Electricity Flashcards
How do you calculate resistivity (formula)?
ρ = RA / l
A = cross-sectional area of wire
Formula for Charge, Time and Current?
I = ΔQ / Δt
Define potential difference.
The difference between the potential electrical energy of the electrons before and after travelling through a component (such as a bulb) per unit of charge is the potential difference.
Formula for Potential difference, Current, Power.
P = IV
How does current change in a series circuit?
It stays the same.
How does current change in a parallel circuit?
Current is split across the different pathways. Where the pathways meet the current is summed.
Why can metals create a current in a circuit?
Most electrons are attached to atoms BUT some are not.
These non-attached electrons are free to flow throughout the metal.
A potential difference will attract electrons to the positive terminal of the metal and create a current.
Why can’t insulators carry a current in a circuit?
All electrons are attched to atoms and so are not free to move.
This means applying a potential difference will not create a current.
How does potential difference change across a series circuit (3 light bulbs)?
The potential difference will decrease between each light bulb.
How does potential difference change across a parallel circuit (3 light bulbs)?
The voltage will be the same across the light bulbs.
Define resistance.
A material’s opposition to the flow of electric current: measured in ohms.
What is ohms law?
Ohms law states that I is proportional to V which is inversely proportional to resistance.
When does Ohms Law hold true?
Under constant physical conditions (such as temperature).
Why does increasing potential difference make a bulb shine brighter?
Higher pd means the energy the electrons transfer to the bulb is greater and therefore the bulb shines brighter.
State the relationship between current and potential difference in a circuit.
Pd and current are proportional to each other as long as temperature is constant.
As pd increases, so does current and vice versa.
What is an NTC thermistor?
Negative temperature coefficient.
As temperature increases, resistance decreases.
What can cause the temperature to change in a circuit?
As current increases, temperature also increases.
Describe the potential difference-current graph for a filament lamp.
A proportional relationship between V and I for lower values of I (straight).
For higher value of I the graph begins to lose proportionality and pd will begin to increase ‘exponentially’ (curve towards pd axis).
This is due to increasing temperatures causing increased resistance.
Describe the potential difference-current graph for a resistor at a constant temperature.
A graph where pd and current are always proportional to one another (straight line).
Describe the potential difference-current graph for a diode.
Pd increases as current stays at 0.
When a high enough pd is reached current will also increase.
Graph is only in positive direction.
Why does a diode have a forward bias?
Because the resistance in the opposing direction is very high and a certain pd is required to conduct.
What is the forward bias of a silicon diode?
0.7 Volts
What is a metallic conductor?
Resitance increases SLOWLY with temperature (PTC)
This is due to positive ions vibrating more with increasing temperature/energy. So electrons can’t pass through easily when pd is applied.
What is a semiconductor?
Resistance decreases QUICKLY with temperature (NTC).
The number of electrons increases far quicker than the impedence caused by vibrating ions.
What are thermistors used for?
Generally used in sensor circuits to set off alarms or to turn devices on or off at certain temperatures.
Used for fans in cooling units of electrical circuits.
What is resistivity?
A measure of the resisting power of a specified material to the flow of an electric current.
What factors affect resistance?
Length of a wire.
Cross-sectional area of wire.
The material (resistivity)
What is resistivity measured in?
Ωm
What things tend to have low resistivity?
Metals and other good conductors.
How do superconductors work?
Superconductive materials can be cooled below a transition temperature below which the resistivity is low enough to be negligible. Therefore, no energy is lost as heat energy.
What is a use of superconductors?
Power transmission in overhead cables creates heating in the cables due to resistance and ∴ a loss of energy.
Superconductive cables don’t have this effect as they reduce resistance and therefore the heating effect.
Why does increased current create increased temperature?
There are more electrons flowing throughout the circuit.
This means more collisions between electrons and positive ions and more energy lost as heat energy.
How do you calculate total resistance across a series circuit?
Rₜ = R₁ + R₂ + R₃…
How do you calculate total resistance across a parallel circuit?
1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃…
What is the EMF of a cell?
The quantity of chemical energy transferred to electrical energy per coulomb of charge.
What is internal resistance?
The opposition to the flow of electric current in the source/cell.
How do you calculate the EMF of a cell?
ɛ = I(R + r) EMF = terminal pd + lost pd
How does total EMF change with more cells (series)?
Total EMF = sum of individual EMFs
How does total EMF change with more identical cells (parallel)?
Stays the same
How is lost pd calculated with multiple identical cells ‘n’ (parallel)?
Lost pd = Ir/n
What is a potential divider?
A circuit that uses resistors to split a source, emf and ∴ supply a variable pd.
Why might two lamps have different power ratings but same light intensity output?
Lamps energy is wasted as heat energy.
Higher power = more wasted energy.
Why does increasing current decrease voltage from a cell?
Terminal pd = ε - lost pd (from internal resistance)
Greater current, greater proportion of emf across internal resistance ∴ more lost pd.
Why is the voltage lower than ε?
Current flows, some energy is transferred into heat energy due to the internal resistance of the cell ∴ some pd is lost.
