3.5 Electricity Flashcards
What is electric current?
The rate of flow of charge
What is potential difference?
The rate of energy transfer (work done) per unit charge when electrical energy is converted into another form of energy
How is volt defined?
The unit of potential difference, work done per unit charge
What is resistance?
The ratio of potential difference over current
What is Ohm’s Law?
The current through an ohmic conductor is directly proportional to the potential difference across it, provided physical conditions (temperature) are constant
What is an ohmic conductor?
A component that follows Ohm’s Law
What is the threshold voltage of a semiconductor diode?
The smallest voltage needed to allow current to flow (forward bias), hence the shape of the I-V graph which has a sudden positive gradient from having no gradient
In the direction of the reverse bias of a semiconductor diode, why can only a very small current flow?
The resistance of the diode is extremely high
Describe a filament lamp.
It contains a length of metal wire (filament) which heats up as current increases, so the resistance of this component increases as current increases
What is a thermistor?
A resistor in which an increase in temperature results in resistance decreasing
What is an LDR?
Light Dependent Resistor - resistor in which an increase in intensity of light decreases resistance
How much resistance should an ideal ammeter have?
Zero, so it doesn’t affect the measurement of current in a circuit at all
How much resistance should an ideal voltmeter have?
Infinite, so no current can flow through it, so the measurement of potential difference across a component is exact
What is resistivity?
- The measure of how easily a material conducts electricity - how much a material opposes the flow of electric current through it
- The value of resistance through a material of length 1m and cross-sectional area 1m^2
What is the relationship between temperature and resistance for a metal conductor?
An increase in temperature of a metal conductor increases its resistance
Why does an increase in the temperature of a metal conductor increase its resistance?
The atoms in the metal gain kinetic energy and move more, causing more collisions between electrons carrying charge and the atoms, slowing the electrons down, decreasing the current and therefore the resistance
What is the relationship between temperature and resistance for a thermistor?
As the temperature of a thermistor increases, its resistance decreases
What is a superconductor?
A material that has zero resistivity below its critical temperature (different for each material)
What are examples of applications of superconductors?
- Power cables - reduces energy loss through heat to zero
- Production of strong magnetic fields - used in maglev trains where there would be no friction between the train and rail, or in certain medical applications
What is power?
The rate of energy transfer (work done)
How is watt defined?
The unit of power, the energy transferred per unit time
What is electrical power?
The rate of electrical energy transfer
What is conventional current?
The flow of positive charge from positive to negative
What is the rule for current in a series circuit?
The current is the same everywhere in the circuit
What is the rule for potential difference in a series circuit?
The EMF is shared across all elements in the circuit, so the total sum of the potential differences across all elements is equal to the EMF
What is Kirchhoff’s First Law?
The total current entering a junction is equal to the total current leaving the junction
What is Kirchhoff’s Second Law?
The total potential difference across the EMF is equal to the sum of potential differences across components in any single loop
What is Kirchhoff’s First Law representative of?
The conservation of charge
What is a potential divider?
A circuit with several resistors in series connected to a power source used to supply constant or variable potential difference
What are examples of components that could be used in a potential divider?
Variable resistors, thermistors, LDRs
What is the terminal potential difference?
The potential difference across a cell
What is EMF?
Electromotive force - the rate of energy transfer per unit charge when one form of energy is converted into electrical energy
What does ‘r’ represent in the emf formula?
Internal resistance
What is internal resistance?
Resistance caused by electrons colliding with atoms inside a battery, causing some energy to be lost before electrons leave the battery
What is the pd across ‘r’ known as?
Lost volts because it is equal to the energy wasted by the cell per unit of charge
