Section 6: Electric & Magnetic Fields Flashcards
What is an electric field?
A region in which another charged object feels a force
What are sparks caused by?
When strong electric fields ionise air particles
When is an electric field created?
Around any electrically charged object
When field lines are close together is the electric field strong or weak?
Strong when electric field lines are close
When field lines are far apart is the electric field strong or weak?
Weak when electric field lines are far apart
What is uniform?
The electric field between two oppositely charged, parallel plates
If the field lines are all equally spaced between parallel plates, what does this mean?
Strength is the same everywhere
Do field lines all point in the same direction or not?
Yes they do
What types of charges attract?
When the objects have opposite charges
What types of charges repel?
When the objects have like charges
How is static charge by friction formed?
- Rub two insulating materials together
- Electrons move from one object to the other
- Both materials become electrically charged, the charge on each material is equal and opposite
What moves in static charge by friction? What doesn’t?
Moves: electrons
Don’t move: protons
When an object loses electrons it become…
Positively charged
When an object gains electrons it become…
Negatively charged
How do objects spark?
- Electric charge builds on the object
- The potential difference between the object and earth increases
- When the potential difference is large enough, electrons will jump the gap - causing a spark
What are examples of sparking?
Lightning
Shocks from clothing etc.
What is earthing?
A process done on objects to prevent sparks
What does earthing do?
- Earthing provides a connection to the ground
- Electrons flow along this connection which discharges static charges
What happens when a static charge is brought near an uncharged object?
The object can have an induced charge
How do electrostatic sprayers work?
- Insecticide is inserted into the sprayer
- The drops of the insecticide are then sprayed - they all have the same type of static charge SO they repel each other
- The insecticides are thus sprayed on crops evenly
What are the dangers of static charges to cars? How can this be prevented?
They can build up whilst fueling a car - a spark could ignite the fuel and cause and explosion
Fuel tanks are earthed so that this is prevented and sparking doesn’t occur
What is a permanent magnet?
A magnet that produces its own magnetic field
What is a magnetic field?
Region where other magnets or magnetic materials experience a force
Where is magnetic field at is strongest? Why?
At the poles - field is stronger where the field lines are closer together
What do field lines show?
Direction force acting on the North Pole - if placed at that point
When does magnetic field decrease?
As distance from magnet increases
What are forces between magnets caused by?
Their magnetic fields interacting
What is magnetic repulsion caused by? Why?
When like poles are close together - they repel
What is magnetic attraction caused by? Why?
When opposite poles are close together - they attract
What is an induced magnet?
A magnetic material that only produces a magnetic field when it’s in another magnetic field
Are permanent and induced magnets attracted to each other or repel from each other?
They’re always attracted to each other
What are the 4 magnetic material?
- Iron
- Steel
- Nickel
- Cobalt
What happens as an induced magnet moves away from a permanent magnet?
It quickly loses all of its magnetism
What are the 4 main uses of magnets?
- Fridge doors
- Separating recycling
- Maglev trains
- Cranes
What do compass needles point to? How?
The direction of the magnetic field that it’s in - the needle itself is a magnet
When a compass isn’t near a magnet where does it point to? Why?
Points north - Earth produces its own magnetic field within its core
What rule do you use to work out the direction of a field?
Right hand thumb rule
What 2 factors does magnetic field strength depend on?
- Size of current - the larger the current, the stronger the field
- Distance from the conductor - the closer to the conductor, the stronger the field
What does the direction of where the thumb points represent in the right hand thumb rule?
The direction of the current
What does the direction of where the other fingers (not thumb) point represent in the right hand thumb rule?
The direction of the magnetic field
What is the motor effect?
When a magnet and a current carrying conductor exert an equal and opposite force on each other
What does the thumb in Flemings left hand rule represent?
Force
What does the index finger in Flemings left hand rule represent?
Magnetic field
What does the middle finger in Flemings left hand rule represent?
Current
What does Flemings left hand rule show?
The direction of force, magnetic field and current in the motor effect
How do electric motors work?
- Direct current is passed through a wire
- Each side of the coil experiences opposite forces
- Coil rotates
What is a solenoid?
A long cylindrical coil of wire
What solenoid an example of?
An electromagnet
What happens when the wire turns inside a solenoid?
The magnetic fields of each turn of the wire add together - forms a strong and almost uniform field
What happens outside a solenoid to the magnetic fields?
The magnetic fields cancel to form weaker fields - these are the same shape as a bar magnets field
What is electromagnetic induction?
The induction of a potential difference (and current if there’s a complete circuit) in a wire which is experiencing a change in magnetic field
What are the two ways to induce potential difference into a wire that’s experiencing a change in magnetic field?
- Move the wire
- Move the magnet
What are the two ways to swap the direction of potential difference in a wire that’s experiencing a change in magnetic field?
- Move the wire in the opposite direction OR start with both magnets the other way around
- Move the magnet in the opposite direction OR start with the magnet the other way around
How can the size of the induced potential difference in a coil that’s experiencing a change in magnetic field increase?
If the speed of movement is increased
OR
If the magnetic field strength is increased
OR
If the turns per unit length are increased
What does an induced current generate?
Its own magnetic field - this always against the change that made it
What does alternators do?
Generate alternating current
How do alternators generate an a.c.?
- A slip rings and brushes - prevents contacts from swapping as it turns
- A force is applied
- Induced a.c. is produced
What does a dynamo generate?
Direct current
How do dynamos generate d.c.?
- A split-ring commutator swaps contacts each half turn
- A force is applied
- Induced d.c. is produced
How do loudspeakers and headphones work?
- Alternating current is sent through a coil
- Current produces magnetic field which produces a force and the coil is within this magnetic field
- Coil moves back and forth as the force on it continuously changes direction
- A paper cone moves back and forth
- Sound waves are thus created as air particles vibrate
How do microphones work?
- Sound waves hit the diaphragm in the microphone
- The diaphragm moves back and forth
- A coil of wire moves back and forth
- Alternating current is generated
How do transformers work?
- Alternating current is passed through a primary coil which has its own magnetic field
- A changing magnetic field is induced in the iron core of the transformer
- Alternating current is then induced in the secondary coil
What is the role of a transformer?
Changes the potential difference of a supply
What has more turns in a step up transformer, the primary coil or the secondary coil?
The secondary coil - it has more turns than the primary coil
What has more turns in a step down transformer, the primary coil or the secondary coil?
The primary coil - it has more turns than the secondary coil
How do power stations generate electricity?
- Fuel is injected into the boiler
- The fuel is burned to heat water to turn it into steam in the boiler
- The steam then turns a turbine
- The turbine then turns the generate which produces a.c.
- This electricity is then sent to the national grid
What is the national grid?
A system of cable and transformers that connect power stations to consumers
What does a step up transformer do?
Increases potential difference to transmit huge amounts of power efficiently as it decreases current and reduces energy loss
What does a step down transformer do?
Decreases potential difference to bring it down to safe, usable levels
Is the electricity transferred in the wires in the national grid process transferred at a high voltage or low voltage? Why is this important?
Transferred at a high potential difference/voltage and thus low current - important as energy loss to thermal stores is reduced making it more efficient as high currents would heat up wires
What is each stage of the national grid process WITH VOLTAGES OF ELECTRICITY AT THE STAGE?
- Power station - 25kV
STEP UP TRANSFORMER… - Pylons - 400kV
STEP DOWN TRANSFORMER… - Heavy industry - 33kV
STEP DOWN TRANSFORMER… - Light industry - 11kV
STEP DOWN TRANSFORMER… - Houses and offices - 240V
Do magnetic fields go from the South Pole to the North Pole OR from the North Pole to the South Pole?
Magnetic fields go from:
North Pole —> South Pole
