16. Magnetism Flashcards
What materials are magnets attracted to?
- iron
- steel
- nickel
- cobalt
What are the forces between magnets?
Same poles repel
Opposite poles attract
Magnetic field lines important points:
- when the magnetic field lines are close together what does this indicate?
- what does the direction of the magnetic field show?
- what should field lines never do?
- Magnetic field is strongest when field lines are closer together
- The direction of the magnetic field shows which direction a compass would point
- Magnetic field lines point away from north and towards south
- Field lines never cross
What is a uniform field?
- constant magnetic field - constant direction + strength
- arrows point in same direction, field lines are equally spaced apart, magnetic field strength = constant
- can be made in the space between opposite poles of a magnet
Describe the properties of magnetically hard and soft materials
- permanent magnets are made of magnetically hard materials eg steel, which retain their magnetism once magnetised
- temporary magnets eg electromagnets are magnetically soft eg iron, which lose their magnetism once they are no longer exposed to a magnetic field and are easy to magnetise and de-magnetise
What does a magnetic field line do?
Indicate the size and direction of the magnetic force
Describe the method for investigating the magnetic field pattern for a single magnet
- Place a bar magnet on the centre of a piece of paper
- Bring a plotting compass close to end of North Pole of magnet but stop it touching magnet
- Mark where the compass needle is pointing next to compass
- Lift up plotting compass and place the centre of it on the mark made on the paper
- Repeat until the South Pole is reached, join up dots
- Repeat for at least another 5 lines, starting each line near the North Pole but at a different place
Describe the method for finding the magnetic field pattern for two interacting bar magnets (north to south)
- Place a sheet of paper on a horizontal surface and place two bar magnets North Pole to South Pole next to each other
- Leave a gap between them at least four times width of plotting compass
- Bring plotting compass close to end of North Pole and draw a small dot next to where arrowhead is pointing
- Move and repeat
- Repeat for at least another four lines from the North Pole
What happens when some materials are placed in a magnetic field?
Magnetism is induced
Why does magnetism occur?
- movement of electrons around atom
- therefore a wire carrying a current (conductor) produces a magnetic field around it
Describe the construction of solenoids
- soft iron core wrapped in a coil of wire
- when current flows through coil of wire it becomes magnetic
How can a coil be strengthened?
- more turns in coil
- iron core
- increase current
What happens when you reverse the current in a solenoid?
-reversing the current reverses the poles of the electromagnet
Why is iron used for the core in a solenoid, not steel?
- iron is magnetically soft/ gains/loses magnetism easily
Describe how a relay works
- Relay = switch controlled by electromagnet
- Electromagnet with small current passing through it closes switch by pushing an armature into the contacts
- Closing the switch using an electromagnet reduces chance of electric shock
Describe how a circuit breaker works
- Current passes through electromagnet
- Magnetic field around electromagnet not strong enough to attract contact
- If there is a surge in current strength of magnetic field increases and electromagnet attracts contact
- Breaks circuit
Describe how a loudspeaker works
- how can the pitch and loudness of the sound be changed?
- Alternating current is passed through electromagnet
- Changes polarity of electromagnet
- Electromagnet attracts and then repels permanent magnet
- Causes magnet and cone to vibrate creating a soundwave
- pitch = change frequency of alternate current which change frequency of vibration
- loudness = increase current in electromagnet, stronger magnetic field around it, larger force of attraction and repulsion between it and fixed magnet -> increased amplitude of vibration
How does an electric bell work?
1.button pressed
2. Current flows
3. Electromagnet operates
4. Attracts armature
5. Strikes gong
6. Circuit broken at screw
7. Electromagnet turns off
What is a conventional current?
Current that flows from + to - battery terminals
What is the motor effect?
- force on charged particle (eg in a wire carrying an electric current) when it moves in a magnetic field as long as its motion is not parallel to the field
- this is because magnetic field produced by moving charges interacts w/ magnetic field of permanent magnet
How can the force on a current carrying conductor in a magnetic field be increased?
- Increasing size of magnetic field
- Increasing size of current
- Making angle between conventional current and magnetic field closer to 90’’ (if current (conducting wire) was moving parallel to field then there would be no force)
- Use longer wire
How can the direction of the force on a current carrying wire be changed?
- can be reversed by reversing current
- reversing magnetic field
How are alpha beta and gamma deflected by force which is perpendicular to a magnetic field?
- alpha deflected in opposite direction (have opposite charge)
- gamma not deflected
- beta is deflected the other way to alpha
What happens to a coil in a magnetic field?
- When a current flows in the coil a force is applied to it
- Current in each side of the loop is in opposite directions
- Force on one side is upwards and on other side it is downwards
- One side of the loop will be forced to the top, the other to the bottom (rotate clockwise)
- The coil will then oscillate and eventually line up vertically with one side at the top and one at the bottom
How can an electric motor’s strength be increased?
- increase current
- increase number of turns in coil
- increase strength of magnetic field
- increase length of wire
How does a simple DC motor work?
- Current flows in a coil of wire
- Creates a magnetic field around coil of wire
- Interacts with field from permanent magnet
- Causes force on loop of wire which causes it to start rotating
- Split ring commutator changes direction of current every half turn as it spins, reversing direction of current and reversing direction of force so it can keep spinning
What is the function of the coil of wire in dc motor
Directs current through magnetic field
What do the brushes do
Prevents wires getting tangled allowing coil of wire to rotate
What does the split ring commutator do
- changes direction of current every half turn
- changes direction of force, allowing it to rotate for more than half a turn
