Electricity Flashcards
Current def
the rate of flow of positive charge carriers
What happens when two oppositely charged conductors are connected together
Charge will flow between them, causing a current
Conventional current def
the flow of positive charge from the positive terminal of a cell to the negative terminal
How is current measured
Using an ammeter
How should ammeters get connected with the part of the circuit you wish to measure the current through
In series
Potential difference def
the work done per unit charge
Resistance def
the opposition to current
Why are wires often made of copper
because copper has a low electrical resistance and resistivity. This is also known as a good conductor as current flows through it easily.
One ohm definition
one volt per ampere
Describe how the resistance controls the size of a current in a circuit
- A higher resistance means a smaller current
- A lower resistance means a larger current
What does Ohm’s law state
For a conductor at a constant temperature, the current through it is proportional to the potential difference across it
Describe how the relations between potential difference across an electrical component (in this case, a fixed resistor) and the current can be investigated through a circuit
How is Ohm’s law represented graphically
if its graph of current against potential difference is a straight line through the origin
Does a filament lamp obey Ohm’s law
No
Draw the current-voltage graphs for a resistor, semiconductor diode and filament lamp
Explain the current -voltage graph for a semiconductor diode
A diode is used in a circuit to allow current to flow only in a specific direction:
- When the current is in the direction of the arrowhead symbol, this is forward bias. This is shown by the sharp increase in potential difference and current on the right side of the graph
- When the diode is switched around, it does not conduct and is called reverse bias. This is shown by a zero reading of current or potential difference on the left side of the graph
Explain the current-voltage graph for a filament lamp
The I–V graph for a filament lamp shows the current increasing at a proportionally slower rate than the potential difference
This is because:
- As the current increases, the temperature of the filament in the lamp increases
- Since the filament is a metal, the higher temperature causes an increase in resistance
- Resistance opposes the current, causing the current to increase at a slower rate
- Where the graph is a straight line, the resistance is constant
- The resistance increases as the graph curves
- The filament lamp obeys Ohm’s Law for small voltages
Explain why resistance exists
- As free electrons move through a metal wire, they collide with ions which get in their way
- As a result, they transfer some, or all, of their kinetic energy on collision, which causes electrical heating
- Since current is the flow of charge, the ions resisting their flow causes resistance
What does resistance depend on
the length of the wire, the cross-sectional area through which the current is passing and the resistivity of the material
What does the resistivity equation show
- The longer the wire, the greater its resistance
- The thicker the wire, the smaller its resistance
What is resistivity
- Resistivity is a property that describes the extent to which a material opposes the flow of electric current through it
- It is a property of the material, and is dependent on temperature
Resistivity unit
Ohm metres
Why does electric current decrease (and resistance increase) as temperature increases
The electrons collide with the vibrating atoms which impede their flow, hence the current decreases
Describe the rules of Ohm’s law on temperature
- An increase in temperature causes an increase in resistance
- A decrease in temperature causes a decrease in resistance
What happens to the resistance of a thermistor as temperature increases
the resistance of the thermistor decreases (and vice versa)
Draw the temperature - resistance graph for a thermistor
Superconductor def
A material with no resistance below a critical temperature
Critical temperature def
The temperature at which a material becomes superconducting
What is a common superconducting material and what is its critical temperature
Mercury - it has a critical temperature of 4.2K
Draw a Resistivity against Temperature graph for a superconductor vs. a normal metal
What is the transition temperature
The temperature threshold before the critical temperature
What are superconductors useful for
for applications that require large electric currents.
For example:
- The production of strong magnetic fields
- The reduction of energy loss / dissipation in the transmission of electric power
Define resistivity
