Current, charge and PD Flashcards
What do Q, I and t stand for in ΔQ = I Δt?
Q stands for Charge, measured in Coulombs, I is the current, measured in Amperes, and t is time, measured in seconds.
In which direction does conventional current flow?
Positive to negative (the opposite way to electron flow).
Define the coulomb.
The amount of charge which passes in 1 second when the current is 1 ampere.
What is the elementary charge (charge on a single electron)?
e= 1.6 x 10-19C
How should you connect an ammeter in a circuit?
In series.
Define ‘drift velocity’.
The average velocity of the electrons (charge carriers).
Fully explain:
I=nAvq
I = electric current in amps
n = number of charge carriers per m3
A = cross-sectional area in m2
v = drift velocity in ms-1
q = charge in C carried by each charge carrier
Describe the charge carriers in a metal.
They are free electrons found in the outer shell of each atom. In the formula I=nAvq there are many charge carriers, making n big. The drift velocity only needs to be small even for a high current.
Describe the charge carriers in a semiconductor.
Semiconductors have fewer charge carriers than metals so the drift velocity will need to be higher if you are going to have the same current.
Describe the charge carriers on a perfect insulator.
A perfect insulator would not have any charge carriers so you would have no current. Real insulators have a very small number of charge carriers per m3
Describe the charge carriers in liquids and gases.
Ionic crystals such as NaCl are insulators. Once molten, the liquid conducts. Positive and negative ions are the charge carriers.
Gases are also insulators but if you apply a high enough voltage electrons are delocalised, causing ions to allow electron flow. There is a spark.
Define ‘Potential Difference’
PD is energy converted per unit charge moved.
V = W/Q (V - voltage, W - energy in Joules,
Q - charge in C)
Define the** Volt**.
1V = 1J/C
The potential difference across a component is 1 Volt when you convert 1 Joule of energy moving 1 Coulomb of charge through the component.
Describe in words how current and charge are related.
Current is the rate of flow of charge.
What is the IV Characteristic for a metallic conductor?
A straight line is the IV characteristic for a metallic conductor, where, at constant temperature, the current is directly proportional to the voltage.
Metallic conductors are Ohmic.
What is the IV Characteristic for a filament lamp?
The IV characteristic for a filament lamp is a curve, which has a steeper gradient closer to the origin, and a lesser gradient further from the origin. This is because a filament lamp changes temperature, so is non-Ohmic.
The resistance of a metal increases as the temperature increases.
What effect does temperature have on charge carriers?
Charge is carried through metals by free electrons in a lattice of positive ions. Increasing the temperature doesn’t change the number of charge carriers present, but does make it more difficult for them to drift freely. The ions vibrate more when the temperature increases, so the electrons collide with them more often, losing energy.
What is the IV Characteristic of an NTC Thermistor?
(You only need to know about negative temperature coefficient thermistors)
The resistance of a thermistor depends on temperature.
The resistance decreases as the temperature increases. The IV Characteristic curves upwards.
When the temperature of the thermistor increases, the electrons in the thermistor gain enough energy to escape their atoms, thus adding more charge carriers. This lowers the resistance.
What is a semiconductor?
A semiconductor has a conductivity between that of a conductor and an insulator.
When a metal’s temperature increases, it becomes a more effective insulator. When a semiconductor’s temperature increases, it becomes a more effective conductor.
