4 Electricity and magnetism Flashcards
Magnetism
The force exerted by magnets when they attract or repel each other.
examples
Magnetic materials
Iron
Nickel
Cobalt
Steel
Magnetic poles
Every magnet has a north and a south pole, positioned at opposite ends of the magnet.
Opposites attract
Same repel
Magnetic fields
The field is NORTH to SOUTH
A magnetic field is a region of space where another magnet or magnetic material experiences a force.
Attraction
field lines point in the same direction (N to S) and flow between the two magnets.
Repulsion
field lines point in opposite directions and bend away from each other.
Induced magnet
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.
Magnetism of magnetic materials
A hard magnetic material , such as steel, is hard to magnetise but also hard to demagnetise.
Steel is used to make permanent magnets in devices that require a constant magnetic field.
A soft magnetic material, such as iron, is easy to magnetise but also easy to demagnetise.
Iron is a good material to use 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.
Difference between Electro and permanent magnets
Permanent magnet
Constant magnetic field
Cannot be switched on or off
North and south poles cannot be swapped
Uses:
guitar pickups
speakers
cupboard latches
Electromagnet
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:
electric door locks
relays
MRI machines
Conductors and insulators
Insulators, such as plastic and wood, do not let electrical charge move freely.
Electrical conductors , such as metals, allow electrical charge to move freely.
Charge
Electrical charge can be positive or negative. Unlike charges attract and like charges repel.
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).
Electric fields
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.
How to read an electric field
Field lines further apart= weaker field
Field lines close together= Stronger field
The arrows on the lines. They always show the direction in which a positive charge will move.
Electric current
Is a measure of the amount of charge passing a point per unit of time
I=Q÷t
I = Current(A)
Q = Charge (C)
t = Time (s)
An ammeter is used to measure current.
How to increase current
Making each charged particle move faster
Increasing the number of charged particles
Increasing the amount of charge each particle carries.
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.
a.c. and d.c.
Alternating current (a.c.): electrons continuously change direction.
Direct current (d.c.): electrons flow in one direction only.
Voltage
Voltage produces the push to move a current
Batteries have an electromotive force(e.m.f.) which is the work done on the charge by the battery.
E = W÷Q
E = E.M.F.(V)
W = Work done on the charge(J)
Q = Charge(C)
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.
Potential difference
The work done by a unit of charge on a component in a circuit.
V = W÷Q
V = Potential difference(V)
W = Work done by charge (J)
Q = Charge (C)
Ohm’s law
V = I x R
V = Voltage(V)
I = Current(A)
R = Resistance (Ω, Ohms)
Series and Parallel Circuit: Voltage laws
Series:
The voltage of each component will add to equal the voltage of the power supply
Parallel:
Each branch of a parallel circuit will receive a voltage to the power supply
Series and Parallel Circuit: Current laws
Series:
The current across components in series is equal
Parallel:
Current will split at the parallel branches based on the resistance of each branch
Resistance
Resistance (Ω) is a measure of how much opposition there is to the flow of current in a circuit
R = V÷I
Resistance of a wire
The longer the wire, the greater its resistance.
A thicker wire has a greater cross-sectional area and a smaller resistance.
Resistance is directly proportional to the length of a wire. If the length doubles, the resistance also doubles.
A wire with a wider diameter gives more room for electrons to flow.
Therefore, resistance is inversely proportional to the cross-sectional area of a wire.
If the cross-sectional area is doubled, the resistance is halved.
A thin wire therefore has greater resistance than a thick wire of the same length.
Resistors in series
When components are connected in series, the total resistance equals to the sum of their individual resistance.
R total = R1+R2+R3
Resistors in parallel
For the components in parallel the following formula applies
1/Rtotal = 1/R1+1/R2+1/R3
Electrical power
Gives us the measurement of the rate at which energy is transformed within a circuit
P = IV
P= Power(w)
I = Current(A)
V= Voltage(V)
Can be useful
P=I2xR
P=V2÷R
Formula
Energy (Electricity)
E=IVt
E= Energy(J)
I= Current(A)
V= Voltage(V)
t= Time(S)
Kilowatt-hours (KWh)
KWh= Kilowatts used every hour, this is a measure of energy (as opposed to using Joules)
Energy (kilowatt hours) = power (kilowatts) × time (hours)
E (kWh) =P (kW)×t (h)
How to draw a circuit
Using straight lines to represent wires
Placing voltmeters in parallel with the components
Placing ammeters in series with components
Using conventional electrical symbols to represent the components, not real-life images.
Diodes
A diode is a type of semiconductor , which allows current to flow in only one direction.
When a diode is placed in a circuit, current will only flow in a positive direction, and only when more than 0.7 V is applied across the diode.
The device then behaves like a normal resistor or wire.
Light-emitting diodes
A light-emitting diode (LED) has the same features of a regular diode, with the addition that it produces light.
A light-emitting diode only produces light of a single colour, rather than a lamp which produces a spectrum of light.
If the current flows through the LED (indicated by the fact that the direction of current flow from the battery and the direction that the diode symbol points are the same), it will light up.
When the battery is connected the other way round current cannot flow as the resistance is too high and the diode will not light.
Variable potential dividers (potentiometers)
A variable potential divider (potentiometer) is simply a long resistor that can be split into two parts
Potential dividers
A potential divider is an electrical component that splits up voltage.
Electrical hazards
Damaged insulation
Overheating of cables and appliances
Overloading sockets
Damp conditions
What is a cable made of
The outside of the cable is made of an insulating plastic and each of the three smaller wires is also insulated to prevent it from touching the other two.
The live wire (brown) carries the current from the mains supply. An on/off switch would be connected to this wire because it is the source of current.
Attaching an on/off switch to any other wire would not guarantee to break the circuit in case of a fault.
The neutral wire (blue) completes the full circuit. It does not supply current.
The earth wire (green and yellow) is a safety feature used to prevent electrocution.
Earthing metal cases
An earth wire is connected to the outer metal casing of an appliance and can prevent a lethal shock if a fault makes the case live.
The earth wire provides a path for current to flow to earth (the ground).
Fuse
A fuse protects a circuit. A fuse has a thin wire inside it that is connected to the live wire.
If too much current flows through the live wire, the wire melts (‘the fuse blows’) and breaks the circuit, which turns off the electrical device.
Trip switch / circuit breaker
If the amount of current flowing between the live and neutral wires increases rapidly, a trip switch detects this and opens a switch to break the circuit.
The rating or value of the trip switch must be slightly above the amount of current the appliance is designed to use.
Faradays law
An e.m.f. will be induced in a conductor in the presence of a changing magnetic field.
2 ways to induce an E.M.F.
Moving a magnet so that its field lines are cut by a wire.
Moving a wire across a magnetic field.
Right-hand grip rule
Used to find the direction of the magnetic field around a current-carrying wire
Thumb represents the direction of the current in the wire
Fingers wrap around and represent the direction of the magnetic field
Fleming’s right-hand rule
When an e.m.f. is induced in a wire, use the right-hand rule to work out the direction of force (motion), magnetic field and induced current
The direction of Motion of the wire (relative to the field) 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 law
The direction of the induced e.m.f. will oppose the change that created it.
AC
Current flow created by electrons continuously changing direction.
DC
Current flow created by electrons always flowing in the same one direction.
AC generators
Magnets: provide a constant magnetic field across the coil
Coil (or armature): usually made from many turns of wire.
It is rectangular so that its sides are perpendicular to the magnetic field
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.)
Carbon brushes: make an electrical connection between the rotating coil and a circuit, avoiding the wires becoming twisted
Generating alternating current
Shown as a Sine graph
0 – The top and bottom sides of the coil are moving parallel to the magnetic field, and so no e.m.f. is induced.
1 – The long sides of the rectangular coil move exactly perpendicular (90°) to the magnetic field, and so maximum e.m.f. is induced.
0 – Again, the top and bottom sides of the coil are moving parallel to the magnetic field, and so no e.m.f. is induced.
-1 – Again, the long sides of the coil move perpendicular (90°) to the field but in the opposite direction. Therefore, the induced e.m.f. is again maximum, but in the opposite direction.
Reasons for alternating current in a AC motor
(as coil rotates) it cuts (magnetic) field between the magnets
This induces an e.m.f. / voltage / p.d. (in the coil)
This produces a current in the (coil transferred to the) galvanometer (via the slip rings and carbon brushes)
Direction of current flow changes with each 180 degree rotation of coil
Magnetic effects of a current in a wire
A magnetic field is created when an electric current (charge) flows through a wire.
A direct current (d.c.) creates a constant magnetic field, while an alternating current (a.c.) creates an alternating magnetic field.
Solenoid
Is a coil of current-carrying wire. The effect of the current is a large magnetic field
Electromagnet
An electromagnet is a solenoid witha soft iron core. The iron core becomes an induced magnet due to the solenoid. this creates a stronger magnet overall.
How to make an electromagnet stronger
Increase coils
Magneti material in the middle
Increase current
Important information about electromagnets
A flow of current results in the generation of a magnetic field around a coil.
The magnetic field will attract a magnetic material, for example an iron bar, and close the circuit.
The closed circuit will now perform an action, such as ringing a bell, turning on a different circuit or locking a door.
Relay
An electronic switch using a magnetising coil (solenoid) to close or open part of a circuit.
Loudspeakers
Sound is caused by vibrations; the force required for the push and pull is caused by the interaction of a coil and a magnet.
The speaker cone can be seen to oscillate left and right when an alternating current is supplied to the coil.
The magnetic field due to the alternating current in the coil either attracts or repels a permanent magnet around it, resulting in the vibrations necessary for sound.
To increase the size of the force on a wire in an electromagnetic 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.
d.c. motors
Coil (or armature): rectangular and often made up of lots of turns of current-carrying wire
Magnets: bar magnets are usually used, producing a field perpendicular to the coil, from N to S
Brushes: allow constant electrical contact with the inside of the split ring while the coil rotates – the wires would get twisted otherwise
Split ring commutator: as the coil rotates, the direction of the current needs to stay the same so that the force also acts in the same direction.
How it works:
This is achieved using a split-ring commutator
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.
Without the split ring, the changing direction of current caused by the coil rotating through 180° would cause the coil to flip backwards and forwards.
To make a motor spin more quickly, you can:
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.
Transformers
A transformer is a device that can increase or decrease the size of an alternating electromotive force (e.m.f.).
There are two types:
A step-up transformer increases voltage
A step-up transformer has more turns on the secondary coil than on the primary coil.
A step-down transformer decreases voltage.
A step-down transformer has fewer turns on the secondary coil than on the primary coil.
A transformer consists of
A primary coil through which alternating current (a.c.) is supplied; this is the energy source of a transformer.
A soft iron core, which is designed to allow the transition of magnetic flux to a secondary coil.
A secondary coil, which is the output of the transformer and will have more or less coils than the primary depending on whether it is a step-up or a step-down transformer.
Transformer formula
Vp÷Vs = Np÷Ns
V= voltage (V)
N= number of turns in the coil
Transformer formula when 100% efficient
Vp x Ip = Vs x Is
V= voltage(V)
I= current(A)
How do transformers work
An alternating current in the primary coil induces a magnetic field in the iron core of the transformer
This magnetic field also alternates
The alternating field induces an alteraning current in the secondary coil
The ratio between the number of turns of wire in the primary and secondary coils determines the size of the change in voltage
Calculating energy loss from transmission
P=I^2R
P is power (W)
I is current (A)
R is the resistance of the wire/cable (Ω)
Efficiency in transformers
efficiency of transformers = P out÷P in = IsVs ÷ IpVp ×100%
The efficiency of a transformer can be increased by:
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; this reduces heating in the iron core and prevents currents from being induced in the core itself (referred to as eddy currents).
thermistor
A resistor that increases resistance as temperature decreases.
