Unit 4 Flashcards
what do opposite poles do
attract
what to do similar poles do
repel
what direction do magnetic field lines always point
north to south
in attraction, how do the field lines point
they point north to south and flow between the 2 points
in repulsion, what is the nature of the field lines
they point in opposite directions and bend away from each other
how is the strength of the magnetic field seen
the density of the field lines
what happens when permanent magnets attract a magnetic materia
a magnetic field is induced in the material
what does hard magnetic materials mean
they are hard to magnetise and also hard to demagnetise
what does soft magnetic materials mean
they are easy to magnetise and demagnetise
what is an electromagnet
when a coil of wire carries current and a magnetic field is induced
what are properties of permanent magnets
constant magnetic field
cannot be switched on and off
north and south poles cannot be switched
made with hard magnetic materials
what are properties of electromagnets
variable strength magnet field
can be switched on and off quickly
north and south poles can be changed by changed the direction of current flow
made from soft magnetic materials
what are the charge carriers in electrical devices
electrons
why are conductors able to conduct
there electrons are free to move around
why are insulators unable to conduct
there are no free electrons to move
what is a charge
an electric charge that can be positive or negative
what do unlike charges and like charges do
unlike charges attract
like charges repel
what happens if there is an equal number of positive and negative charges
it is neutral
what happens when an object loses electrons
it becomes positively charged
what happens when an object gains electrons
it becomes negatively charged
what is the symbol for electrical charge and how is it measured
symbol is Q
measured in coulombs(C)
what is static electricity
occurs when friction between 2 insulators causes electrons to be transferred from one surface to another
what are electrical fields
when 2 charged particles approach each other, they experience a force
the space in which the electric charge experiences a force is called the electric field
what is current
the measure of the flow of charge
also known as rate of flow of charge
how can current be increased
current flow can be increased by:
making each charged particle move faster
increase the number of charged particles
increased the amount of charge each particle carries
formula for current
I = Q/T
current = charge/time
what is used to measure current
an ammeter
what is conventional current
electrons are repelled from the negative terminal of the battery and attracted to the positive terminal
conventional current is the opposite(positive to negative)
what are alternating currents(AC)
when the electrons continously change direction
what are direct currents(DC)
electrons flow in one direction
what is electromotive force
the phenomena that enables charge to flow around the whole circuit
determined by the battery voltage
what is electromotive force measured in
volts
what is potential difference
the energy needed per charge to flow between two points in a circuit
work done per unit charge
what is potential difference measured with
measured with a voltmeter and always placed parallel across a component
formula for potential difference and electromotive force
V = W/Q
v = voltage
w = energy
q = charge
what is resistance
the measure of how much opposition there is to the flow of current
measured in ohms
what is the formula for resistance
R = V/I
r = resistance
v = potential difference
i = current
how is the length of the wire related to resistance
resistance is directly proportional to the length of the wire
how is the thickness of the wire related to resistance
it is inversely proportionate
thicker the wire, smaller the resistance
what is ohms law
electric current is directly proportionate to voltage and inversely proportionate to resistance
how can electrical power be calculated
P = IV
p = power
i = current
v = potential difference
what is a diode
only allows current to flow in one direction
requires a small amount of power to activate
non ohmic device
what is a light emitting diode(LED)
regular diode that lights up when current flows through
what is a series circuit
the current flows in one loop and voltage is shared throughout. if there are multiple lamps, they will all be dimmer
what is a parallel circuit
there is more than one loop of current
voltage is the same for all loops
advantages of parallel circuits
the potential difference across each lamp will be equal to the e.m.f. means every lamp will be brighter
each loop can be turned off separately
featurs of series circuits
the current will be the same at all points
the potential difference across all resistors is shared
sum of the pd = battery power
how to find total resistance in series circuits
by adding resistance of all the components up
features of parallel circuits
the current flowing into a junction is equal to the sum of currents flowing out of a junction
each resistor experiences the same p.d as the power supply
how to find total resistance in parallel circuits
1/total resistance = 1/resistance of component 1 + 1/resistance of component 2
what is Faradays law
a wire close to a changing magnetic field will experience an induced electromotive force
how can EMF be induced
moving a magnet so that its field lines are cut by the wire
moving a wire across a magnetic field
what is flemings right hand rule
thumb is direction of motion of the wire
first finger is direction of magnetic field
second finger is direction of the current
what is Lenz’s law
the direction of the induced emf opposes the charge that creates it
how can the strength of the induced emf be increased
increasing the force or speed at which the wire or hte magnet is moved
increasing the strength of the magnet
what happens when the wire moves parallel to the magnetic field
an emf is not induced
how can the induced emf be decreased
decreasing the number of turns in the wire
how do AC generators work
a rectangular coil of wire continously rotates within a magnetic field and generates an induced emf
the slip rings allow the direction of the induced emf to alternate and cause an ac currect
what is the purpose of carbon brushes in AC generator
make an electrical connection between the rotating coil and circuit
what will the voltage output look like in an AC generator
the voltage output will have peaks and troughs as the magnet turns from being parallel to perpendicular
what magnetic field does a direct current create
it creates a constant magnetic field
what magnetic field does an alternating current create
an alternating magnetic field
what is the magnetic field around a current carrying wire
circular
whats a solenoid
when wires are arranged in a coil, the resulting magnet is the same is that of a bar magnet
how can the magnetic field of a solenoid be made stronger
increase the current through the solenoid
increase the number of turn in the wire
what happens when the solenoid is supplied with an alternating current
changes the direction of the magnetic field every half cycle
what happens when a current carrying wire is placed within a magnetic field
the wire experiences a force and can move
how can the size of the force be increases when a current carrying wire is placed in a magnetic field
increase the current in the wire
increase the number of individual wires
increase the strength of magnetic field
increase the length of the wire
what is the right hand rule used for
used to find emf induction
what is left hand rule used for
used for DC motors
what is the difference between AC and DC motors
DC motors have a cell that supply it power
AC motors get their current through an induced emf
how can DC motors spin quicker
increase the strength of the magnetic field
increase the number of turns of wire in the coil
increase the current to the coil from the power supply
what are transformers
device that can increase or decrease the size of an alternating electromotive force
what do step up transformers do
increase the voltage
what do step down transformers do
decrease the voltage
what do transformers consist of
a primary coil- alternating current supplied through her
soft iron core-allows transition of magnetic flux to a secondary coil
a secondary coil-the output of the transformer and will have more or less coils
formula for transformers
Vp/Vs = Np/Ns
Vp-voltage applied to primary coil
Np-number of turns in the coil
Vs-voltage applied to secondary coil
Ns- number of turns
how many turns does the secondary coil have in a step up
the secondary coil has more turns than the primary coil
how many turns does the secondary coil have in a step down transformer
has fewer turns than the primary coil
what current do transformers require
they require alternating currents
what is formula for 100% efficiency in a transformer
IpVp = IsVs
how can the efficiency of a transformer increase
using low resistance coils to reduce power wasted
using a laminated core which consists of layers of iron separated by layers of insulation
how can power loss in a wire be calculated
P=1^2R
