TOPIC 6- Electric And Magnetic Fields Flashcards
How’s static electricity built up?
When insulating materials are rubbed together and electrons scrape off one to the other and are stuck (aren’t free to move as are insulators), building up a charge.
Eg polythene and acetate rods rubbed w cloth duster
How do like charges act?
Repel each other
How do opposite charges act?
Attract
What happens to electric charges’ forces as they go further apart?
Forces get weaker.
How is static electricity induced in uncharged objects?
Rubbing a balloon on clothes transfers electrons to balloon (negative). Then moving the charged object near an uncharged object will cause the negative charges in the uncharged object to repel the charged object’s negative charge, leaving the previously uncharged object with a positive charge (attracting negative object).
How are sparks caused from static?
Charge builds on object so potential difference between earth and object increases, if large enough, electrons can jump the gap between the charged object and the earth or to any earthed conductor nearby.
What is static electricity used for?
Electrostatic sprayers (insecticide / paint)
Photocopiers
Reducing dust and smoke from industrial chimneys
How do paint sprayers work?
The charged spray gun has charged particles of paint (all same charge so repel) so spray evenly over the oppositely charged object to attract it. < even coat and v little is wasted.
How do insecticide sprayers work in comparison to paint sprayers?
Plants sprayed aren’t given opposite charge so plants charge by induction.
How can static electricity be dangerous?
When refuelling cars, static can build up via friction from flow out pipe, leading to spark (explosion).
Friction between air and plane charges the plane, causing a buildup of static charge that can interfere w communication equipment.
Raindrops and ice create positive charge at top of cloud and negative at bottom, creating big potential difference and so big spark (lightning).
How do you prevent electrostatic charge build up?
EARTHING (connecting charged object to ground w conductor)
Provides easy route for static charge to travel into ground.
Electrons flow down to ground and positive charge flows from ground upwards.
Electric field
Region around a charged object where, if a second charged object was placed inside it, a force would be exerted on both of the charges.
What happens as you get closer to a charged object?
The electric field grows stronger.
What direction do electric field lines go in?
Positive to negative.
At a right angle from the surface.
What is indicated by closer together field lines?
The stronger the field is.
What happens when a charged object is placed inside an electric field? Why?
The charged object feels a force caused by electric fields around 2 objects reacting.
What happens if the field lines of 2 charged objects point in the same direction?
The field lines join up and objects are attracted to each other.
What happens when the field lines of 2 charged objects point in opposite directions?
The field lines ‘push against’ each other and the objects repel.
What do field lines look like around 2 oppositely charged parallel plates?
A uniform field occurs w an equal strength and direction anywhere between the 2 plates.
How do electric fields cause sparking?
A statically charged object forms an electric field which (if interacted with) can cause sparks.
High potential difference causes a strong electric field between charged and earthed object, causing ionisation of air so current can flow through it.
2 poles of magnets
North and south
Magnetic field
A region where other magnets/magnetic materials experience a force.
Which way do magnetic field lines go?
North to South
Closer magnetic field lines?
What happens to this the further you get from a magnet?
Stronger magnetic field
Gets weaker as you move further away.
Where’s a magnetic field strongest?
At the poles of the magnet.
How so 2 same poles act?
Repel
How do 2 opposite poles act?
Attract
How do you create a uniform magnetic field?
How strong is the field?
How do you draw one
By placing north and south poles of 2 bar magnets near each other.
Same strength everywhere.
How does a compass work?
Tiny bar magnet (needle) in compass lines up w the magnetic field it’s in.
How do you use a compass to see what the magnetic field around it looks like?
Draw around magnet on paper
Place magnet near, needle will point in direction of field line, Mark direction w 2 dots at each end of needle.
Move compass so ends of needle have switched, repeat and join up marks made (1 field line).
Repeat method at many points to get field lines.
What direction does a compass needle point normally?
Why?
Towards the North Pole (magnetic South Pole)
Earth generates own magnetic field, showing core of earth must be magnetic.
Main 3 magnetic elements
An alloy?
Iron
Nickel
cobalt
Steel (contains iron)
Permanent magnets
Produce own magnetic field all the time
Induced/temporary magnets
Only produce magnetic field when in another magnetic field.
Magnetic material
Becomes induced magnet when placed in magnetic field.
What’s the force between a magnet and a magnetic material?
Why?
Always attractive
Because South Pole of magnet induced north in the material (vice versa)
MAGNETIC INDUCTION
Magnetically ‘soft’ materials
Lose magnetism quickly
Magnetically ‘hard’ materials
Lose magnetism slowly
Make permanent magnets
Uses of magnetic materials
Fridge doors
Cranes (induced electromagnets attract/move magnetic materials)
Doorbells (electromagnets switch on and off rapidly)
Magnetic separators (recycling plants sort items)
Maglev trains (magnetic repulsion causes train to float, reducing friction, propelling it along)
MRI (magnetic fields create images inside body)
Speakers/microphones
How does a moving charge create an electric field?
Current flowing through long straight conductor causes a magnetic field to be created around it.
Concentric circles perpendicular to wire w wire in centre.
Rule to see which way currents and magnetic field is going
Right thumb rule
Thumb points in direction of current and fingers curl in direction of field.
What happens to the magnetic field when you change the direction of a current?
The direction of the magnetic field is changed.
What affects the strength of a magnetic field around a flowing current?
Larger current through wire, closer to wire
Stronger field
What happens when a current-carrying conductor is pit between magnetic poles?
Magnetic fields interact, resulting in a force on the wire.
Wire also exerts equal and opposite force on wire
At what angle will the wire feel the full force of the magnetic fields?
At 90° (as opposed to following in direction of field lines)
Flemings left hand rule
What’s the thumb?
What’s the first finger?
What’s the second finger?
Left hand,
First finger points in direction of FIELD
Second finger points in direction of CURRENT
Thumb points in direction of motion
What does the force acting on a conductor in a magnetic field depend on?
Magnetic flux density
Current size
Length of conductor used in magnetic field
When the current is at 90° to magnetic field what equation can be used to find the force?
F=BIl
Force (N) = magnetic flux density (T/ N/Am) *current (A) * length (m)
How does a basic d.c motor work?
Forces act on 2 side arms of split ring commutator connected to wire carrying current, causing rotation as contacts (poles) swap every half-turn to rotate in same direction.
How do you reverse the motor direction of a d.c motor?
Swap polarity of d.c supply (reverse current)
Swap magnetic poles.
Solenoid
Single coiled loop of current-carrying wire with a magnetic field surrounding it.
How do you increase the strength of the magnetic field produced by a solenoid?
Putting a block of iron in center (becoming induced magnet)
Magnetic field inside solenoid
Strong and almost uniform
Magnetic field outside the coil
Overlapping field lines cancel each other out w weak field (except ends of solenoid)
Electromagnet
A magnet with a magnetic field that can be turned on and off using an electric current.
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.
2 examples of electromagnetic induction
Electrical conductor/ magnetic field moving relative to one another. (Eg. Moving/rotating either a magnet in a coil of wire or conductor in a magnetic field)
Change of of a magnetic field through an electrical conductor. (Eg transformer)
How do you reverse the current in a magnetic field/coil moving relative to each other ?
Moving/rotating a magnet in the opposite direction.
Reversing polarity of magnet.
How do you produce an AC through electromagnetic induction?
Keep the magnet/coil moving backwards and forwards or rotating in the same direction.
How do you increase the size of an induced pd?
Increasing strength of magnetic field
Increasing speed of movement/change of field
More turns per unit length on the coil of wire
Transformers
Use induction to change the size of the potential difference of an alternating current.
What are transformers made up of?
2 coils of wire (primary and secondary coils) joined with an iron core.
How do transformers work?
Alternating p.d applied across the primary coil produces an alternating magnetic field. Magnetisation in the core therefore also alternates, inducing a p.d in the secondary coil. The number of turns on the primary/secondary coils determine what happens to the potential difference.
Step-up transformers
Step potential difference up by having more turns on the secondary coil than the primary coil.
Step-down transformers
Step the potent difference down by having more turns on the secondary coil than primary coil.
Efficiency of transformers
Almost 100% efficient
Equation for input power = output power
Vp * Ip = VsIs
Voltage through primary coilcurrent through primary coil = voltage through secondary coil* current through secondary coil
Dynamos
Generate direct current by applying a force to rotate a coil in a magnetic field (or vice versa).
As coil rotates, a current is induced in the coil, changing direction every half turn. Connection is swapped every half turn to keep the current flowing in the same direction.
Alternators
(Basically dynamos)
Produce an alternating current by having slip rings and brushes so the contacts don’t swap every half turn, producing an alternating pd and therefore ac.
Microphones
(Use electromagnetic induction to generate electrical signal)
Sound waves hit flexible diaphragm attached to a coil of wire. Coil of wire surrounds 1 pole of a permanent magnet and is surrounded by the other pole.
As diaphragm moves, a current is generated in the coil, movement depending on properties of sound wave.
Loudspeaker
Diaphragm replaced w paper cone and coil wrapped around pole of permanent magnet so a.c signal causes a force on the coil.
When current is reversed, force acts in opposite direction, making cone vibrate and so air particles vibrate (variations in pressure that cause a sound wave).
How is electricity generated from power stations ?
Burning fuel heats water and converts it to steam, turning a turbine connected to a powerful magnet (usu. electromagnet) inside a generator (cylinder wound w copper wire)
As turbine spins, so does magnet, inducing a large p.d/alternating current in coils.
Coils are joined in parallel to produce single output from generator.
Power generation not requiring a turbine/generator
Solar
National grid
A network of wires and transformers that connect uk power stations to consumers.
Power
Energy/time
Current * potential difference
Why does the national grid not want a high current?
Makes the wire heat up, wasting energy to thermal stores
Power loser due to resistive heating
Current (squared) * resistance
P= I^2R
How does the national grid reduce energy losses?
More efficient, high voltage, low-resistance cables and transformers are used.
What do step-up transformers do at power stations?
Boost p.d v high (400000V) and keep current low.
What do step-down transformers do?
Bring voltage back down to a safe / usable level at the consumer’s end.
Ratio between potential differences on primary and secondary coils?
Vp/Vs = Np/Ns
Input p.d/output p.d = number of primary coil turns/ number of secondary coil turns
