Electricity Flashcards
Define electric current and give a symbol and unit
Electric current as the rate of flow of charge, measured in A (I)
Define potential difference and give a symbol and unit
Potential difference as work done per unit charge (V)
How do insulators work
electron attached to atom and cant move away
How do metallic conductors work
some electrons unattched so carry charge to positive terminal
Semiconductor
number of charge carriers increases with temperature therefore resistance decreases as temperature increases
Ohms law states that
I is directly proportional to the pd (V)
EMF =
electrical energy produced per unit charge passing through a point
When plotting current (x) against pd (y) for an ohmic conductor the gradient =
1/R
resistivity(p) =
resistance * cross sectional area / length
Superconductor and applications =
Superconductivity as a property of certain materials which have zero resistivity at and below a critical temperature which depends on the material. Applications (e.g. very strong electromagnets, power cables as can transfer electrical energy without wasting energy).
At any junction in a circuit….
the total current leaving the junction = the total current entering the junction
The current passing through two or more components in series is
the same throughout each component
Potential difference =
energy transfer per coulomb of charge
For 2 or more components in series, the total potential difference across all the components is
equal to the sum of the potential difference across each component
In series, V0 =
V1 + V2 + V3
In parallel, V0 =
V1 = V2 = V3
For any complete loop of a circuit, the sum of the emfs round the loop is
equal to the sum of the potential drops around the loop
In series, R0 =
R1 + R2 + R3 same current different pd
In parallel, 1/R =
1/R1 + 1/R2 same pd different current
Rate of heat transfer =
RI^2
E=IVt
P=IV
P=RI^2
Emf =
Electrical energy, E / Charge, Q
IR +Ir = terminal pd + lost pd (pd across internal resistance)
V1 - V2 / I2 - I1
Internal resistance is
the loss of potential difference per unit current
the resistance of the battery itself within the circuit (lost volts)
Circuit with emf, e and two resistors r and R, current =
I = e/R + r
When plotting Current (x) against terminal voltage (y) what can you get
y intercept = emf
gradient = negative internal resistance
Application of internal resistance
Car batteries have low internal resistance and low voltage so to provide energy needed to ignite the petrol in a short time a high current is needed
Cell current =
net emf / total circuit resistance
Pd across each resistor in series within the cell =
I * resistance of each resistor
Current through each resistor =
pd across parallel combination / resistors resistance
If cells are connected in the same direction
the net emf is the sum of the individual emfs
emf0 = emf1 + emf2 ….
If cells are connected in the opposite direction
the net emf is the difference of the individual emf’s
For a circuit with n identical cells in parallel the current through each cell is
I/n
For a circuit with n identical cells in parallel the lost potential difference in each cell is
Ir/n = V/n
Solar panels consist of
many parallel rows of identical solar cells ins series
A diode has a forward pd of ____ with a current passing through it
0.6V
A diode has a resistance of ___ in the reverse direction or when _____
infinite ohms
less than 0.6V
Potential divider =
2+ resistors in series can be used to supply a pd which is fixed at any value between 0 and the source pd
- supply a variable pd
- supply a pd that varies with a physical condition such as temperature
Current =
V/R
pd across resistors / total resistance
V0 / R1 +R2…
Potential difference =
IR
V0R/R1+R2…
Ratio of pds across each resistor =
ratio of resistance across 2 resistors
V1/V2 = R1/R2
A variable potential divider can be used
as volume or brightness control
Sensor circuits produce output pd which changes a result of a change in a physical variable such as light intensity or temperature
a change in a physical variable such as light intensity or temperature
Light sensor structure
A light sensor uses a LDR and variable resistor
Temperature sensor circuit
Temperature sensors consist of a potential divider made using a thermistor and a variable resistor
The resistance of a metal increases with temperature because
as the positive ions in the conductor vibrate more due to the increase in thermal energy it becomes harder for the conduction electrons to pass through the metal when a potential difference is applied
Metals have a ___ temperature coefficient because
Positive
Temperature causes resistance to increase
Semiconductors have a ____ temperature coefficient because and why
Negative
Temperature causes resistance to decrease because as the number of conduction electrons increases do does temperature and so as the current increases the resistance decreases
Variable resistors and potential dividers measure
the variation of current with potential difference (with potential dividers current and pd can = 0 but not with variable resistors)
Ammeter symbol and function
—–(A)—– Measure current (in series in Amps)
Voltmeter symbol and function
—–(V)—– Measure potential difference/voltage (in parallel in volts)
Cell symbol and function
—–| l—– source of energy
Bulb symbol and function
—–(X)—– light source / indicator of flowing current
Diode symbol and function
—–l>—- allows current to flow in a single direction
Light emitting diode symbol and function
—–l>—- with two arrows pointing north east from the top right - emits light when conducts
Resistor symbol and function
___
—-|___|—- Designed to have a specific resistance
Variable resistor symbol and function
___
—-|___|—- with single arrow going from bottom left to top right - measure variation of current with pd
Thermistor symbol and function
___
—-|___|—- with line from bottom left to top right that going horizontal when out of rectangle /—-
Light dependent resistor symbol and function
___
—-|___|—- with two arrows coming in from top left - as light intensity increases resistance decreases
Heater symbol and function
—-[ | | | ]—- (rectangle with 3 lines in) A transducer which converts electrical energy to heat.
Electric motor symbol and function
—–(M)—– A transducer which converts electrical energy to kinetic energy (motion).
Graph of voltage (x) against current (y) for a wire
graph of y=x, directly proportional with gradient 1/R
Graph of voltage (x) against current (y) for a lamp
stretched S shape (sinx graph for -90 < x <90)
Graph of voltage (x) against current (y) for a thermistor or LDR
y = 2x (steeper) for higher temperature/light intensity
y=x/2 (shallower) for lower temperature/light intensity
Graph of voltage (x) against current (y) for a diode
Current = 0 until voltage = 0.6V then increases almost vertically but not too far over 0.6V
State and explain one reason why it is an advantage for a rechargeable battery to have a
low internal resistance
Internal resistance limits current so low ir can provide higher current
Charges quicker as current higher and less energy wasted
State what is meant by the emf of a battery
Energy transferred to electrical energy per unit charge coulomb passing through
(pd across terminals when no current passing through cell)
(word done by the battery per unit charge)
When the switch is open the voltmeter reads 6.0 V and when it is closed it reads
5.8 V.
Explain why the readings are different
when switch is closed a current flows (through the battery)
hence a pd/lost volts develops across the internal resistance
State and explain why it is important for car batteries to have a very low internal resistance
Need large current to start a car, internal resistance limits current
Define internal resistance
Resistance of materials within battery
State and explain the effect of attempting to use a battery with a much higher internal
resistance to start the car
Car probably wont start as battery cant provide enough current
Explain why the terminal pd decreases as the current increases
emf > V
pd across internal resistance increases with current
In graph of current (x) against terminal pd what is the emf and the internal resistance
y intercept
gradient
Circuit: battery, switch, lamp, voltmeter
Why is the voltmeter reading less than the emf of the battery?
battery has internal resistance
current passes through (this resistance)
work done/voltage lost, which reduces the value of the emf
The reading on the voltmeter is less than the emf. Explain why this is so
there is a current (through the battery)
voltage ‘lost’ across the internal resistance
what will happen to the reading on the voltmeter as the temperature of the thermistor
increases.
reading will increase
resistance (of thermistor) decreases (as temperature increases)
current in circuit increase (so pd across R1
increases)
The filament of the lamp in X breaks and the lamp no longer conducts. It is observed that
the voltmeter reading decreases and lamp Y glows more brightly.
(i) Explain without calculation why the voltmeter reading decreases
(ii) Explain without calculation why the lamp Y glows more brightly.
(circuit) resistance increases
current is lower (reducing voltmeter reading
pd across/current through Y increases
hence power greater or temperature of lamp
increases
A student wishes to investigate how the resistance of a thermistor changes with
temperature.
Describe the procedure the student would follow in order to obtain accurate and
reliable measurements of the resistance of the thermistor at different temperatures
Thermistor connected with voltmeter and ammeter
Thermistor heated in beaker of water and thermometer used to measure temperature at small regular intervals
Resistance calculated at various temperatures
Repeat experiment
Plot graph of resistance agaisnt temperature
State and explain the effect on the voltmeter reading if the internal resistance of the battery
in the circuit was not negligible.
less current now flows/terminal pd lower
hence voltage across resistor will decrease
Some materials exhibit the property of superconductivity under certain conditions.
• State what is meant by superconductivity.
• Explain the required conditions for the material to become superconducting
superconductivity means a material has zero resistivity/resistance (1)
resistivity decreases with temperature or idea of cooling (1)
becomes superconducting when you reach the critical/certain/
transition temperature
State and explain what happens to the resistance of the cable when the embedded
filaments of wire are made superconducting.
the resistance decreases (to zero) (1) copper still has resistance (1) but this is in parallel with filaments (which have zero resistance) (1) hence total resistance is zero (1) current goes through filaments
describe how you
would use a voltmeter, ammeter and other standard laboratory equipment to determine a
value for the resistivity
length with a ruler
• thickness/diameter with vernier callipers/micrometer
• measure voltage
• measure current
• calculate resistance
• use of graph, eg I-V or resistance against length
• use of diameter to calculate cross-sectional area
• mention of precision, eg vernier callipers or full scale readings
for V and I
`Describe the procedure the student would follow in order to obtain an I-V curve for
the semiconductor diode.
connect circuit up (1)
measure current (I) and pd/voltage (V) (1)
vary resistance/voltage (1)
obtain a range of results (1)
reverse connections to power supply (and repeat) (1)
plot a graph (of pd against current) (1)
mention of significance of 0.6V or disconnect between readings
or change range on meters when doing reverse bias (1)
The figure below shows the I – V characteristic for a filament lamp. Explain the shape of
the characteristic.
at low V, I increases proportionally (or Ohm’s law obeyed) (1)
(as V increases) greater I heats filament/wire
(or temp of filament/wire increases) (1)
resistance increases (1)
rate of increase of I with V decreases [or ref. to gradient = 1/R] (1)
reference to same form of the curve in negative quadrant (
Internal resistance of a source of electricity is due to
the opposition to the flow of charge through the source causing produced electrical energy to dissipate when charge flows through it
Potential difference is also known as
Voltage
Emf is
Electrical energy produced per unit charge passing through a point
Internal resistance is
Work done by the battery
Opposition to flow of charge through the source causing emf to dissipate (produced electrical energy to dissipate)
Resistance is
Ratio of potential difference to current
Current is
The rate of flow of charge
Voltage is
Work done per unit charge
Potential divider is
combination of resistors in series connected across a voltage source to produce a required a potential differnce
