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
Give examples of appliances that contain an electric motor?
hairdryer, electric shaver, a refrigerator pump, and a computer hard drive
Why does an electric motor work?
a force can act on a wire (or any other conductor) in a magnetic field when a current is passed through the wire
What is the motor effect?
when a current is passed along a wire in a magnetic field, and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field
What 2 ways can the size of the force acted upon the wire be increased by?
- increasing the current
- using a stronger magnet
What does the size of the force depend on?
the angle between the wire and the magnetic field lines
When is the size of the force of the magnetic field greatest?
when the wire is perpendicular to the magnetic field
When is the size of the force of the magnetic field zero?
when the wire is parallel to the magnetic field lines
The direction of the force is always at what to the wire and field lines?
at right angles
What happens when the direction of the force is reversed?
the direction of the current or the magnetic field is reversed
What is the magnetic flux density of a magnetic field?
the measure of the strength of the magnetic field
What is the symbol B used for and what is its unit?
magnetic flux density, tesla (T)
The size of the force on the conductor depends on?
- the current of the conductor
- the length of the conductor
- the magnetic flux density of the magnetic field
How to work out force of a magnetic field?
magnetic flux density x current x length
An electric motor is designed to use what?
the motor effect
How can you control the speed of an electric motor?
by changing the current
How can you reverse the direction the motor turns?
by reversing the current
Why does the coil spin when a current is passed through the coil?
- a force acts on each side of the coil due the motor effect
- the force on one side is in the opposite direction due to the force on the other side
Why does a force act on each side of the coil?
motor effect
What is the motor effect?
when a conductor such as a wire is carrying a current, it experiences a force in the magnetic field
how may we determine the direction of the force?
using Fleming’s left hand rule
The size of the force can be increased by?
- increasing the current
2. Using a stronger magnet
The size of the force depends on?
depeds n the angle betwee nthe wire and the magnetic field lines
When is the force acting on the wire greatest?
when the wire is perpendicular to the magnetic field
Why does an electric motor work?
a force can act on a wire (or any other conductor) in a magnetic field when a current is passed through the wire
When is the force acting on the wire zero?
when the wire is parallel to the magnetic field lines
The direction of the force is always at what to the wire and the field lines?
right angles
When is the direction of the force reversed?
when the direction of the current or the magnetic field is reversed
What is the magnetic flux density of a magnetic field
the measure of the strength of the magnetic field
The side of the force on the conductor depends on?
- the current i in the conductor
- the length l of the conductor
- the magnetic flux density B of the magnetic field
What is the unit of force?
newtons
What is an electrc motor designed to use?
motor effect
How can you control the speed of an electric motor?
changing the current
How can you reverse the direction the motor turns in?
by reversing the current
What is something a simple motor has that is forced to rotate?
a rectangular coil of insulated wire (armature coil)
In a simple motor, what is the armature coil (insulated wire) connected to?
the battery by two metal or graphite brushes
What do the two metal or graphite brushes do in a simple motor
press onto a split-ring commutator fixed into the coil
What is graphite?
a form of carbon that conducts electricity and is very slippery
How much friction does graphite cause when it is in contact with the rotating commutator?
very little
What is a split-ring commutator?
metal contacts on the coil of a direct current motor that connects the rotating coil continuously to its electrical power supply
Why does the coil spin when a curent is passed through the coil?
- a force acts on each side of the coil due to the motor effect
- the force on one side is in the opposite direction to the force on the other side
Every half-turn of the coil, what happens?
the split-ring commutator reverses the current around the coil
Why is the coil pushed in the same direction every half-turn?
the sides swap over each half-turn
A hospoital has its own what?
own electricity generator that is always on standby in case of a power cut
What happens if there was no standby generator in the hospital?
parents’ lives would be put at risk if the mains electricity supply failed
What does a generator contain?
coils of wire that spin in a magnetic field
When the wire crosses through the magnetic field lines, what happens?
A potential difference (voltage) is created, or induced across the ends of the wire
What is electromagnetic induction?
When any conductor crosses through magnetic field and a voltage is produced
What happens if the conductor is part of a complete circuit
the induced potential difference makes an electric current pass around the circuit
What is the generator effect?
when the induced potential difference makes an electric current pass through the circuit
What happens to the ammeter pointer when one end of a bar magnet is pushed into the coil?
the ammeter pointer deflects
Why does the ammeter pointer deflect when one end of a bar magnet is pushed into the coil in the generator test?
-the movement of the bar causes an induced potential difference in the coil which causes a current, becausr the coil is part of a compete circuit
If a stronger magnet is used, what happens to the current and induced potential difference?
it will be bigger
What happens to the ammeter pointer if the bar magnet is then withdrawn from the coil?
it deflects in the opposite direction
Why does the ammeter pointer deflect in the opposite direction when the bar magnet is withdrawn from the coil?
the induced potential difference acts in the opposite direction, so the induced current is in the opposite direction
if the direction of movement in the motor effect switches, what happens?
direction of the current switches
If the movement stops what happens to the current?
the current stops
when do we only see the generator effect?
if the wire passes through the magnetic field
if the wire moves along the magnetic field, what happens?
we do not get an induced potential difference or current
The direction of the current changes when?
the direction of movement changes
What is a way of switching the direction of the induced current?
switching the poles of the magnet
What happens when we move a magnet into a coil of wire?
a current is induced in the wire
Induced currents can create its own what?
magnetic field
What is the correlation between the magnetic field produced by an induced current and the movement of the magnet?
the magnetic field opposes the movement of the magnet
What happens when we insert the North pole of a bar magnet into the end of a coil?
that end of the coil also ecomes a North pole
What happens to the magnet when the North pole of a bar magnet is inserted into the end of the coil?
the magnet is repelled by the end of the coil being North. making it harder to push the magnet in
