Electricity And Magnetism Flashcards
Types of circuits (3)
Combination
Parallel
Series
Potential divider
A series of resistors connected to a fixed power supply which is capable of splitting the input voltage into a fraction of the voltage
Ratio of resistances in potential divider
R1/R2 = V1/V2
Why is combined resistance of resisters in parallel less than one of the resistance alone
There is more than 1 path for the current to flow through
Real life eg of potential divider + description
Rheostat- type of potential divider that can adjust the ratio of R1 to R2 by moving a central sliding contact along a coil of wire. (Therefore adjusts V1 to V2)
EMF
Electromotive Force - energy converted to electrical charge per unit of charge
(Eg. Chemical to electrical in a battery)
Potential Difference
Aka Voltage
Work done per unit charge (transfer of energy)
(Eg. Electrical to thermal in a lightbulb)
Potential divider experiment
Vary the position of a rheostat slider and record the current through the potential divider and voltage across the terminals
Do parallel or series circuits use less EMF
Parallel
Law of magnetic poles
Like poles repel, unlike oples attract
The force between magnetic poles decrease as their separation increases
Experiment to identify the pattern + direction of magnetic fields (5)
Lay bar magnet on a sheet of paper
Place a plotting compass near one pole of the magnet
Mark direction of the compass
Move the compass to find the magnetic field line
Repeat until the other pole is reached
What is the point inbetween to interacting north poles called
Neutral point
Methods of magnetisation (3)
Stroking with a magnet
Hammering in a magnetic field
Using direct current in a coil
Methods of demagnetisation
Hammering
Heating
Using alternating current in a coil
Hard magnetic materials
Make permanent magnets
Steel
Magnetically hard to retain magnetisation
Hard iron
Have to store magnets in correct orientation
Electromagnet
Insulated wire coil carring current wrapped around an iron core
How to increase magnetic affect in an electromagnet (3)
Increasing the number of coils
Introducing an iron core
Increasing the current
Uses of electromagnets
Scrap yard cranes
Circuit breakers
Electric bells
Relay switch
What physical factors affect the strength of an electromagnet (2)
Density - higher=stronger
Amount of FeNiCo=the more the stronger
Why is a full wave rectifyer used
To stop half the current being lost
Full wave rectifyer
Combination of 4 diodes arranged in a wheatstone bridge, allowinmg multiple paths of varied lengths for current to flow. When these currents are blended they make a smooth DC current
Hazardous conditions regarding electricity (3)
Damaged insulation around a wire - coming in contact with direct wire can cause electrocution
Drawing too much current through a cable - causes overheating and can start a fire
Damp conditions around electricity - electricity can be conducted through water leading to electrocution
3 ways of electrical safety
Fuses - thin piece of metal designed to melt when too much current flows through it (breaks the circuit)
Circuit breakers - designed to detect when too much current is flowing or if current is leaking out of the circuit
Earth wire - connected to DV board and then burried underground to that the electricity from the metal case can dissipate into the ground
Electric field
A region in which an electric charge experiences a force
What does charging of solids by friction involve
Transfer of negative charge (electrons)
Charge measuring unit
Coulombs
What does a mains circuit consist of (3)
Live wire
Neutral wire
Earth wire
Why must a switch be connected to a live wire
To be able to safely turn off a circuit
What does a fuse without an earth wire do
Protects the circuit and cabling for a double insulated appliance
Hazard of overcrowding plugs
The plug in the wall socket could overheat causing a fire
Why must the outer casing of an electrical appliance be either non conducting or earthed
If something goes wrong and the live wire comes loose, it could touch the metal case causing the person using it to be electrocuted
What happens to the resistance in a long wire
Resistance is greater as each electron will collide with more ions
What happens to resistance in a thick wire
It decreases as the wider the diameter the more space there is for electrons to flow
Mains electricity
Electricity generated by power stations and transported around the country through the national grid
What type of current comes out the wall
AC current
How to choose a fuse rating
Calculate the current through an appliance (I=P/V)
Fuse should have a rating higher than the current so choose the next size up
What magnetic forces are due to
Interactions between magnetic fields
Magnetic field
Region in which a magnetic pole experiences a force
Describe the forces between magnetic poles
The forces are due to interactions between magnetic fields. Like poles repel, opposite poles attract
Forces between magnets and magnetic materials
Magnetic materials are attracted to magnets and can be magnetised (FeNiCo)
Forces between magnets and non-magnetic materials
None, non-magnetic materials are not attracted to magnets and cannot be magnetised
Induced magnetism
Way of inducing a magnetic current in a magnetic material. Done through stroking with a magnet, hammering in a magnetic field or putting them inside a coil with a direct current passing through.
Differences between properties of temporary and permanent magnets
Temporary magnets made of soft iron are described as magnetically soft meaning they are not permanently magnetic
Permanent magnets are made of magnetically hard materials (steel) and retain their magnetic properties
Plotting compasses
Small compasses which show the direction and shape of a magnetic field
What the direction of a magnetic field line shows
The direction of the force on a North Pole at that point
How is the relative strength of a magnetic field represented
Through the spacing of the magnetic field lines
How to plot magnetic field lines (2)
Using a plotting compass - where the north side points to is the South Pole of the magnet, make dots and draw
Using iron filings - place a sheet of paper of top of the magnet, pour iron filings and lightly tap
Uses of permanent magnets
In situations where they don’t need to be turned off such as a fridge magnet
What does charging a body involve
The addition or removal of electrons
Difference between conductors and insulators
Conductors (metals, used as wires in circuits) allow electrons to flow through them
Insulators impede the flow of electrons
How can charge be detected + show
Using a gold leaf electroscope. Rub a rod with a cloth, this positively charges the rod as the electrons are transferred onto the cloth. When this rod is brought near to the top of the gold leaf electroscope, the electrons will become attracted to the top of the disk. The 2 gold leaf sheets will be repelled as they have both become positively charged. The leafs can also be negatively charged by induction through someone touching the disk allowing electrons from the ground to flow into the disk as they are attracted to the rod.
Electric field
A regional in which a charge experiences a force
Direction of an electric field at a point
Direction of the force on a positive charge at that point
Conventional current vs free flow of electrons
Rate of flow of positive charge from positive to negative
E;entrons flow from negative to positive
Describe the difference between electrical conductors and insulators using a simple electron model + e.g
Electrical conductors are materials with delocalised electrons meaning they are free to move. The easier it is for electrons and therefore current to flow the better the conductor. (Metal)
Insulators don’t have free charges (glass, rubber, wood)
Use of ammeters
To measure electric current
Electric current
Charge passing a point per unit time
Q=It
E.M.F
Electromotive force - electrical work done by a source moving in a unit charge around a complete circuit measured in volts
P.D
Potential difference - work done by a unit charge passing through a component measured in volts
What do electric circuits do
Transfer energy from a source of electrical energy (mains supply/electrical cell) to the circuit components and then into the surroundings
Kilowatt-hour
A unit of energy equivalent to one kilowatt of power expended for one hour
What an electrical appliances power rating tells a customer
The amount of energy transferred to the device every second
Compare the combined resistance of 2 resistors in parallel to one of the resistors by themselves
Combined resistance in parallel is less than that of either resistor by itself
Action of a variable potential divider
Splits the potential difference of a power source between 2 components by moving a sliding contact over a coil of wire (potentiometer) closer to terminal A or B
Direction of an induced emf
Opposes the change that is causing it
How to induce emf in a conductor
By moving the conductor across a magnetic field/changing a magnetic field linking with a conductor
Factors affecting the magnitude of an induced emf (3)
Strength of magnetic field
Speed at which the wire, coil or magnet is moved
Increasing the length of the wire in the magnetic field
Why are power losses in cables smaller when the voltage is greater
P=I^2 R
By lowering the voltage you lower the current, thus reducing the amount of power lost due to the wires heating up. Basically same amount of power can be transmitted over long distances with minimal energy losses to heat
Form of A.C generator
Coil is made of insulated copper wire and is rotated by turning the shaft. Slip rings rotate with it and are fixed to the coil. Brushes (carbon) are 2 contacts which rub against the slip rings and keep the coil connected to the outer part of the circuit.
How to increase turning effect of a current-carrying coil in a magnetic field (3)
Increase the number of turns on the coil
Increase the current
Increase the strength of the magnetic field
What is it called when the voltage increases in a transformer
Step-up
Advantage of high voltage transmission (2)
Increased transmission efficiency
Reduced power loss
Construction + operation of a simple transformer
Soft iron core
Used for voltage transformations
When an AC current is supplied to the primary coil, a changing magnetic field is produced which passes through the core to the secondary coil. The changing field in the secondary coil induces an emf, which is also alternating and has the same frequency as the original current.
Use of transformers in high-voltage transmissions of electricity
Electricity is transmitted at high voltages in overhead cables. A step-up transformer is used to raise the potential difference before transmissions and a step-down transformer is used to lower it again when it reaches its destination
Operation of an electric motor
When a current passes through the coil it creates a magnetic field around the coil which interacts with the field of the magnets, exerting a force on the coil. This pushes one side of the coil up, (left hand rule) the other down, causing it to spin. The split ring commutator (2 half copper rings and 2 brushes) reverses the direction of the current in the coil every half turn, reversing the direction of the force, keeping the coil spinning.
Force on a current carrying inductor hand rule
Flemmings left hand rule
Thumb=force
Index=magnetic field
Middle = current
Hand rule solenoid
Right hand rule
Hand rule particles in a magnetic field
Left hand rule
Describe pattern + direction of magnetic field due to currents in straight wire
Right hand rule (wrap around wire) - find the direction using a plotting compass
Pattern is made up of concentric circles which spread out the further away you are from the wire
Applications of magnetic effect (2)
Relays
Loudspeakers
How magnetic effect of a current is used in relays
Relay switches open/close as a result of magnetic attraction to the electromagnet
Application of relays (2)
Electric bells/scrap yard cranes
How magnetic effect of a current is used in loud speakers
loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet
AC passes through coil of loud speaker, creating a constantly changing magnetic field around the coil which interacts with the magnetic field of the permanent magnet, exerting a force on the coil (FLHR). Since the magnetic fields are constantly changing the direction of the force will as well, causing the coil and then the speaker coil to oscillate. These oscillations make the air oscillate and cause sound waves.
Application of magnetic effects in loudspeakers (2)
Speaker connected to a microphone
Headphones
Describe the pattern of the magnetic field due to currents in solenoids
Outside the solenoid, the magnetic field pattern is that of a bar magnet. When wrapping the right hand around the solenoid with the fingers in the direction of the current, the thumb points in the direction of the magnetic north pole (the solenoid rule). And the other side will be the south pole. Inside the solenoid, the magnetic field is uniform because the field has the same strength except at the ends where the field gets weaker.
