the coil
Generator effect: How can you reverse the polarity of the induced p.d. and direction of induced current? (name 2 ways)
- Reverse the direction of motion
- OR reverse polarity of the magnet
Induced current ___ the change that made it
Opposes
Explain what is meant by: ‘an induced current opposes the change that made it’
- Change in magnetic field (moving magnet in and out of coil) = induces a current in wire
- But when current flows through wire = magnetic field is created around wire
- Magnetic field created by induced current opposes field of permanent magnet (as it tries to return things to way they were)
What do alternators do?
Generate alternating current
What do generators do?
Rotate a coil in a magnetic field (or a magnet in coil)
How does an alternator work?
- As coil (or magnet) spins = current induced in coil
- Current changes direction every half turn
- Instead of split-ring commutator = alternators have slip rings & brushes so contacts don’t swap every half turn
- Means they produce alternating pd
What does a dynamo do?
Generate direct current
How does a dynamo work?
- Dynamos work in same as alternators BUT have spilt-ring commutator instead of slip rings
- This swaps connection every half turn to keep current flowing in same direction
What does an oscilloscope show?
How pd generated in coil changes over time
In an oscilloscope, what is the ac trace like?
Line goes up & down, crossing horizontal axis
In an oscilloscope, what is the dc trace like?
Line isn’t straight but it stays above the axis (pd is always positive)
In an oscilloscope diagram, what does the height of the line at given point represent?
The generated pd at that time
In an oscilloscope diagram, what does increasing frequency of
revolutions do?
Increases overall pd & creates more peaks too
How power stations generate electricity?
Use turbines to turn wire coils between magnets
Loudspeakers work because of the ___ ___
Motor Effect
Loudspeaker & headphones both use ________
electromagnets
Explain how loudspeaker work
- AC is sent though coil of wire attached to base of a paper cone
- Coil surrounds 1 pole of a permanent magnet, and is surrounded by the other pole = so current causes force on coil (which causes it to move)
- When current reverses = force acts in opposite direction = causes cone to move in opposite direction
- So variations in current make cone vibrate = makes air around cone vibrate & creates variations in pressure = sound wave
- Frequency of sound waves is same as the frequency of ac
- Controlling frequency of ac = can alter sound wave produced
Explain how microphones work
- Sound waves hit flexible diaphragm that’s attached to coil of wire, wrapped around magnet
- Causes coil of wire to move in magnetic field = generates current
- Movement of coil (& generated current) depends on properties of sound waves
- (Louder sounds = diaphragm move further)
- This is how microphones convert pressure of variations of sound wave into variations in current in electrical circuit
What do transformers do?
Change size of potential difference of an alternating current
What do transformers consist of?
Have 2 coils of wire, primary & secondary, joined with iron core
Explain how transformers work
- When alternating pd is applied across primary coil = iron core magnetises and demagnetises quickly
- Changing magnetic field induces an alternating pd in secondary coil
When is current induced in a transformer?
If second coil is part of complete circuit
Ratio between number of turns on primary and secondary coils = …
Ratio between primary and secondary potential differences
