electricity Flashcards
what is current?
the rate of flow of charge
definition of a coulomb
the amount of charge that passes in 1 second when the current is 1A
what measures current through a circuit?
ammeter
where would you attach an ammeter in a circuit?
always in series
what is potential difference between two points?
the work done in moving a unit charge between two points
what measures pd in a circuit?
voltmetre
where do you attach a voltmeter to measure pd?
across the component in parallel
how do you draw a fuse on a circuit diagram?
rectangle with line going straight through it in horizontal direction
how do you draw a thermistor on a circuit diagram?
draw a resistor then horizontal line directly under start of rectangle, turned 45 degrees into a horizontal line through rectangle like variable resistor
how do you draw a diode on a circuit diagram?
horizontal line, draw a circle with line going through centre, draw a triangle with a line at the narrowest part inside circle
how to draw an LED on a circuit diagram
diode but with two arrows pointing north east from circle
how to draw a LDR (light dependant resistor) on a circuit diagram?
resistor with a circle around the rectangle and two arrows pointing south east into the circle
what is power?
the rate of transfer of energy
measured in Watts
what are the four equations for power?
power=energy (J) / time
power= current x pd
power= pd squared/ resistance
power= current squared x resistance
what are the three equations for total electrical energy transferred?
E= I x t x V
E= (pd squared/ resistance) x t
E= current squared x resistance x t
what is internal resistance
in a battery, chemical energy is used to make electrons move
as they move they collide with atoms inside the battery so they will have some resistance called internal resistance
what is load/ external resistance?
the total resistance of all the components in a circuit
what is emf?
electromotive force
the amount of electrical energy the battery produces and transfers to each coulomb of charge
measured in volts
what is terminal potential difference?
the energy transferred when one coulomb of charge flows through a resistor
with no internal resistance its the same as emf
what are lost volts?
the energy wasted per coulomb overcoming internal resistance
how can you measure internal resistance and emf of a cell or battery? RP6
set up circuit- battery in series with ammeter and voltmeter. variable resistor in parallel with switch.
- set the variable resistor to the highest resistance
- close the switch and record the current and pd
- open the switch and close it again 2 more times to calculate mean current and pd
- decrease the resistance of the variable resistor and then repeat previous steps with this resistance
- keep decreasing the resistance till you have 10 readings
- plot a V-I graph for mean data and draw line of best fit
is charge conserved in a circuit?
yes
what’s Kirchhoff’s first law?
current entering a junction = current exiting the junction
what’s Kirchhoff’s second law?
the total emf around a series circuit = the sum of pd across each component
how do you work out total resistance in series circuits?
add them all together
how do you work out total resistance in parallel circuits?
1/total R =1/r1 + 1/r2…
what is a potential divider?
a circuit with a voltage source (battery) and a couple of resistors in parallel
when does a light dependant resistor have a very high resistance?
in the dark
when does a thermistor have a high resistance?
low temperatures
what is a potentiometer
it has a variable resistor which replaces R1 and R2 of the potential divider
you move the knob to adjust the sizes of the resistors
useful for constant change in voltage eg radio
what is ohm’s law?
it states that providing the physical conditions, such as temperature and light, remain constant, the current through an ohmic conductor is directly proportional to the potential difference between it
what is resistance?
a measure of how difficult it is to get current to flow through
what are I-V characteristics?
a graph showing how current changing through a component changes with the pd of a component
how can you tell what has the highest resistance from a I-V graph?
component with the smallest gradient
how can you find the I-V characteristics of a component?
set up a circuit- power pack, variable resistor and ammeter in series then voltmeter in parallel with component
by changing the resistance, you can change the pd and current
then plot a I-V graph
ideally, how much resistance should a voltmeter and ammeter have?
voltmeter- infinite (no current through)
ammeter- no resistance (no pd across)
what are often ohmic conductors?
metallic conductors
what does a characteristic I-V graph look like for a filament lamp?
curve that starts steep and gets shallower as voltage increases
why is a filament lamp not a ohmic conductor?
the current flowing through the lamp increases the temperature which increases the resistance
how does temperature increase in a filament lamp?
contains a thin coil of metal wire inside. as a current flows through this, some of the electrical energy is transferred to heat energy and causes the metal to heat up
how does increased resistance of a filament lamp lead to increased temperature?
the extra heat energy causes the particles in the metal to vibrate more. these vibrations make it more difficult for the charge-carrying electrons to get through the resistor- current can’t flow easily so resistance increases
how does amount of current effect temperature and resistance?
increased current, increases temperature which increases resistance so current will decrease again
what are diodes?
made from semiconductors and designed to let current flow in one direction only
what is forward bias?
the direction which current is allowed to flow in a diode
how would you draw a diode if you want current to go left to right?
triangle first then line on point of triangle crossing wire and circle around all of it
what is the threshold voltage of a diode?
0.6V
the voltage they need in the forward direction in order to conduct
what is resistivity?
a measure of how much a particular material resists current flow
its a property of the material
what’s resistivity dependant on?
the structure of the material as well as on environmentally factors such as temperature and light intensity
also area and length of wire and resistance
how does the resistivity of a material show its ability to conduct electricity?
the lower the resistivity, the better it is at conducting electricity
what are semi-conductors?
a group of materials that aren’t as good as conducting electricity as metals because they have far fewer charge carriers available
if energy is supplied to a semi-conductor what happens to the resistivity and why?
resistivity of the material decreases because by increasing temperature, electrons can be released
what is an NTC thermistor?
‘negative temperature coefficient’
as temperature increases, resistance decreases
how does increasing the temperature of the thermistor lead to decreased resistance?
it gives more electrons enough energy to escape from their atoms so more charge carriers are available and resistance decreases
how can you investigate how a thermistor’s resistance changes with temperature?
power supply- ammeter- thermistor in series
if thermistor is waterproof- use water bath
1. place the thermistor in a beaker
2. pour enough boiling water over the thermistor to cover it
3. measure and record the temperature of the water using a thermometer
4. measure the current through the circuit
4. keep recording these for every drop in 5 degree c
5. pd needs to be kept constant, use these to work out resistance
what is transitional temperature?
the critical temperature at when you cool materials below this, their resistivity completely disappears
what is a superconductor?
materials that have been cooled below their transitional temperature so have no resistivity
why can we not realistically make all materials superconductors?
most normal conductors (metals) have really low critical temperatures below 10K so its too difficult and expensive getting them that cold
what are some uses of superconductors?
superconducting wires can be used to make
-power cables that transmit electricity without any power loss
-really strong electromagnets (used in medicine/maglev trains)
-electrical circuits that work really fast with no energy loss
finding the resistivity of a material experiment RP5
- set up a circuit, power supply connected to an ammeter, the test wire is clamped to a ruler, the other side of the power supply is attached to a switch and a flying lead. voltmeter in parallel
- calculate the cross sectional area of the test wire using a micrometer
- attach the flying lead to the end of the test wire and measure the length of the test wire
- close the switch and measure current and pd
- open the switch again and calculate resistance for current and pd
- repeat this and take a mean
- then repeat this at different lengths of wire
- plot a graph resistance-length and draw line of best fit
- find resistivity by multiplying gradient by cross-section area of wire
