4. Electricity and Magnetism Flashcards
Magnets
Can exert a force (attraction or repelling) on another magnet or on magnetic materials
They have a permanent magnetic force.
Attract (magnets)
A force that pulls objects together.
Repel (magnets)
A force that pushes objects apart.
Magnetic materials (Examples)
- Iron
- Nickel
- Cobalt
- Steel (component of steel is Iron)
Magnetic materials
Not magnets, but they are attracted to magnets.
(If connected to a permanent magnet a temporary magnetic field will be induced making it magnetic. –> However, when removed from the magnet the magnetic field is removed and the magnetic material can no longer attract another magnetic material)
Non-magnetic
Do not experience a force when in a magnetic field. Plastic is an example of a non-magnetic material.
Magnetic forces (attraction/ repelling)
Opposites attracts
Similars repel
Magnetic poles
- Every magnet has a north and a south pole, positioned at opposite ends of the magnet.
- If you cut a magnet in half, you get two smaller magnets, each with a north (N) and a south (S) pole. (North pole will still be on the same side as the previous north side, just smaller)
Magnetic field
A region of space where another magnet or magnetic material experiences a force.
Represented by magnetic field lines, ALWAYS going from north to south.
- Field lines never cross
- The strength of the magnetic field is indicated by the density of field lines
The arrow on the magnetic field always shows the direction of force on the N-pole of compass, meaning the direction a compass will point.
Magnetic fields (attraction/ repulsion)
Attraction (North pole next to south pole) : field lines point in the same direction (N to S) and flow between the two magnets.
Repulsion (North pole next to North pole) : field lines point in opposite directions and bend away from each other.
Induced magnetism
- Magnetic materials can attract each other, but only when a permanent magnet is present.
- A permanent magnet always has a magnetic field.
- When a permanent magnet attracts a magnetic material, it induces a magnetic field in the material.
Induced magnetism - Eg. nickel coin on a horseshoe magnet
- When the magnetic material (the nickel coin) is placed in the magnetic field of the permanent magnet (the horseshoe magnet), magnetism is induced in the coin and it becomes attached to the permanent magnet.
- The nickel coin is attracted to either the north or the south pole of the permanent magnet.
- When a coin is attracted to the south pole, a north pole is induced at the top of the coin and a south pole at the bottom.
- The opposite is true when a coin is attracted to the north pole.
- When a third nickel coin is placed in the magnetic field (below the nickel coin), a south pole is induced at the top of this coin and a north pole at the bottom.
Demagnetised
The process of removing magnetism. Also known as unmagnetised.
Eg. When a magnetic material is removed from the magnetic field of the permanent magnet.
Demagnitism - Difference in materials
- A hard magnetic material , such as steel, is hard to magnetise but also hard to demagnetise/unmagnetise. (Used to make permanent magnets in devices that require a constant magnetic field)
- A soft magnetic material (eg. iron) is easy to magnetise but also easy to demagnetise/unmagnetise. (Used for temporary magnets. Iron is used in electronic door locks as it gains and loses its magnetism quickly.
Electromagnet
A magnet caused by the flow of current in a coil. It only creates a magnetic field when current passes through it.
Any time a coil of wire carries current, a magnetic field is induced
Properties of permanent magnets
Constant magnetic field
Cannot be switched on or off
North and south poles cannot be swapped
Properties of electromagnets
Variable strength magnetic field
Can be switched on and off quickly
North and south poles can be changed by changing the direction of current flow
Uses of permanent magents
Guitar pickups
Speakers
Cupboard latches
Uses of electromagnets
Electric door locks
Relays
MRI machines
Conductor
A material through which charges can flow freely.
Insulator
A material that does not allow the free movement of charges.
Charges - Net overall charge /removal/ addition
Most everyday objects have no net overall charge (neutral)
- It is possible to add or remove charges from the surface of an object to make it either positive or negative overall.
- This is due to the transfer of electrons, which are negatively charged particles.
An object that loses electrons will become positively charged, while an object that gains electrons will become negatively charged.
What is electrical charge measured in?
Electrical charge (Q) is measured in coulombs (C).
- A single electron carries a very small charge of 1.6 × 10−19 C.
Static electricity
- Static electricity occurs when friction between two insulators causes electrons to be transferred from one surface to another
- One insulator gains electrons (and becomes negatively charged) while the other loses electrons (and becomes positively charged).
- Insulators with net overall charge can attract to neutral objects by repelling like charges and being relatively more charged.
Static electricity - Eg. Balloon
- Rubbing the balloon causes the transfer of electrons from the jumper, leading to an imbalance of charges.
- As one surface is negative and the other is positive, they attract.
- Rubbing two balloons on the jumper causes the balloons to repel each other because they both have like (negative) charges.
- Balloon is attracted to neutral wall bc relative to wall the ballon is more positive/ negative making the charges opposite.
- Also the postive charged balloon will repel positive charges making the wall more negative + vice versa with negative balloon.
Electric field
When two charged particles approach each other, they experience a force.
The space in which an electric charge experiences a force is called an electric field.
An electric field always points in the direction that a positive charge experiences a force.
Uniform field
Parallel electric field lines betewen two flat surfaces.
Current
Measure of amount/rate of the flow of charges (eg. electrons).
When charges are stationary, there is no current; when they move, there is a current.
How to increase the current
Making each charged particle move faster
Increasing the number of charged particles
Increasing the amount of charge each particle carries.
Current (formula)
I = Q/t
I - Current (A, Amps)
Q - Charge (C, Coulombs)
t - Time (s, seconds)
I = V/R
V - Potential difference (V)
R - Resistance (ohms)
How to measure current
Placing an ammeter IN the circuiot BETWEEN componenets/ the battery etc.
Ammeters can be analogue (needle pointer) or digital (numbers).
Conventional current
- Conventional current is imagined flowing out of the positive terminal of a battery, around the circuit, to the negative terminal.
- The charge carriers, however, are electrons, which have a negative charge. Electrons are repelled from the negative terminal of the battery and attracted to the positive terminal.
- Electron flow is always in the opposite direction to conventional current.
AC/DC - General
- Alternating current (a.c.): electrons continuously change direction.
- The voltage must reduce to zero and then become negative, causing the electrons to move in the opposite direction –> moving back and forth.
- Direct current (d.c.): electrons flow in one direction only.
- One level of voltage –> electrons flowing in one direction only
Voltage
Measure of how much E.M.F. a battery has.
Electromotive Force
The work done or energy per unit charge around the whole circuit by an energy source, such as a battery. Measured in volts.
- Phenomena that enables a charge to flow –> the electrical work done by a source in moving a charge around a complete circuit.
Potential difference
The energy needed per charge to flow between 2 points in a cirvuit.
Work done per unit charge passing through a component.
EMF vs PD
(If circuit stays the same)
EMF = Constant
PD = Variable depending on points chosen within circuit
Calculating EMF in a circuit/ Voltage of a battery (Formula)
EMF (V, Volts) = Work done (J) / Charge (C)
E = W/Q or V= W/Q
Measuring EMF
Voltmeter placed parallel aroudn a component
- Can be analogue (needle-pointer) or digital
PD - Voltmeter = parallel around points.
EMF - Voltmeter = parallel around battery/ energy source.
Resistance
A measure of the opposition to current flow within a circuit.
Measured in ohms (Ω)
Resistance - Formula
Resistance (Ω) = Potential difference (V)/ Current (A)
R = V/I
Factors affecting resistance
Length / Cross-sectional area
If length doubles –> resistance doubles (directly proportional)
If cross-sectional area doubles –> resistance halves (Inversely proportional)
Ohms law
V = IR
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
Energy in circuits –> power
Power - Measure pf transfer rate of energy
Original energy source in a circuit = chemical energy in battery.
Electricity used to transfer chemical energy in battery to heat/ light/ kinetic energy.
Power (electricity) formula
P = IV
P = Power (W or J/s)
I = Current (A)
V = Potential Difference (V)
Other power electricity formulas
E = IVt
(bc P = E/t –> sub into P = IV formula)
P = I²R
(substituting V = IR into P = IV)
P = V²/R
(substituting I = V/R into P = IV)
Kilowatts
1 kW = 1000 W
E (J) = P (W) * T (s)
E (kWh) = P (kW) * T (h)
(used when units are too high)
How to draw circuits
- Use straight lines to represent wires
- Place voltmeters in parallel with the components
- Place ammeters in series with components
- Use conventional electrical symbols to represent the components, not real-life images.
Switch
Function
- Opens/ closes circuit
Application
- Turns devices on/ off
Cell
Function
- Provides direct current (d.c.)
Application
- Store of energy
Battery
Function
- Series of cells
Application
- Larger store of energy than one cell
Lamp
Function
- Transfers energy into light
Application
- Lighting
Fuse
Function
- Breaks circuit if too much current
Application
- Safety device
Voltmeter
Function
- Measures potential difference
Ammeter
Function
- Measures the current
Fixed resistor
Function
- Resists flow of electrons
Application
- Limits the current
Variable resistor
Function
- Changes resistance and thereby changes current
Application
- Dimmer switches
Thermistor
Function
- Resistance decreases as temperature increases
Application
- Temperature switches or thermostats
Heater (electrical circuits)
Function
- Transfers electrical energy into heat energy
Application
- Heating a room/space
Light dependent resistor (LDR)
Function
- As light intensity increases, resistance decreases
Application
- Light sensors
Relay coil (electrical circuits)
Function
- Switch operated by a magnet
Application
- Control high-power circuits with low-power inputs
Transformer (electrical circuits)
Function
- Increases or decreases voltage
Application
- Efficient electricity supply
Variable potential divider (potentiometer)
Function
- Varies resistance at a point with a sliding contact
Application
- Sensory circuits
Magnetising coil (electrical circuit)
Function
- Produces a magnetic field when current flows
Application
- Relays, electromagnets, solenoids
A.C. Power supply (electrical circuit)
Function
- Provides an alternating current
Application
- A store of energy
Motor (electrical circuit)
Function
- Uses current to make some spinning motion
Application
- Store of kinetic energy
Generator (electrical circuit)
Function
- Uses kinetic energy to produce a current
Application
- Generating electricity
Diode (electrical circuit)
Function
- Allows current to flow in only one direction
Application
- Safety devices
Light-emitting diode (LED) (electrical circuits)
Function
- Emits light of a specific colour when current flows.
Application
- Low energy lighting
Series circuit - What it is.
One loop of wire –> no brances
Series circuit - Voltage
Voltage of each component adds up to voltage of battery
Vtotal = V1 + V2 + V3
eg.
Battery = 6V
Light bulb 1 = 2V
Ligh bulb 2 = 4V
(difference between 1 and 2 is due to resistance of components)
Series circuit - Current
Current is constant across circuit.
Itotal = I1 = I2 = I3 etc.
Series circuit - Resistance
Resistance of componenets adds up to toal resistance
Rtotal = R1 + R2 etc.
Cells in series
EMF added up to form total voltage for battery (multiple cells)
Parallel circuit - Basic
Circuit with loops. Branches divide into two wires forming two loops.
Parallel circuit - Voltage
Each branch of the parallel circuit recieves voltge equal to the power supply.
Vtotal = V1 = V2 = V3
(if V1, V2 and V3 are located on different loops with only one component on each loop)
Parallel circuit - Current
Current splits at each branch in the circuit based on the resistance of the branch.
Itotal = I1 + I2
(total current flowing into junction = total current flowing out of junction –> bc current ‘joins up’ again)
Parallel circuit - Resistance
1/Rtotal = 1/R1 + 1/R2 + 1/R3
Faraday’s law
An e.m.f. will be induced in a conductor in the presence of a changing magnetic field.
Electromagnetic induction
- When a coil of wire is placed around a magnetic material, the current flowing through the magnetic material, aligns the domains temporarily, making it magnetic.
- A wire close to a changing magnetic field will experience an induced EMF force. (magnetic field creates a force on the electrons creating induced EMF + current)
How can an emf be induced with a stationary magnet)
- Moving a magnet so its field lines are cut by wire.
- Moving a wire across a magnetic field.
Flemings right hand rule
- Current, Motion of wire, Direction of mag. field.
Thumb + pointer + middle finger perpendicular to each other.
thuMb represents Motion of wire (relative to field <– ask Mr. Gardiner what that means)
First finger represents direction of magnetic Field
seCond finger represents direction of Current (conventional)
Bc current used in conventional, to work out direction of electron flow –> reverse current.
Dots (magnetic field)
Coming out of page
Crosses (magnetic field)
Going into page
Magnetic effects of a current
- A magnetic field is created when an electric current (charge) flows through a wire.
- DC –> constant magnetic field
- AC –> alternating magnetic field
- Magnetic field around a current carrying wire is circular + perpendicular to wire (get’s weaker the further away from wire)
Right hand grip rule
Thumb pointing upwards - Direction of current
Fingers curled inwards (curling around wire) - Direction of magnetic field
Current used is conventional. To figure out direction of electron flow, reverse current.
AC generator (set-up)
- Two stationary magnetic poles
- Soft iron core with copper wire coil (armature/coil) found in between magnetic poles)
- Slip ring attatched to coil that rotates with it.
- Carbon brushes in contact with slip ring at all times
AC generator (how it works)
- Kinetic energy causes the armature to spin.
- An EMF is induced when a wire moves through a magnetic field.
- Therefore as the armature spins, an EMF is induced due to the movement through the magnetic field.
- The EMF pushes the electrons, causing them to move. This creates a current flow through the armature.
- The current flows through the armature into the slip rings and then through the carbon brushes when in contact.
- The changing direction of the armature results in a positive and negative EMF, which results in an alternating current.
AC generator (position of armature, related to EMF size)
During a full rotation of the armature, a large range of EMF’s are produced.
At 0 turns (Sides of armature are moving parallel to mag. field), the EMF is 0
At 1/4 of a turn (Sides of the armature are moving perpendicular to the mag. field) = Max EMF produced
At 1/2 of a turn (Sides of armature are moving parallel to mag. field) = EMF is 0
At 3/4 of a turn (Long sides of armature are moving perpendicular to mag. field but in opposite direction. –> Max EMF produced but in opposite direction (caused change in current direction)
At 1 turn (Sides of armature are parallel to mag. field) –> EMF is 0.
AC generator (Magnets)
provide a constant magnetic field across the coil
AC generator (Coil/Armature)
- Usually made from many turns of wire (Iron core with a copper wire wrapped around it)
- It is rectangular so that its sides are perpendicular to the magnetic field.
AC generator (Slip rings)
- Cylindrical conductors that make constant contact with the coil during rotation
- They allow the direction of the induced electromotive force (e.m.f.) to alternate and therefore cause an alternating current (a.c.)
AC generator (Carbon Brushes)
Make an electrical connection between the rotating coil and a circuit, avoiding the wires becoming twisted
Electrical hazards - List
- Damaged insulation
- Overheating of cables and appliances
- Overloading sockets
- Damp conditions
Electrical hazards - Damaged insulation
- Electrical wires + cables are covered with an insulator (eg. PVC plastic).
- If the insulator is damaged and you touch live wire you cna be electrocuted
Electrical hazards - Overheating of cables + appliances
Large current running through appliances = heat.
Too much heat can cause a fire.
Electrical hazards - Overloading sockets
- Too many appliances / too many extension adapters draws too much current through one socket.
- Can overload the socket which can result in a fire.
Electrical hazards - Damp conditions
- Water conducts elecrticity
- By touching an lecetrical appliance when wet or with damp gans you can be electrocuted.
(Electrial devices in damp conditions need to be specifically designed for such)
Protecting yourself from electrocution
- Always use appliances such as pliers/wire cutter with PLASTIC HANDLES (used for relatvely small voltags)
- Never move any object close to a large potential difference.
Mains supply
- Circuit distributing electricity around your house.
- Outside of the cable is made of an insulating plastic + contains three smaller wires with insulating plastic around them too.
- Live wire
- Earth wire
- Neutral wire
(The wires have different colours due to Europe guidelines + regualtions)
Live wire
Brown
- Carries current from the main supply.
- On/Off switch connected to this wire bc it’s the source of current.
Neutral wire
Blue
- Completes the full circuit
- Doesn’t supply current
Earth wire
Green + Yellow striped
- Safety feature used to prevent electrocution
Earthing metal cases
- Earth wire is connected to the outer metal casing of an appliance + prevent lethal shock if a fault makes teh casing live.
- Earth wire provides a path for current to flow to earth (ground)
- This happens bc the wire has a lower resistance than the person.
Fuse
- Protects a circuit
- Has a thin wire inside it connected to the live wire.
- If too much current flows through teh live wire, the wire melts (the ‘fuse blows’) and breaks the circuit (turning off the electrical device)
- Fuses are designed to blow at different current ratings.
(The rating of the fuse must be the lowest value that is still greater than the amount of current the appliance is designed to use)
Trip switches/circuit breaker
- Safety device similar to dues
- If the amount of current flowing between the live and neutral wires increases rapidly –> trip switch detects this + opens a switch to break the circuit.
- Rating/ value of trip switch has to be slightly above the amount of current the appliance is designed to use.
- Trip switch breaks circuit quickly (unlike fuse)
- Can also be reused by switching it back on.
Solenoid
Wire arranged as a coil –> causes magnetic field bc of the magnetic fields created by current flowing through a wire.
The magnetic fields ‘combine’ and create a magnetic field identical to a bar magnet
(The field is strongest inside the solenoid (coil) bc the field lines are densest there)
To increase the strength of the magnetic field in a solenoid…
- Increase the current through the solenoid.
- Increase the no. of turns of wire in the solenoid
- Placing a soft iron core in the middle (concept of electromagnets)
AC in a solenoid
Supplying a coil with AC –> results in changing the direction of the current –> thus diretion of the magnetic field every half cycle.
Increasing size of force on the wire due to it moving through a magnetic field.
- Increase current in the wire.
- Increase no. of individual wires.
- Increase strength of magnetic fields.
- Increase length of the wire within the magnetic field.
(Changing the direction of the magnetic field or current changes the direction of the force)
Flemings left-hand rule
- Direction of Force, Current, Field
First finger = Field (N to S)
seCond finger = Current (conventional)
thuMb = Movement (Force)
If calculating force on negatively charged particle, reverse current direction.
DC motor - Set up
North + south poles of a magnet with gap between.
Armature/ coil of wire in the middle.
Coil connected to split ring.
Split ring connected to carbon brushes.
Carbon brushes connected to power supply.
DC motor- How it works
When a wire moves through a magnetic field a force is induced –> causing the wire to move.
There are two magnetic poles with a coil between them.
- The direction of the mag. field stays constant.
- If the side of the coil closest to the north pole, is constantly supplied with current in one direciton, it will constantly experience a force in one direction.
Due to the split ring this can happen and this causes the armature to spin, thus converting electrical energy into kinetic energy.
Split ring/ Commutator
As the coil rotates, the direction of current needs to stay the same so that the force also acts in the same direction. (Fleming’s left hand rule)
- The commutator spins with the wires.
- Brushes make contact with part of the split ring, when it spins the brushes make contact with other part of split ring.
Once the coil has rotated through 180°, current continues to flow in the original clockwise direction causing the force on the left side to be up, and the force on the right side to be down
- This means the north side of the DC motor always gets teh current in one direction, meaning that side will always have a downwards force.
Increasing turning force of a motor
- Increase the current
- Increase teh strength of the magnetic field
- Increase the no. of turns in the coil.
Transformer
A device that can increase size of an alternating electromotive force (e.m.f)
Two types of transformers
- Step-up transformer
- Step-down transformer
Step-up transformer
Increases voltage
More turns on the secondary coil than on the primary coil
Step-down transformer
Decreases voltage
More turns on the primary coil than on the secondary coil.
Transformer - Consists of…
- Primary coil through which A.C. is supplied (energy source of transformer)
- Soft iron core –> allows induction of alternating magnetical field which can induce an AC current in secondary coil.
- Secondary coil is the output of the transformer. No of coils decides the type of transformer.
No. of coils in a transformer affecting voltage transfer - Formula
Vp/Vs = Np/Ns
Vp - Voltage in primary coil
Vs - Voltage in secondary coil
Np - No. of turns in the primary coil
Ns - No. of turns in the secondary coil.
Voltage in coils on transformer affecting current - Formula
VpIp = VsIs
Vp = Voltage in primary coil
Ip = Current in primary coil
Vs = Voltage in secondary coil
Is = Current in secondary coil
Transformers –> how they work
- The primary coil is supplied with an alternating current and behaves like a bar magnet that is constantly switching its poles due to the constantly changing current (a bit like a rotating bar magnet).
- A constantly changing magnetic field is transferred to the secondary coil via the soft iron core.
- Because we now have a conductor (secondary coil) in the presence of a changing magnetic field, an electromotive force (e.m.f.) is induced across the secondary coil.
- The e.m.f. induced across the secondary coil is also constantly changing and therefore provides an alternating current.
Efficiency of transformers
Efficiency of transformers = Pout/Pin
= (IsVs/IpVp) * 100%
Increasing efficiency of transformers
- Using low resistance coils to reduce the power wasted due to the heating effect of the current.
- Using a laminated core (Layers of iron separated by layers of insulation) –> reduces heating in the iron core + prevents currents from being induced in the core itself (referred to as eddy currents).
Why is it more efficient to transfer electricity in wires at a high voltage.
- Power has to be conserved
- P = IV
Therefore if voltage increases, current decreases to compensate. Therefore at a high voltage, current is low.
Decreasing current, decreases heat loss.
