Chapter 9, Energy, Power And Resistance Flashcards
What is the equation for potential difference in terms of charge?
V = W(energy transferred) / Q(charge)
What property would an ideal voltmeter have in order to function most efficiently?
It would have infinite resistance so when connected, no current would actually pass through
What is voltage in terms of work done by charge carriers?
Voltage is work done by the charge carriers, thus, the charge carriers are losing energy as they pass through components
Def electromotive force? (e.m.f.)
e.m.f. Is when work is done on the charge carriers, essentially the charge carriers gain energy as they travel through a component such as a cell, battery or power pack
What is the equation for the electromotive force?
E (e.m.f.) = W (energy transferred) / Q (charge)
What is the name of the process when electrons gain enough energy to escape the surface of a metal?
Thermionic emission - the emission of electrons through the action of heat
How does an electron gun work?
A heated filament is placed in a vacuum and a high p.d. Is applied between the filament and an anode, the filament then acts as a cathode and the freed electrons accelerate towards the anode gaining kinetic energy as they go. The electrons then go through a small hole in the anode creating a beam of electrons with a specific kinetic energy
What is the equation for calculating the work done on a single electron travelling from the cathode to the anode in the electron gun experiment? (Hint, not kinetic energy)
W (work done) = e x v (accelerating p.d.)
What is the equation for the work done on an electron to increase its kinetic energy?
eV (work done on an electron) = 1/2 x m (mass) x v^2 (velocity)
What assumption is being made when calculating the work done on an electron to increase it’s kinetic energy (in the electron gun experiment)
The electrons have negligible kinetic energy at the cathode (initial energy is almost 0)
How does increasing the accelerating p.d. In the electron gun experiment affect the velocity of the electrons?
It increases the kinetic energy (more energy being transferred to the electrons) so they have a greater velocity.
What is the equation for resistance?
R (resistance) = p.d. (Voltage) / Current
V = IR
What is the definition of an ohm?
The resistance of a component when a p.d. of 1 is produced per amp of current
What is Ohm’s law?
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.
Why does resistance increase when heat increases?
When heat increases, the positive ions in the metal gain more energy thus vibrate more about their mean positions so the frequency of collisions between the positive ions and charge carriers increases so the charge carriers do more work (transfer more energy) as they travel through the wire.
What does the I-V graph for a fixed resistor look like?
Straight line through the origin
What is a resistor called if it obeys ohms law?
It is an ohmic conductor.
How does a resistor behave under reversed polarity?
The same
What is the relationship between the voltage and current in a filament lamp I-V graph and what does this say about the lamp?
Voltage is not directly proportional to current so is a non ohmic conductor and its resistance is not constant.
How does the behaviour of a filament lamp change with polarity?
It behaves the same
What is a main reason that LEDs are more effective than other bulbs?
They do not get hot so do not have much resistance and draw much less power
What does the I-V graph of a diode look like?
Flat up until the origin where it starts to curve upwards before increasing at a linear rate
What is the relationship between the p.d. and current in an LED and what does this say about and LED?
P.d. Is not directly proportional to current in an LED and so can be described as a non ohmic conductor
How does the behaviour of an LED change with polarity?
A negative voltage will cause an infinite resistance in the LED so it will not work
What is the name of the point on an I-V graph for an LED where the resistance starts to decrease?
The threshold p.d.
What three factors (aside from temperature) affect resistance?
The material
The length of the wire L
The cross-sectional area of the wire A
What is the relationship between Resistance and the length of a wire?
Resistance is directly proportional to the length of a wire (double R = double L)
How is resistance related to cross-sectional area?
Resistance is inversely proportional to cross sectional area (2R = 1/2A)
What is the equation for resistivity?
R (resistance) = (P (resistivity) x Length) / A (cross-sectional area)
What does it mean if a material has a negative temperature coefficient?
It’s resistance drops as the temperature increases
What is a thermistor made of?
A material with a negative temperature coefficient
Where are thermistors used?
Thermometers
Thermostats
Engine temperature monitors
What does the I-V graph for a thermistor look like?
It is an upwards curve (downwards cure in the negative V)
How does the resistance of an LDR change with surrounding conditions?
As the surroundings get brighter, resistance decreases
How does an LDR work?
When the light intensity increases, the resistance decreases because the number density of charge carriers increases.
