4.2 energy,power & resistance Flashcards
what does a switch do?
turns an electric current on and off
what does a cell do?
provides the circuit with a source of energy or emf
what does a lamp do?
transforms electrical energy to light energy as a useful form
what does a variable resistor do?
controls the amount of electric current flowing in a circuit or through a component but different to a normal resistor because the resistance can be controlled
what does a fuse do?
acts as a safety device - will melt or ‘blow’ if the current gets too high
what does a battery do? how is it different to a cell?
provides the circuit with a source of energy or emf, a battery is two or more cells
what is an ammeter?
measures the amount of electrical current flowing in a circuit or through an electrical component, measured in amps
what is a thermistor?
a electric component that responds to the temperature of the environment and changes its resistance as a result
what is a voltmeter?
measures the size of the potential difference in a circuit or through an electrical component, measured in amps
what is a diode?
a component that allows current to flow only in one direction, often used to protect delicate components from having large currents through them
what does LDR stand for?
light dependent resistor
what does LED stand for?
light-emitting diode
what does a resistor do?
controls the amount of electric current flowing in a circuit or through a component
what does a LED do?
like a diode, allows electrical current to flow through it in only one direction, when current flows through it, it emits light (lamp)
what is the unit for potential difference?
volts, V
define potential difference or voltage (p.d)
the p.d measured across a component is the energy lost per unit charge by the charges passing through the component (transfers electrical energy to other forms of energy), is it measured in volts
define electromotive force (e.m.f)
the e.m.f of a supply is the energy gained per unit charge by charges passing through the supply, when a form of energy is transferred to electrical energy carried by the charges, measured in volts
what is an example of a source of emf?
a cell or battery
what is the formula for emf?
emf or voltage = energy transferred to charge / charge
V = E / Q
what is the distinction between emf and p.d in terms of energy transfer?
p.d = transfers electrical energy into other sources
emf = transfers energy from other sources into electrical energy
how must you connect a voltmeter in a circuit?
in parallel across the component
what is Ohm’s law?
ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided that the physical conditions, such as temperature, remain constant
how must you connect an ammeter in a circuit?
in series
what happens to the electrons in a conductor when a potential difference is applied across the conductor?
the electrons are accelerated and gain kinetic energy
what is the equation for the energy transferred to each charged particle? (kinetic energy)
eV = 0.5mv^2
where e = charge
V = potential difference
m = mass of the electron
v = velocity of the electron
what is resistance measured in?
ohms, Ω
what is the equation for resistance?
resistance = voltage / current
what does it mean if a component as a resistance of 1Ω?
a component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it
what factors affect resistance? (think of the equation)
length - the longer the wire the more difficult it is to make a current flow
cross-sectional area - the wider the easier it will be for the electrons to pass along it
resistivity - this depends on the material
what is resistivity affected by?
charge is carried through metals by free electrons in a lattice of positive ions
-heating up a metal makes it harder for electrons to move about, the ions vibrate more when heated, so the electrons collide with them more often losing energy to other forms
RESISTIVITY OF A METAL INCREASES AS THE TEMP INCREASES
what does an I-V graph look like for an ohmic conductor (at a fixed temp)?
straight line with constant gradient through the origin
(obeys Ohm’s Law, current and voltage are directly proportional), RESISTANCE IS CONSTANT
what does an I-V graph look like for LED or diode?
the line is flat until threshold voltage (usually around 0.6V) and then shoots up straight with constant gradient
diodes and LEDs only let current flow in one direction (from positive to negative, this is called the FORWARD BIAS direction)
REVERSE BIAS is the reverse direction where the resistance is almost infinite meaning no current, regardless of voltage
what does an I-V graph look like for filament lamp?
-line starts straight, through the origin then starts to curve and flatten as gradient gets close to 0 (S shape)
-at low temp, filament lamp behaves in accordance to Ohm’s
-as the current increases, its temperature increases due to self-heating affect which in turn increases its resistance
(resistance of a filament lamp increases as its temp increases)
what does an resistance-temp graph look like for NTC thermistor? and why does it look like this?
exponential curve downwards with negative gradient
an NTC thermistor changes its resistance as temperature changes
at LOW temperatures, resistance is HIGH, as temperature increases resistance decreases
this is because an NTC is a semiconductor and warming it up gives more electrons enough energy to be released, this means that there are more charge carriers available so resistivity and resistance is lower
what does an NTC thermistor stand for?
negative temperature coefficient resistor
what does an I-V graph look for a LDR (light dependent resistor)?
-exponential curve downwards with negative gradient
-at low light intensity (DARK), resistance is high
- at high light intensity (BRIGHT), resistance is low
(useful for switching components on/off under certain conditions of brightness)
-this is because an LDR is a semiconductor and warming it up gives more electrons enough energy to be released, this means that there are more charge carriers available so resistivity and resistance is lower
what is the equation that links resistivity of a material and its temperature?
ρT = ρo ( 1 + α ( T - To )
where ρT = the resistivity of a material at a temperature T
ρo = the resistivity value at To (usually room temp.)
α = the temp. coefficient
T = the temperature of the material (in degrees or Celsius)
To = the reference temperature at which the resistivity of the material is quoted
what is the equation for resistance?
R = ρL / A
where R = resistance of the component
ρ = the resistivity of the component
L = length of the component
A = cross sectional area
what are the equations for power?
P = IV
P = WD / t
by substituting V = IR
P = I^2 R
P = V^2 / R
give some examples of semiconductors?
thermistors, LDRs, and diodes
what is resistivity? and what are the units?
an intrinsic property that isn’t affected by physical dimensions of the material (only affected by temp.) its units are ohm metres Ωm
what is the unit for power?
watts, W
what is power defined as?
the rate of doing work
what is 1kWh equal to in joules?
1000joules x 3600 seconds (in 1 hour)
3.6 million joules
what is the common unit for energy used in households when charging for electricity?
kilowatt hours
why are kilowatt hours used?
more sensible, appropriate unit as large amounts of energy are used in households
