Unit 4 - Electricity and magnetism Flashcards
Attract
A force that pulls objects together
Repel
A force that pushes objects apart
Poles of a magnet
North & South poles
Magnetic field
A region of space where another magnet or magnetic material experiences a force
Attraction displayed in a diagram
Field lines point in the same direction (N to S) and flow between the two magnets
Repulsion displayed in a diagram
Field lines point in opposite directions and bend away from each other
Attraction of poles
Opposite poles attract e.g. (N&S) hence same poles repel
Non-magnetic materials
Do not experience a force when in a magnetic field
Property of field lines that represent the field strength
Density of the lines
Permanent magnet
Always has a magnetic field
Induced magnetism
When a magnet material, in the presence of a permanent magnet becomes attached to a magnet and temporarily induces a North and South Pole until later becoming demagnetized
Materials that keep their magnetism after the external field is removed
- Hard magnetic material e.g. steel - hard to magnetize and demagnetize - used to make permanent magnets
- Soft magnetic material e.g. iron - easy to magnetize and demagnetize - good for temporary magnetics
Properties of a permanent magnet
- Constant magnetic field
- Cannot be switched on or off
- North and south poles cannot be swapped
- Used for guitar pickups, speakers & cupboard latches
Properties of an electromagnet
- Variable strength magnetic field
- Can be switched on and off quickly
- North and south poles can be changed by changing the direction of the current flow
- Used for electric door locks, relays & MRI machines
Insulators
Do not let electrical charge / electrons to move freely
Conductors
Allow electrical charge / electrons to move freely
Charge carriers in electrical devices
Electrons
Two types of electrical charge
- Positive
- Negative
Unit of electrical charge (Q)
Coulombs (C) - one electron carries the charge of 1.6x10^-19 C
Condition for static electricity
Occurs when friction between two insulators causes electrons to be transferred from one surface to another making one insulator gain electrons and become negative and the other to become positive through the loss
Electric field
The space in which an electric charge experiences a force
Direction that an electric field points
in the direction that a positive charge experiences a force
Uniform field
When the field lined are parallel between parallel plates
Pattern of field lines used around conducting spheres or point charges
Radial patterns
Current
A measure of the flow of charge
Ways to increase current
- Making each charged particle move faster
- Increasing the number of charged particles
- Increasing the amount of charge each particle carries
Unit of current
Amps (A)
Device used to measure current
Ammeter
Current equation
- Current (I) = Charge (Q) ÷ Time (T)
- Amps = Coulombs ÷ Seconds
Direction of conventional current
Flowing out of the positive terminal of the battery
Correlation between conventional current and electron flow
both act in the opposite direction to each other
Alternating current (a.c.)
Electrons continuously change direction
Direct current (d.c.)
Electrons flow in one direction only
Condition of voltage source to make alternative current (a.c.)
- Source must cause electrons to move back and forth
Condition of voltage source to make direct current (d.c.)
Electrons must flow in one direction
Voltage
The unit of potential difference
Unit of measurement for voltage
Volts (V)
Electromotive force (e.m.f)
The work done by a source in moving a unit charge around a complete circuit
Unit of measurement for electromotive force (e.m.f)
Volts (V)
Potential difference (p.d.)
The energy needed per charge to flow between two points in a circuit / the work done per unit charge passing through a component
Device used to measure volts
Voltmeter
Type of circuit voltmeters are used in
Parallel circuits
Voltage equivalent in joules
-1 V is equivalent to giving 1 J of energy to 1 C of charge
- 1 V = 1 J/C
Electromotive force (e.m.f) equation
- Electromotive force (V) = Work done (J) ÷ Charge (C)
- E = W/Q & V = W/Q
Resistance
Measure of how much opposition there is to the flow of current in a circuit
Resistance equation
- Resistance = Voltage ÷ Current
- R = V/I
Unit of measurement of resistance
Ohms (Ω)
Correlation of resistance and the length of a wire
- Directly proportional e.g. if length doubles resistance doubles
Correlation of resistance and the width of a wire
- Inversely proportional e.g. if the cross-section doubles in width resistance is halved
Ohm’s law
- V = IR
- Relationship between p.d. and current is directly proportional e.g. if voltage doubles current doubles
Negative current or voltage
Achieved by changing the direction of the battery
Plotting of I-V graphs
Current and voltage are plotted for both positive and negative values
Ohmic device
A device where under constant temperature the current and voltage are directly proportional e.g. a resistor or wire
Non-ohmic device
e.g. a filament lamp where when large voltage is applied there is only a small increase in current causing an increase in heat and atoms gaining kinetic energy and colliding, causing an increase in resistance
Diode
- A circuit component that only allows current to flow in one direction but requires a small voltage to ‘switch on’
- Non-ohmic device since there is always zero negative current
- Only works when more than 0.7 V is applied
- In circuit, diode symbol points in the direction of conventional current flow
Power
A measure of how quickly energy is transferred
Unit of power
Watt (W) = 1J/s
Electrical power equation
- P = IV
- or:
- P = V^2 ÷ R
- or:
- P = I^2R
Energy equation
- E = Pt
- E = IVt
Common unit of energy
Kilowatt hours
- Energy (kWh) = P(kW) x t(h)
Cost of electricity equation
Cost of electricity = number of kilowatt hours x cost per kilowatt-hour / cost per unit
Conventions for a circuit diagram
- Straight lines for wires
- Placing voltmeters in parallel with the components
- Placing ammeters in series with components
- using conventional electrical symbols
Switch symbol
Straight line with an opening
Cell symbol
Straight line with two vertical lines in the middle, a longer one on the left with a positive sign and a shorter on the right
Battery symbol
Straight line with two vertical lines alike a cell on the right, and another two alike a cell on the left, with a dotted line connecting them
Lamp symbol
A circle with an X inside
Fuse symbol
A rectangle with a straight line running through it
Voltmeter symbol
a circle with a V inside it
Ammeter symbol
A circle with an A inside it
Fixed resistor symbol
An empty rectangle
Variable resistor symbol
a rectangle with a diagonal arrow running through pointing to the top right
Thermistor symbol
A rectangle with a diagonal line running through to the top left, with a straight hook at the bottom left
Heater symbol
A rectangle divided into four squares
Light dependent resistor (LDR) symbol
A rectangle with two arrows pointing at it from the top left
Relay coil symbol
A rectangle cutting through a vertical line
Transformer symbol
Two rectangles with vertical coils separated by a vertical line
Variable potential divider (potentiometer) symbol
A rectangle with an arrow pointing in the middle in a straight direction
Magnetizing coil symbol
A coil
a.c. power supply symbol
Two white dots with a squiggle in between
Motor symbol
A circle with an M inside of it
Generator symbol
A circle with a G inside of it
Diode (extended only) symbol
A straight line with a 90˚ clockwise rotated equilateral triangle pointing at it
Light-emitting diode (LED) (extended only) symbol
A straight line with a 90˚ clockwise rotated equilateral triangle pointing at it and two arrows pointing at it from the top right
Light-emitting diodes (LED)
- Same features as a regular diode but produces a single color of light
- Current flow from battery and direction of the diode are the same - other way around current cannot flow because resistance is too high
Property of a thermistor that changes with temperature
Resistance increases with increased temperature
Properties of a series circuit
- Current flows in one loop - os same at all points
- Voltage is shared - hence multiple lamps are dimmer than one
- Total resistance is sum of resistors
- Electromotive forces of cells added together to find total e.m.f.
- p.d. across resistors is shared - sum of p.d.s across the resistors is qual to the battery power (e.m.f.)
Properties of a parallel circuit
-More than one loop of current - current is shared - total current flowing into a junction is equal to the sum of the currents flowing out of a junction
- Voltage is the same through each loop - hence p.d. of each lamp is equal to electromotive force (e.m.f.) of the battery
- Loops can be turned on or off independently of each other
- Combined resistance of two resistors in parallel is less than that of either resistor by itself - 1/Rtotal = 1/R1 + 1/R2 - each resistor experiences the same p.d. as the power supply
Advantages of parallel
- One light can go out without others going out - one but in series would cause the whole circuit to fail
- Less resistance than in series
Variable potential divider use
- Long resistor that can be split in two parts
- Arrow in the centre represents an adjustable wiper
- Wiper can be moved to adjust ratios of resistance - e.g. if wiper is moved to the left right side has more resistance
Potential divider use
- Splits up voltage
- Seen as two resistors in series
- R1/R2 = V1/V2
Electrical hazards
- Damaged insulation - damaged insulation leads to live wires that can electrocute - repair with insulating tape
- Overheating of cables and applianced - can cause fire- switch off any appliance that feels hot
- Overloading sockets - can draw too much current to one socket - can result in a fire
- Damp conditions - Water conducts electricity there can electrocute - keep applianced dry and have dry hands
Circuit used for a house
Mains supply
Safety devices with electricity
- Earthing metal cases - Earth wire is connected to the outer metal casing of an appliance - can prevent a lethal shock because current flows to earth because resistance is lower than a person
- Fuses - protect a circuit - thin wire inside that connects to the live wire - too much current and the wire melts and circuit breaks - rating of a fuse must be the lowest value greater than the current needed for an appliance
- Trip switches - circuit breaker - safety device - if too much current is flowing between the live and neutral wires a switch opens to break the circuit - relay in reverse - can be used more than once - value is lowest value that is greater than the current needed for the appliance
Three wires of a three-core electric cable
- The Live Wire - brown - carries the current from the mains supply and an on/off switch would be connected
- The neutral wire - blue - completes the full circuit - does not supply current
- The earth wire - Green & yellow - safety feature to prevent electrocution
Faraday’s Law
A wire close to a changing magnetic field will experience an induced electromotive force
Two ways to induce e.m.f
- Moving a magnet so that its field lines are cut by a wire
- Moving a wire across a magnetic field
Fleming’s right-hand rule
- First and second finger and thumb are perpendicular
- Line up hand with the diagram with and the two quantities given
- Observe the direction of the third quantity by:
- The direction of the motion of the wire is represented by the thumb
- The direction of the magnetic field is represented by the first finger
- The direction of the current is represented by the second finger
Lenz’s law
The direction of the induced e.m.f opposes the change that creates it
Components of an a.c. generator
- Magnets - provide a constant magnetic field across the coil
- Coil (or armature) - made from many turn of wire - rectangular so that its sides are perpendicular to the magnetic field
- Slip rings - cylindrical conductors that make constant contact with the coil during rotation - allow the direction of induced e.m.f. to alternate and cause an alternating current
- Carbon brushes - make an electrical connection between the rotating coil and a circuit - avoiding the wires becoming twisted
Stages of rotating generator coil
1 - top and bottom sides of the coil are moving parallel to the magnetic field and no e.m.f is induced
2 - Long sides of the rectangular coil move exactly perpendicular to the magnetic field and maximum e.m.f. is induced
3 - Again the top and bottom sides of the coil are moving parallel to the magnetic field and no e.m.f is induced
4 - Again the long sides move perpendicular to the field in the opposite direction - maximum e.m.f.
Magnetic fields created by types of currents
Direct current - constant magnetic field
- Alternating current - alternating magnetic field
Solenoid
When the wire is arranged as a coil the resulting magnetic field is the same as that of a bar magnet
Increasing the strength of a magnetic field inside a solenoid
- Increase the current through the solenoid
- Increase the number of turn of wire in the solenoid
How electromagnets work
- A flow of current results in the generation of a magnetic field around a coil
- The magnetic field will attract a magnetic material close to the circuit
- The closed circuit will now perform an action
Function of a relay
A device used to switch between separate circuits
How loudspeakers function
- Speaker cone oscillates left and right when an alternating current is supplied to the coil
- Magnetic field due to alternating current in the coil either attracts or repels a permanent magnet around in resulting in vibrations needed for sound
Corkscrew rule
- Thumb points in the direction of current flow and fingertips in the direction of the magnetic field
- Used to find out the direction of the magnetic field around a straight wire
Ways to increase the force experienced by a wire within another magnetic field
- Increase the current in the wire
- Increase the number of individual wires
- Increase the strength of the magnetic field
- Increase the length of the wire within the magnetic field
Fleming’s left-hand rule
- Direction of force, current and field are perpendicular
- If field and current are parallel, no force will be acting
Flow of charged particles
- Conventional currents and positive charge flow from positive to negative
- Negative charge movies in the opposite direction to current flow
How to make a d.c. motor spin more quickly
- Increase the strength of the magnets and thus the magnetic field
- Increase the number of turns of wire in the coil
- Increase the current to the coil from the power supply
How to increase the turning force of a d.c. motor
- Increase the current
- Increase the strength of the magnetic field
- Increase the number of turns in the coil
Components of a d.c. motor
- Coil (or armature) - rectangular and often made up of lots of turns of current-carrying wire
- Magnets - bar magnets used making a field perpendicular to the coil N-S
- Brushes - allow constant electrical contact with the inside of the split ring while the coil rotates otherwise wires would get twisted
- Split ring commutator - As coil rotates the direction of the current needs to stay the same so that the force also acts un the same direction - once the coil rotates through 180˚ current continues to flow in the original clockwise direction causing the force on the left side to be up and force on the right side to be down - if not present the coil would flip backwards and forwards
Function of a transformer
A device that can increase or decrease the size of an alternative e.m.f.
Two types of transformers
- Step-up transformer - increases voltage - has more turns on the secondary coil than the primary coil
- Step-down transformer - decreases voltage - has fewer turns on the secondary coil than on the primary coil
Parts of a transformer
- Primary coil - where a.c. is supplied - energy source of the transformer
- Soft iron core - designed to allow the transition of magnetic flux to a secondary coil
- Secondary coil - the output of the transformer
Transformer notations
- Primary coil:
- Np - number of turns on the primary coil
- Vp - voltage applied to the primary coil
- Ip - current in the primary coil
- Secondary coil:
- Ns - number of turns on the secondary coil
- Vs - voltage induced on the secondary coil
- Is - current in the secondary coil
Equation for coil turns and voltage in transformers
Vp/Vs = Np/Ns
Use of transformers
- Designed to increase the voltage which electricity is transmitted - current in cables is lower and less energy is wasted as heat - cables with large cross-sectional area lowers the resistance and energy wasted
How transformers function
- Only work with a.c.
- Primary coil is supplied with an a.c. and behaves like a bar magnet that is constantly switching its poles due to changing current
- Constantly changing magnetic field is transferred to the secondary coil via soft iron core
- Secondary coil acting as a conductor is the presence of a changing magnetic field induces e.m.f across the secondary coil
- e.m.f. induced across the secondary coil is constantly changing and therefore provides and alternating current
Conservation of energy in a transformer
- Power in is equal to the power out
- Pp = Ps
- VpIp = VsIs
Equation of the efficient of transformers
Efficiency of transformers = Pout / Pin or IsVs / IpVp
How to increase the efficiency of a transformer
- Using low resistance coils to reduce the power wasted due to the heating effect of the current
- Using a laminated core which consists of layers of iron separated by layers of insulation - reduces heat in the iron core and prevents eddy currents (currents induced in the core itself_
Main energy loss in transmission of electricity
- Heat
Equation to calculate power loss in a wire
P = I^2R
- Power (W) = Current ^2 (A) x resistance (Ω)
