Electricity Flashcards
Define current
The rate of flow of charge
Define Potential Difference
The work done per unit charge
What does Ohm’s Law State
Provided physical conditions remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it
Ohms Law states that physical conditions must stay constant, what are some of these physical conditions
- Temperature
- Pressure
- Light Level
What does an ideal ammeter mean
Having no resistance
What does an ideal voltmeter mean
Having infinite resistance
For an ohmic conductor describe the Current vs Voltage Graph
A straight line passing through the origin
-Constant gradient, as R is constant
I-V Characteristics of a semi-conducting diode
- Not as good at conducting as metals, as there are fewer charge carriers
- But if energy is supplied to the semiconductor more charge carriers are released
I-V Characteristics of a filament lamp
- Non-ohmic component
- Produce a curve on an I-V graph, passing through the origin
- The curve starts steep but gets shallower as voltage rises
Why does a filament lamp not produce a straight line I-V graph
- Because its resistance isn’t constant
- As the current increases the temperature of the wire inside the lamp increases
- As temperature increases resistance increase, as higher temperatures cause the atoms in the wire to vibrate more, leading to more collisions between the electrons and the atoms
Characteristic of a negative temperature coefficient thermistor
As temperature increases, resistance decreases. Because as temperature increase more electrons have enough energy to escape from their atoms, meaning there are more charge carriers available
define resistivity
the resistivity of a material is defined as the resistance of a 1m length with a 1m^2 cross sectional area. Measured in ohm-metres
What does resistivity depend on
- Length of wire- the longer the wire the more difficult it is for current to flow
- Cross Sectional area of wire- the wider the wire the easier it is for electrons to flow
- Resistance of a material
Describe the effect of temperature on the resistance of metal conductors.
As temperature increases resistance increase becauseatoms in the maerial have more energy and therefore vibrate more leading to a higher number of collisions
Applications of thermistors
Temperature Sensors
Define Superconductivity
Superconductivity as a property of certain materials which have zero resistivity at or below the critical temperature.
Use of Superconductors
- Produce strong electromagnetics that don’t need a constant power source, used in medical applications
- In power cables, to reduce energy losses in the transmission of electrical power
What is the issue with superconductors
That the critical temperature is generally around -263 degrees celcius
How can a potential divider provide a constant or variable potential difference from a power supply.
- You can use a potential divider to supply a potential difference to the circuit between zero and the potential difference across the voltage source as you can vary the resistance of the circuit
Define EMF
Energy provided by a cell per coulomb of charge passing through it
Define Terminal pd
The energy transferred per coulomb of charge to the components
State Kirchoffs first law
the sum of the currents arriving at the junction = the sum of the currents leaving the junction.
State Kirchoffs second law
energy supplied by a power supply is equal to the sum of the energies of power dissipation in a circuit’
2 ways of measuring emf and internal resistance
- The emf is equal to the potential difference across the terminals of a battery. If you connect a voltmeter across the terminals you will get a value very close to the emf as little current will low
- If you vary the current and measure the pd, then plot a graph of V against I then the gradient is equal to the internal resistance and the y-intercept is equal to the emf
2 ways of measuring emf and internal resistance
- The emf is equal to the potential difference across the terminals of a battery. If you connect a voltmeter across the terminals you will get a value very close to the emf as little current will low
- If you vary the current and measure the pd, then plot a graph of V against I then the gradient is equal to the internal resistance and the y-intercept is equal to the emf
Advantages of a low internal resistance
- Internal resistance limits current hence can provide a higher current
- Energy wasted in battery is lower
- A battery charges quicker as current is higher
- Higher terminal voltage as less lost volts
Characteristics of diodes
- Only let current flow in one direction
- Forward bias= direction current is allowed to flow in
- Have a threshold voltage of around 0.6v in the forward direction before they will conduct
- In reverse bias the resistance of the diode is very high and the current that flows is very tiny
