Physics - Electricity Flashcards
what is an electrostatic conductor
material that is able to disperse electrical current (electrons) easily
what is an electrostatic insulator
do not disperse electrical charge (electrons) easily
what happens when two electrostatic insulators are rubbed together
there is a flow of charge from one insulator to another
insulator that loses electrons becomes positively charged
insulator that gains electrons becomes negatively charged
induction
place a neutral object next to charged object causes it to become charged
if one end of this newly charged object is momentarily earthed, the object becomes permanently charged
what happens is a newly charged object is momentarily earthed
becomes permanently charged
example of static electricity in industry
spray paint
paint droplets have opposite charges to the object being painted meaning they are attracted to the object and less paint is wasted
why can static electricity be dangerous
sparks can form upon spontaneous dissipation of electrical charge which can lead to a fire
what can reduce the risk of a spark
earthing an object
what are the two ways current can be supplied
direct
alternating
direct current
current always supplied in same direction
examples of uses of direct current
batteries or cells supply
alternating current
current that repeatedly changes direction
examples of uses of alternating current
power stations output and the mains
what is a waveform
current changes at certain regular frequency which produces a waveform
what is the frequency of mains current in UK and Europe
50Hz
50 oscillations or 100 changes in direction per second
what is current
flow of charged particles through an electrical conductor
what symbol represents current
I
what is current measured in
amperes (A)
how to calculate current
current = charge / time
current = amperes (A)
charge = coulomb (C)
time = second (s)
how to measure current
ammeter placed in series with components of series
what is potential difference (voltage)
work done to move charge between two points
what symbol represents voltage
V
what is voltage measured in
volts (V)
how to calculate voltage
voltage = work done / charge
voltage = volts (V)
work done = joules (J)
charge = coulomb (C)
how to measure voltage
voltmeter which is always placed in parallel with the component whose potential difference you are trying to measure
what is resistance
opposition to the flow of current (electrons) through a conductor
all conductors carry a certain resistance which is directly proportional to the length of the conductor and inversely proportional to the cross sectional area
what symbol represents resistance
R
what units are used for resistance
ohms (Ω)
how to calculate resistance
resistance = voltage / current
resistance = ohms (Ω)
voltage = volts (V)
current = amperes (A)
what is 1 ampere
1 coulomb of charge flows through a conductor per second
what is 1 volt
work done moving 1 coulomb of charge between two points
what is 1 ohm
resistance of a conductor when a potential difference of 1 volt produces a current of 1 ampere
voltage-current graphs are linked by which equation
voltage = current x resistance
how is a voltage-current graph plotted
current on y axis
voltage on x axis
what is an ohmic conductor
a fixed resistor maintains constant resistance with different currents flowing through it
ohmic conductors
what is current and voltage relationship
directly proportional
how to calculate resistance on voltage-current graph
1/gradient
what are non-ohmic conductors
resistance alters with varying current flowing through them and varying potential differences across them
non-ohmic conductors
what is current and voltage relationship
sigmoidal curve
example of non-ohmic conductor
filament lamp
why is filament lamp non-ohmic
as more current flows through filament lamp, it begins to heat up due to increased electron collisions
heat generated leads to an increased resistance through filament lamp as particles vibrate with greater amplitude, making it harder for the free electrons to flow through the conductor
this leads to a decrease in current
thermistors
thermistors are components whose resistance is dependent on temperature
example of thermistor
negative coefficient (NTC) thermistor’s resistance decreases as its temperature increases
light dependent resistors
components whose resistance is dependent on light intensity incident on it
resistance of LDR decreases as light intensity increases
ideal diodes
components that only allow current to flow in one direction which is shown by the arrowhead in the circuit symbol for the diode
what can diodes require before current flows through them sometimes
forward junction potential
what is a diode’s breakdown voltage
in the reverse direction
causes negative current to flow through them
series circuit
components placed on one long continuous path and current flows through every component
current in a series circuit
it is the same no matter where in the circuit it is measured from
potential difference in a series circuit
equal to the sum of potential difference across each of the components that the series circuit contains
this is because the energy carried by each coulomb of charge must be shared as it travels through each component
VT = V1 + V2 + V3
resistance in series circuit
sum of all components in a circuit
RT = R1 + R2 + R3
how to calculate the voltage in a cell or battery in series
VT = RT x IT
what is a parallel circuit
components are arranged in several ‘parallel’ branches
(can be in a series of series and parallel)
potential difference in parallel circuit
same across each loop in a parallel circuit
due to the fact that the electrons entering each loop have the same amount of energy
the total voltage supplied by the cell is equal to the voltage across each of the loops in the parallel circuit
VT = V1 = V2 = V3
current in parallel circuit
shared between branches in ratio of resistance od components in each branch
therefore, at a branch point the current will be equal to the sum of the currents going into each branch
IT = I1 + I2 + I3
resistance in parallel circuit
reciprocal law
1/RT = 1/R1 + 1/R2 + 1/R3
dealing with circuits that consist of a combination of series and parallel resistors, use the reciprocal rule to calculate the total resistance of the parallel portion before adding the series resistances
electrical power equation
power = energy / time
power in watts (W)
energy in joules (J)
time in seconds (s)
transfer of electrical power equation
(using V and I)
power = voltage x current
power in watts (W)
voltage in volts (V)
current in amperes (A)
transfer of electrical power equation
(using I and R)
power = current^2 x resistance
power in watts (W)
current in amperes (A)
resistance in ohms (Ω)
transfer of electrical power equation
(using V and R)
power = voltage^2 / resistance
power in watts (W)
voltage in volts (V)
resistance in ohms (Ω)
energy equation
(using V I and T)
energy = voltage x current x time
energy in joules (J)
voltage in volts (V)
current in amperes (A)
time in seconds (s)
what do transformers do
devices that have the ability to alter voltage and current
what do step up transformers do
increase voltage and decrease current
useful when transferring electricity from power stations to national grid in order to reduce energy losses
what do step down transformers do
decrease voltage and increase current
more safe use in our homes and reduces risk of electric shocks
what does a transformer consist of and why
primary coil wrapped around coil
when a current is allowed to flow through the primary coil, it creates a magnetic current which induces a current in a secondary coil
current and voltage can then be altered by varying the ratio of turns in the primary and secondary coils
(v in secondary coil / v in primary coil)
(turns on secondary coil / turns on primary coil)
power calculation using primary voltage and primary current
power = primary voltage x primary current
power calculation using secondary voltage and secondary current
power = secondary voltage x secondary current
generator effect
generation of a current by the movement of an electrical conductor relative to a magnetic field
also : keeping stationary conductor in a fluctuating magnetic field
what does a generator consist of and why
wire rotating in a magnetic field
- as it rotates each side of the coil moves through the magnetic field in two different directions withe each 360 degree rotation
- wire simultaneously moving in opposite direction through the magnetic field, produces alternating current
