Topic 3 - Electricity Flashcards
What is the definition of current?
Current is the rate of flow of charge
What is the formula for curent?
I = Q/t
What is potential difference?
Potential difference is the energy transferred per unit charge between two points in a circuit.
What is the formula for p.d?
V = W/Q
What is resistance and what is the formula?
Resistance is a measure of how difficult it is for charge carriers to pass through a component.
R = V/I
What does ohm’s law state?
Ohm’s law states that for a ohmic conductor, current is directly proportional to p.d across it given that physical conditions e.g temperature is kept constant.
What is the principle of charge conservation?
Charge conservation is the principle which states that the total electric charge in a closed system does not change.
e.g total current flowing into a junction is equal to total current flowing out of the junction.
Due to charge conservation, how do series and parallel circuits differ?
In a series circuit –> current is the same everywhere
In a parallel circuit –> sum of currents in each parallel set of branches is equal to total current.
What are Kirchoff’s laws?
1st first law
-total current flowing into a junction is equal to total current flowing out of junction.
Second law
- Sum of all voltages in a series circuit is equal to the battery voltage.
What is the principle of energy conservation?
Energy conservation is the principle which states that energy cannot be formed or destroyed; total energy in a system stays constant.
Due to energy conservation, how do series circuits and parallel circuits differ?
Series
- Battery p.d is shared across all elements in the circuit; total sum of voltages across all elements = supply p.d
Parallel
- p.d across each branch is the same.
How do you work out total resistance in a series circuit?
Add resistances of all components together.
R₁ + R₂ + R₃ + … = Rₜ
What is the definition and formula of power?
Power is the rate of transfer of energy.
P = VI
How do you work out total resistance in a parallel circuit?
1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ + …
What will the graph for a ohmic conductor look like?
Ohmic conductors’ current is proportional to voltage; straight line through the origin.
What is the formula for energy transferred using power?
Power x time = energy transferred
W = Pt
What would the IV graph for a diode look like?
Diode only allows current to flow in one direction, forward bias is the direction in which it will allow current to flow easily past the threshold voltage
Before point of threshold voltage, line will be near x axis; when threshold voltage is met, current increases sharply.
What is the definition and formula of resistivity ?
Resistivity is a measure of how easily a metal conducts electricity.
ρ = RA/l
What other factors affect current?
- Charge carrier density (n)
- Speed at which charged particles are travelling. (drift velocity)
- The charge a single charged particle carries
What is drift velocity?
Charged particles are constantly colliding with other particles in the conductor; they do not travel in a straight line. The average speed they move along the conductor is the drift velocity.
What formula can be used to calculate current passing through a conductor?
I = nqvA
What is a potential divider?
Potential divider is a circuit with several resistors in series connected across a voltage source.
What happens to LDR resistance as light intensity increases?
As light intensity increases, LDR resistance decreases
Why do voltmeters have high resistance?
Limit current draw
How does Ω change in a wire as temperature increases?
As temp increases, KE of lattice ions increases
Frequent collisions with electrons and lattice ions
Drift velocity of electron decreases so current decreases
Resistance increases
How does resistance change when temperature increases in a solid?
- As temp increases, intensity of vibration of lattice structure increases
- More intense lattice vibrations; likely to collide with vibrating atoms
- Harder for electrons to pass through solid
- Resistance increases
How does resistance change when temperature increases in a metal?
- Temp of metal increases, atoms gain energy
- Once they gain enough energy, they release electrons (thermionic emissions)
- Increases number of charge carriers
- Resistance decreases
How do negative temperature coefficient thermistors work?
They release a large amount of charge carriers when temp increases; outweighs effect of lattice vibrations.
Temp increase = Resistance decrease
How does resistance change in a metallic conductor?
- As temperature increases, lattice vibrations increase
- Electrons released but not quick enough to counter disruptive effect of lattice vibration.
- As temp increases, resistance increases.
How does resistance change in a light dependant resistor?
LDR made of photoconductive materials
- As light intensity increases, LDR releases electrons
- Increases number of charge carriers available to conduct electricity
- Resistance of LDR decreases
