Electricity Flashcards
(32 cards)
Define current, conventional current and electron flow
-current is the rate of flow of charge
-conventional current, current flows from + to -
-electron flow, current flows from - to +
Define a coulomb and a volt
One coulomb (C) is defined as the amount of charge that passes in 1 second if the current is 1 ampere (A)
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component.
define potential difference
Potential difference (pd) or voltage is defined as the work done (energy converted) per unit charge moved
How do you make electric charge flow through a conductor, how do you measure the current flowing though a circuit, how do you measure the potential difference across a component
To make electric charge flow through a conductor, you need to do work on it
Measure current flow using an ammeter in series
Measure the potential difference using a voltmeter in parallel
Define resistance
Resistance is a measure of how difficult it is to get a current to flow through
A component has a resistance of 1 ohm if a potential difference of 1 volt makes a current of 1 amp flow through it
Unless specifically stated in questions what do you assume about ammeters and voltmeters
That they are ideal
Voltmeters have infinite resistance and ammeters have zero resistance
State ohms law
The current through an ohmic conductor is directly proportional to the potential difference across it, provided the physical conditions, such as temperature, remain constant
-the graph of current against voltage is a straight line through the origin
-ohms law is only true for ohmic conductors under physical conditions
What do I/V graphs show and what do I/V graphs for an ohmic conductor look like
-they show how resistance varies
-how current (I) flowing through a component changes as the potential difference (V) across it is increased
-current is directly proportional to voltage (R is constant)
-at a constant temperature:
-I/V graph is a straight line through origin (steep means low resistance)
-V/I graph is a straight line through origin (shallow means low resistance)
I/V graph for a filament lamp
A curve that starts steep but gets shallower as the voltage rises
-the filament is just a coiled up length of metal wire, not the same graph for metallic conductor (ohmic graph) as it heats up (temp not constant)
-current flowing through the lamp increases its temperature
-the resistance of a metal increases as the temperature increases
What are semiconductors and what are they used for
Semiconductors are not as good at conducting electricity as metals as there are fewer charge carriers available.
If energy is supplied to a semiconductor, more charge carriers can be released
Therefore, they make excellent sensors for detecting changes to their environment
A thermistor is a semiconductor. Describe how it works and the graphs for it
-a thermistor is a resistor with a resistance that depends on temperature
-a NTC (Negative Temperature Coefficient) means that the resistance decreases as the temperature goes up (curved)
-the I/V graph for an NTC is a curve that has an increasing gradient for + and - current
-warming the thermistor gives more electrons enough energy to escape from their atoms
-this means that there are more charge carriers available, so the resistance is lower
A diode is a semiconductor. Describe how it works
-diodes (eg LEDs) are designed to allow current to flow in one direction only
-forward bias is the direction which the current is allowed to flow
-most diodes require a threshold voltage of about 0.6V in the forward direction before they conduct
-in reverse bias, the resistance of the diode is very and the current that flows is vertically tiny
What does resistance depend on
-length- the longer the wire, the more difficult it is to make current flow
-area- the wider the wire, the easier it will be for the electron to pass along it
-resistivity- depends on the material. The structure may make it easy or difficult for charge to flow. Also depends on environmental factors: temperature and light intensity
Define resistivity of a material
The resistance of a 1m length with a 1m^2 cross-sectional area
-typical values of resistivity of conductors are really small
What is superconductivity and how do you make a superconductor
-superconductivity has zero resistance
-resistance is when electricity flows through them, they heat up, some electrical energy is wasted as thermal energy
-you can lower resistivity by cooling them down
-if you cool some materials down below a ‘critical temperature’ their resistivity disappears entirely and they become a superconductor
-‘critical temperature’ for some materials is really low, might be expensive to cool down that much
Use of superconductors (what could you makes with superconducting wires)
-power cables that transmit electricity without any loss of power
-strong electromagnets that don’t need a constant power source (maglev trains)
-electronic circuits that work really fast, because there’s no resistance to slow them down
Define Power and give the formula for power and total energy transferred
The rate of transfer of energy
Power: P=E/t
Total energy transferred: E=VIt
Explain why batteries have resistance
Resistance comes from electrons colliding with atoms and losing energy to other forms
In a battery, chemical energy is used to make electrons move. As they move they collide with atoms inside the battery- so batteries must have resistance
So batteries have an internal resistance
What is internal resistance and load resistance
Internal Resistance: The resistance to the flow of charge within a source
Load Resistance: the total resistance of all the components in the external circuit
Define emf of a cell
The amount of electrical energy the batter produces for each coloumb of charge
Define terminal pd (V= ε-v)
The potential difference across the load resistance- the energy transferred when one coulomb of charge flows through the load resistance
If there is no internal resistance, the terminal pd is the same as the emf
Define lost volts
The energy wasted per coulomb overcoming the internal resistance
What does conservation of energy say energy per coulomb supplied by the source
Energy per coulomb transferred in load resistance + energy per coulomb wasted in internal resistance
Formulas with emf and lost volts
ε =V + v
V= ε - v
v= lost volts
