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.
