P15- Electromagnetism Flashcards
What is a magnetic compass?
a tiny magnetic need;e pivoted at its centre
Why does one end of the compass always point north, and the other end south?
Because of the Earth’s magnetic field
Like poles repel. Unlike poles?
attract
What can be magnetised?
any iron or steel object
What is an example of only a few other materials that can be magnetised and demagnetised
cobalt and nickel
Permanent magnets are made up of what and why ?
steel
-magnetised steel does not lose its magnetism easily
What happens if a sheet of paper is placed over a bar magnet and iron fillings are sprinkled onto the paper?
the filings form a pattern of lines
The region around the magnet is called a?
magnetic field
What does the any other other magnetic material placed in the magnetic field experience?
a force caused by the first magnet
Magnetic field lines are also called?
lines of force
Where are the lines of force more concentrated and why?
at the poles because that is where the field is strongest
Why is the direction of a line of force is always from the north pole of the magnet to its south pole?
a plotting compass placed in the magnetic field aligns itself along a magnetic field line ( pointing direction away from the B-pole and towards the magnet’s S-pole)
The further the plotting compass is from the magnet…
the less effect the magnet has on the plotting compass
Why is there less effect on a magnet the bigger the distance?
the greater the distance from the magnet, the weaker the magnetic field
How can an unmagnetised magnetic material be magnetised?
by placing it in a magnetic field
By placing an unmagnetised magnetic material in a magnetic field, what is the magnetic field said to do?
induce magnetism in the material
Give an example where a magnetic field is said to induce magnetism?
an unmagnetised iron rod placed in line with a bar magnet becomes a magnet with poles at each end.
The nearest poles of the rod and the bar magnet always have opposite polarity
Induced magnetism will cause a what?
a force of attraction between any unmagnetised magnetic material placed near one end of a bar magnet
The force of attraction created by induced magnetism is always a what force and when?
an attractive force, whichever end of the bar magnet is nearest to the material
Why do magnetic poles move about gradually?
the Earth is partly molten inside
When an electric current passes along a wire, what happens?
a magnetic field is set up around the wire
The lines of force caused by a straight current-carrying wire are a series of what?
concentric circles
What are concentric circles?
circles centered on the wire in a plane that is perpendicular to the wire
Suggest a way to find the pattern of the magnetic field?
finding the pattern of the magnetic field around a long straight wire by using a plotting compass
Why may one want to use a corkscrew to find the pattern of a magnetic field?
to remember the direction of the magnetic field for each direction of the current
What does reversing the direction of the current do?
reverse the direction of the magnetic field
What is a solenoid?
a long coil of insulated wire
Solenoids are used in a lot of…
devices where a strong magnetic field needs to be produced
Where is the magnetic field produced when a current is passed through the wire?
in and around the solenoid
What happens to the magnetic field when the current is increased?
increases in strength
What happens to the magnetic field when the current is reversed?
the magnetic field reverses its direction
What would happen to the magnetic field if the wire was straight?
the magnetic field is much stronger
The magnetic field lines are to what direction with the solenoid?
parallel to the axis, all in the same direction (uniform)
Describe the magnetic field inside a solenoid?
strong and uniform
What are the magnetic field lines like outside the solenoid?
the magnetic field lines bend around from one end of the solenoid to the other end of the solenoid
Why is the magnetic field outside the solenoid like the field of a bar magnet, except that each field line is a complete loop?
it passes through the inside of the solenoid
What is an electromagnet?
a solenoid in which the insulated wire is wrapped around an iron bar (the core)
What happens when a current is passed along the wire of an electromagnet?
a magnetic field is created around the wire
How does an electromagnet work?
- a current is passed along a wire
- magnetic field is created around the wire
- the magnetic field of the wire magnetises the iron bar
- when the current is switched off, the iron bar loses most of its magnetism
How does a scrapyard crane work?
scrap vehicles- lifted using powerful electromagnets attached to cranes
-steel frame of a wehicle sticks to the electromagnet when current passes through the coil
How does a circuit breaker work?
- is a switch in series with an electromagnet
- switch is held closed by a spring
- when the current is too large, switch is pulled open by the electromagnet and it stays open until it is rest manually
How does an electric bell work?
- electric bell is connected to a battery, iron armature is pulled on to the electromagnet
- opens the make-and-break switch, electromagnet is switched off
- the armature springs back and the make-and-break switch closes again, so the whole cycle repeats itself
How does the relay work?
- used to switch an electrical machine on or off
- small current through the coil of the electromagnet magnetises the iron core, which then pulls armature onto the electromagnet
- this closes the switch gap and switches the machine on
- a small current (in the coil) is used to switch on a machine with a much bigger current