Module 4 Energy, Power And Resistance Flashcards
What is the difference between P.d and E.m.f?
P.D is the electrical energy transferred per unit charge by the charges flowing in a circuit, to other forms.
E.M.F is the electrical energy gained per unit charge, by the charges while flowing through a power source.
How many electrons are in 1 C of charge?
6.25 x 10^18 electrons
State the base units for volt
J/C
Give formulae for energy in a charge “W”
W=VQ or W=eV
What is the value of one eV in Joules?
1.6x10^-19 J
Do electrons in metals maintain their initial Ke throughout? Why?
No they don’t, they collide with the positive metal ions in the crystal lattice
What’s the velocity of an electron that has been accelerated through a p.d of 5V.
Give formulaes.
Ke=1/2 m v^2 Energy gained by electrons equals initial kinetic energy. 1/2 m v^2 = eV v =(2eV/m)^1/2 v = 1.3 x 10^6 m/s
State Ohm’s Law.
Ohm’s law states that current flowing through a conductor is directly proportional to the potential difference applied across it, considering other physical factors like temperature remain constant.
How can you test Ohm’s law yourself with basic equipment.
Also state what precautions you should take to ensure validity of results.
Set up a power pack and connect to a fixed length of wire in series. Place an ammeter in series and voltmeter parallel to this wire. Increase and measure pd using equipment and measure the change in current. The graph of Current against Pd should be linear and pass through the origin.
Make sure to keep the amount of current small as this will prevent heating of wire, also you can allow the wire to cool between each reading to increase validity.
What are the 4 properties that affect the resistance of a material?
The number of free charge carriers per unit volume, the higher the number density of the charge carriers the lower the resistance.
The length of the material, the bigger the length the higher the resistance as there will be more positive ions in the length of a wire, increasing number of collisions of conduction electrons with positive metal ions. Or the voltage applied per meter would decrease and the mean drift velocity of the electrons will fall causing a lower current and since R is inversely proportional to I, R will increase.
The Cross sectional area of the wire is inversely proportional to the Resistance. This is because the voltage per meter stays the same but the volume of electrons flowing increases for the same drift velocity. This lowers the resistance.
An increase in temperature will increase the resistance as the effective cross sectional area of positive metal ions in a wire increase, as they vibrate more with higher temperature. This will increase rate of collision between conduction electrons and positive metal ions, hence increasing resistance.
What property is unique to thermistors? What are they made of?
Thermistors drop resistance as temperature increases. They are made of semi-conductors.
What are ohmic conductors?
Conductors that show a direct proportionality between current and voltage are ohmic conductors.
Describe the I/V graph of an ohmic and non ohmic conductor.
For an ohmic conductor the gradient is a straight line passing through the origin.
For a non ohmic conductor, specifically a filament lamp.
The graph passes through the origin and is more or less, linear for a range of pd but then the gradient falls very quickly, because the mean drift v increases and the rate of collisions increase causing a higher resistance.
What is the “forward bias” direction of an Led?
What is the other direction called?
Describe it’s I/V graph.
Forward bias is the only direction current can flow through a Diode. (Positive to negative)
The other direction is called reverse bias.
No gradient when values of I and V are negative but then gradient increases from zero after a certain amount of pd is applied.
Describe the similarity and differences between I/Temperature and I/Light intensity graphs of thermistors and ldr ‘s.
Both them have a negative gradient but ldr have a more gradual negative gradient, with a slight curve. Whereas thermistor’s graph is more curvy showing that Current passing decreases faster with increase in temperature, ipfor Thermistor’s, than it does for decrease in light intensity, for Ldr ‘s.
