Electromotive force + Internal Resistance Flashcards
Explain what is meant by a potential divider.
A combination of resistors in series connected across a voltage source (to produce a
required pd)
State and explain what happens to the voltmeter reading when the resistance of R
is increased while the temperature is kept constant.
When R increases, pd across R increases ✓
Pd across R + pd across T = supply pd ✓
So pd across T / voltmeter reading decreases
State and explain what happens to the ammeter reading when the temperature of
the thermistor increases.
At higher temp, resistance of T is lower ✓
1
So circuit resistance is lower, so current / ammeter reading increases
Define the volt.
1 joule per coulomb
State what is meant by electromotive force
emf is the work done / energy transferred by a voltage source / battery / cell ✓per
unit charge
Describe how you would make a direct measurement of the emf ɛ of a cell,
Voltmeter across terminals with nothing else connected to battery / no additional
load.
Explain why this meter must have a very high resistance
This will give zero / virtually no current
The overhead cables used to transmit electrical power by the National Grid usually consist
of a central core of steel cables surrounded by a sheath of cables of low resistivity
material, such as aluminium.
What is the main purpose of the steel core?
To increase the mechanical strength of the cables.
State the significance of the critical temperature of a material.
below the critical temperature / maximum temperature which resistivity /
resistance ✓
is zero / becomes superconductor
Explain why the efficiency of electrical power transmission is improved when
conventional wires are replaced with superconducting wires.
(with no resistance there can be) no power loss
State what is meant by a superconducting material.
no resistance
M1
(at or) below critical temperature
State an application of a superconductor and explain why it is useful in this
application.
MRI scanner, strong magnetic field, no energy dissipation
Define the term electromotive force (emf).
joules per coulomb (of charge)/work done per unit charge
(treat reference to force as neutral)
M1
where charge moved (whole way) round circuit
State what happens to the resistance of the filament lamp as the current
increases.
resistance (of filament lamp) increases
State and explain whether a filament lamp is an ohmic or non−ohmic
conductor up to its working power
filament lamp is a non-ohmic conductor as current is not (directly)
proportional to voltage / resistance is not constant
A student decides to use two potato cells in series as a power supply for a light
emitting diode (LED). In order for the LED to work as required, it needs a voltage of
at least 1.6 V and a current of 20 mA.
Explain whether the LED will work as required.
total emf is above 1.6 V✓
but will not work as current not high enough / less than 20 mA
Define resistance.
ratio of voltage (across component) to current (through
component) or R = V/I with terms defined and R as subject
A semiconducting diode is an example of a non-ohmic component. State what
is meant by a non-ohmic component.
a non-ohmic conductor does not have a constant resistance
