What is electric current?
- I = ΔQ/Δt where Q is charge and t is time.
- the rate of flow of positive charge
What is potential difference?
- The energy transferred per unit charge BETWEEN TWO POINTS IN A CIRCUIT.
- This can either be to the charge carriers OR to the component from the carriers
- V = W/Q where W is energy transferred and Q is charge.
What is resistance?
R = V/I in ohms
- measure of how difficult is for charge carriers to pass through a component
What is required for current to flow?
- a closed circuit with a source of emf (electromotive force) and a complete path of conducting material
What is the difference between charge carriers and electrons?
- Conduction electrons are the charge carries in metals which move while repeatedly colliding with positive ions
- in solutions, ions are the charge carriers
How does conventional current flow and how do electrons flow?
- Conventional current from + to -
- Electron flow from - to +
How many electrons are in a Coulomb?
- 6 x 10^18
How does electric current flow in a conductor?
- Electrons experience a force due to the electric field
- This causes them to accelerate
- However, they are slowed in contact with ions
- greater force from field (due to higher p.d.) leads to greater acceleration AND more frequent collisions
What is Ohm’s law?
- for an ohmic conductor, current is directly proportional to potential difference
- given constant physical conditions (e.g. temp)
What does the I-V graph of a diode look like?
- Extremely high resistance and therefore very low current in reverse bias
- For forward bias, current is able to flow with low resistance past the threshold voltage (the smallest voltage needed to allow current to flow
- For an ideal diode, once threshold voltage is reached, there is an no change in V regardless of I so causes an infinitely variable resistance.
What are the I-V characteristics of a filament lamp?
- initially obeys Ohm’s law
- heats up as current increases
- causes resistance to increase
- as current increases, the graph begins to curve due to increasing resistance
Explain the process of increasing resistance due to temperature.
- When temperature increases, the amplitude of the vibrations of the lattice ions increase
- This causes electrons to collide more frequently
- The rate of energy transfer to ions increases
- Therefore, to maintain current, more p.d. is needed
- Therefore R=V/I so R increases
What are ideal voltmeters and ammeters?
Ideal voltmeters have infinite resistance, ideal ammeters have zero resistance.
What is superconductivity?
- At a critical temperature, electrical resistivity drops to 0
- This change is abrupt and complete
- Used for generating very strong magnetic fields (medical applications) and transferring electrical energy without dissipation through heating
What is resistivity and how is it found?
- The resisitivity of a material is the resistance that 1 metre long of that material with a cross-sectional area of 1m^2 has.
- ρ = RA/l
- where R is the resistance, A is the cross-sectional area, and l is the length in m
- resistivity is measured in ohm metres
What control factors are needed when measuring resistivity?
- temperature
- material
- cross-sectional area (take repeats along diameter and average)
What are the characteristics of thermistors?
- NTC Thermistors decrease resistance as temperature increases
- temperature causes electrons to be emitted from atoms and increase charge carriers
- used in a temperature sensing circuit e.g. heating
What are LDR’s and what are their characteristics?
- LDR’s or Light-Dependent Resistors decrease resistance with increasing light intensity incident upon the resistors
- similar graph to thermistors
- used for automatic security lights
What is temperature coefficient?
Positive temperature coefficient in metals to increase resistance with temperature.
Negative temperature coefficient in thermistors to decrease resistance with temperature.
What is electrical power in circuits and how is it calculated?
- Power is the energy transferred over time
- Work done is found through W = IVt
- Therefore, power is found as P = IV
- Alternately, P = I^2 x R or P = V^2 / R
What is the total resistance in series and parallel circuits?
Series:
Rtotal = R1 + R2 +R3
Parallel:
1/Rtotal = 1/R1 + 1/R2 + 1/R3
What is Kirchoff’s first law?
- At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction
- proves that no charge is lost
What is the difference between a series and a parallel circuits?
- In series circuits, there is only one path for the current to flow, so the current is the same everywhere and the potential difference is divided between components
- In parallel circuits, the total current is the sum of the current in each branch, which all have the same potential difference.
What happens to the p.d. when connecting cells in series as compared to in parallel?
Series:
Vtotal = V1 + V2 + V3
Parallel: (if identical cells)
Vtotal = V1 = V2 = V3
- this is because current is split between branches so total p.d. same as if flowing through one cell
