Module 4 - electricity Flashcards
What is mean drift velocity?
The free electrons make collisions with the metal ions and this gives them a random velocity.
The free electrons drift towards the positive end of the supply along the length of the wire.
The distance travelled per unit time along the length of the wire is called the mean drift velocity of the electrons
What is Ohm’s law?
For ohmic conductor at a constant temperature, the current is directly proportional to the p.d across it.
Explain the I-V characteristics for a filament lamp?
As the current increase the resistance increases.
Resistivity increases with temperature: heating metal causes ions to vibrate move, electrons collide with them more losing energy.
What is the conservation of charge?
An electrical charge cannot be created or destroyed - total amount of charge is constant.
What is Kirchhoff 1st law?
For any point in a circuit, sum of the current entering the point is equal to the sum of current leaving the point.
What does number density mean?
Number of electrons per m^3.
What affects the mean drift velocity?
The narrower the wire —> HIGHER mean drift velocity .
What is potential difference?
Work done by charge carriers
What is e.m.f?
Work done on charge carriers per unit charge.
What are 3 things that determine the resistance of a component?
- The 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 electrons to pass along it
- Resistivity - this depends on the material the wires made from, as the structure of the material may make it easy or difficult for charge to flow.
What are the I-V characteristics for a metallic conductor?
- At constant temperature, the current through a metallic conductor is directly proportional to the potential difference.
- They are ohmic - they have constant resistance provided the temperature doesn’t change.
What is internal resistance in a battery?
Resistance comes from electrons colliding with atoms and losing energy. In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery - this is internal resistance.
What is Kirchhoff second law?
The total e.m.f = the sum of the p.d around a closed loop
Explain the properties of an LDR?
It has a very high resistance in the dark, but a lower resistance in the light.
Explain the properties of a NTC thermistor?
It has a high resistance at low temperatures, but a much lower resistance at high temperatures
What is thermionic emission
The emission of electrons from the surface of a hot metal wire.
Describe an experiment to obtain IV characteristics?
- measure current through and voltage across
- adjust the potential divider (rheostat) to give a range from 0-6V.
- plot graphs I against V - gradient =R
Explain how you would carry out a resistivity experiment?
- measure the diameter using a micrometer.
- clamp the wire into circuit
- obtain values for p.d. across different lengths of wire.
- if the current in each wire is measured too, we can use R=V/I to calculate the resistance for each length.
- as resistance ∝ length. plot a graph for resistance and length and the gradient = ρ/A.
What three factors affect the resistance of a wire?
- the material of the wire
- the length of the wire
- the cross sectional area of the wire.
Examples of where thermistors are used?
- in simple thermometers
- in thermostats to control A/C and heating
- to monitor the temperarure of components inside electrical devices.
- to monitor engine temperarures to ensure engines don’t overheat
Explain how you could carry out a simple investigation inro how the resistance of a thermistor changes with temperature?
- use a water bath to control the temperature of a thermistor and an ohmmeter for a quick and simple recording of the resistance .
Explain the I-V characteristics of Thermistors?
- with a thermistor as the current increases the temperature increase. however unlike a lamp this temperature increase leads to a drop in resistance because the number density of charge carriers increases.
what is electrical power?
the rate of energy transfer by each electrical component is called the electrical power
What are the 4 equations for power in electricity?
P=VI
P=I²R
P = VQ/t
What two factors does the energy transferred to each individual electrical device depend on?
- the power of the device
- how long the device is used for
Definition of kilowatt-hour?
the energy transferred by a device with a power of 1kW operating for a time of 1 hour.
What is conventional current?
It’s a model used to describe electric currents.in a circuit.
It travels from the positive to negatives it is the direction in which positive charges would travel.
