Section 6 - Electric and Magnetic fields Flashcards
How is the build up of static caused by friction?
- When certain insulating materials are rubbed together, - charged electrons will transfer from one to the other
- The charges electrons are not free to move (due to insulators) so the charge builds up, causing static electricity
- both materials become electrically charged, with a + static charge on one, and a - static charge on the other
How do electric charges (or things with them) repel/attract?
Electrically charges objects exert a force on one another
they can also attract uncharged objects
How does attraction by induction work?
Electrically charged object attracting an uncharged one
A negative electrically charged object will repel the negative electrons in the unchraged object. This causes a positive charge on the surface of the uncharged object, which attracts the negatively charged object
How can static cause sparks?
As an electric charge builds on an object, potential difference between the object and the earth increases
-> If the p.d. gets large enough, the electrons can jump across the gap between the charged object and the earth (or any earthed conductor) - this is the spark
What are some examples of uses of static electricity?
Photocopiers - to copy imapes onto a charged plate before printing them
- To reduce dust and smoke that rises out of industrial chimneys
- Electrostatic sprayers - Painting / insecticide
How do electrostatic sprayers work?
The spray gun is charged, which charges up the particles of paint. This makes them repel eachother, creating a fine, dispersed spray. - even coat, no waste
The object to be painted is given the opposite chrage to attract the spray.
Insecticide works like this but by opposite induction not given charge
e.g. Bikes, cars
How can static electricity be dangerous when refuelling cars?
As fuel flows out of a filler pipe, e.g. into an aircraft or tanker, static can build up. This can lead to a spark which may cause an explosion in dusty or fumey places e.g. at a petrol station
These are earthed to prevent this
How is the static on aeroplanes dangerous?
Friction between the air and the plane causes the plane to become charged.
-> The build up of static can interfere with communication equipment
How is lightning formed?
Raindrops and ice bump together inside storm clouds, leaving the top of the cloud positive and the bottom of the cloud negative. This creates a huge voltage and a big spark, which can damage homes or cause fires when it strikes the ground
What is earthing?
Connecting a charged object to the ground using a conductor (e.g. a copper wire)
How does earthing prevent dangerous sparks?
It provides an easy route for static charges to travel into the ground. this prevents a build up of charge causing a spark
Electrons flow down the conductor to the ground if the charge is negative and flow up the ground through the conductor if the charge is positive
What is an electric field
a region around any electrically charged object
the closer you get to the object, the stronger the field
What are the rules for electric field diagrams?
Go from + to -
theyre at right angles to the surface
The closer the lines, the stronger the field
With 2 objects:
- If the field lines point in the same direction, the lines join up and the objects are attracted to each other.
- If the field lines are in opposite directions, they push against eachother and the objects repel
- The strength and direction are the same everywhere between 2 plates
What happens when a charged object is placed in an electric field?
It feels a force
-> This is caused by the electric fields around the two object interacting
How is sparking explained by electric fields
Hair comb and paper
After its run through the hair, its charged and so produces an electric field. This electric field interacts with the peice of paper and so the paper feels a force. This causes it to move towards the comb
How do sparks relate to potential difference
Sparks are caused when there is a high enough p.d. between a charged object and the earth. A high p.d. causes a strong electric field between the two. A strong electric field causes electrons in the air to be removed (ionisation). This causes the normally insulatory air to be conductive, allowing a current (spark) to flow through it
What is a magnet and a magnetic field?
Magnet - a thing with 2 poles
Magnetic field - a region where other magents or magnetic materials experience a force
What are the rules for drawing magnetic fields?
Lines always go from North to South and display which direction a north pole would feel force at each point in the field
Closer lines = stronger field
Further away = weaker field
Magnetic (field and forces) are strongest at the poles
Where do compasses point when not near magnets?
The Earth’s north pole (which is actually a magnetic south pole)
What are the main 3 magnetic elements?
Iron, Nickel and Cobalt
-> Some of their alloys as well e.g. Steel
What is the force between a magnetic material and a magnet?
Always Attractive
What do permanent magnets do?
Produce their own magnetic field all the time
e.g. bar magnets
What do induced/temporary magnets do?
Only produce a magnetic field whilst their in another magnetic field
How does magnetic induction work?
A permanent magnet placed next to a magnetic material will induce opposite magnetic poles.
N—-S ____ N—-S (induced)
When you take the Permanent magnetic away, the induced magnets return to normal and stop producing a magnetic field
-> This is why the force is always attractive
How is - How quickly an induced magnet loses their magnetism - mesured?
Magnetically “hard” or “soft”
What are some examples of magnetically “soft” materials
Materials that loose their magnetism very quickly
Pure iron and Nickel-iron alloys
What are some examples of magnetically “hard” materials
Materials that loose their magnetism slowly -Permanent magnets come from
Steel
How is magnetism used in fridge doors?
There is a permanent magnet strip in the fridge door to keep it closed
How is magnetism used in cranes?
These use induced electromagnets to attract and move magnetic materials e.g. moving scrap metal in junk yards
How is magnetism used in doorbells?
These use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise
How are magnetic seperators used?
Used in recycling plants to seperate items (like cans)
How is magnetism used in maglev trains?
These use magnetic repulsion to make trains float slightly above the track ( to reduce losses from friction) and to propel them along
How is magnetism used in MRI machines?
These use magnetic fields to create images of the inside of your body without having to use ionising radiation
How is magnetism used in media?
In speakers and microphones
What does a moving charge cause?
A magnetic field
-> when a current flows through a long, straight conductor (e.g. a wire), a magnetic field is created around it
How is a magnetic field created by a current directionally proportional to it?
The field is made up of concentric perpendicular to the wire, with the wire in the centre
This is shown by the right-hand thumb rule
What is the motor effect?
A current in a magnetic field experiences a force
- When a current-carrying conductor (e.g. a wire) is put between two magnetic poles, the two magnetic fields interact. - this results in a force
What angle does the wire have to be to the magnetic field to experience full force during the motor effect?
90°
-> if it runs along the magnetic field it wont feel any force, some angles feel a partial force
What is flemmings left hand rule?
The directional relationship between components in the motor effect
- Thumb - F (force)
- 1st Finger - B (Magnetic field/flux dens)
- 2nd Finger - I (Current)
What is magnetic flux density and length in FBII
FBII only used when force in 90° to the field
MFD = How many field lines there are in a region (shows strength of field)
length = Length of conductor thats IN the magnetic field
How can the direction of a motor be switched?
swapping the polarity of the d.c. supply (reversing current) or swapping the magnetic poles over (reversing field)
How do you increase the magnetic field strength of a loop of wire?
Make more loops - turning it into a solenoid
How is a motor kept going the same direction?
Split-ring commutator - swaps contacts every half turn to keep the motor rotating in the same direction
How do the field lines in a solenoid work?
The field lines outside the coil overlap and cancel eachother out - so the field is weak apart from at the ends of the solenoid
Inside the solenoid, you get lots of field lines pointing in the same direction - the magnetic field is strong and almost uniform
What is a solenoid an example of?
An electromagnet
How can you increase the strength of a solenoid?
Put a block of iron in the centre of the coils - this becomes an induced magnet whenever current is flowing
