[old][custom] 4.2 test Flashcards
Define potential difference.
What would a potential difference of 1V be defined as?
What is the equation relevant here?
The energy transferred from electrical energy to other forms, per unit charge.
A potential difference of 1V is defined as 1J of energy transferred per coulomb.
V = W/Q
What is potential difference used for?
Used to measure the work done by charge carriers, which lose energy as they pass through the components in a circuit.
Define electromotive force.
The energy transferred from chemical energy into electrical energy per unit charge.
E = W/Q
What is EMF used for?
Used to measure the work done to charge carriers, when they gain energy as they pass through a cell or a power supply.
What is resistance?
Define resistance.
A measure of how difficult it is for current to flow through a component.
The potential difference across a component, divided by the current in that component.
How do we determine the resistance of a component?
Setup a circuit with a variable power supply, an ammeter in series with the component, and a voltmeter in parallel with the component.
By varying the power supply, we can vary the potential difference across the component, and record the PD and current to calculate the resistance. The resistance is equal to 1 over the gradient of the IV graph.
State Ohm’s law
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the potential difference across it.
Where Ohm’s law is true, components are considered ____, and their IV characteristics graph will have a _____ ______ gradient.
Where Ohm’s law is true, components are considered OHMIC, and their IV characteristics graph will have a CONSTANT LINEAR gradient.
What happens to make some components non-ohmic?
When the current across the component increases, the metal ions are heated, gaining kinetic energy and vibrate more around their fixed points in the metallic lattice.
This increases the frequency of collisions with electrons, so more work is done on the charge carriers, increasing the resistance.
Describe fixed resistors
Designed so that their resistance is always constant, despite environmental changes.
The PD is proportional to the current, so the component follows Ohms law.
(All wires can be considered as fixed resistors, just with very low values of resistance)
`What do the IV characteristics of a filament lamp show?
They show that the component is non-ohmic.
For small values of current, the filament lamp acts like an ohmic component, but as the magnitude of the current increases, the heating effect on the metal ions causes the resistance of the component to increase.
Describe diodes.
Components made from semiconductors, which allow current to flow only in one direction.
The diode is a non-ohmic component.
When a diode is reverse biased, it does not conduct at all, and the resistance across it is infinite.
The diode does not conduct until a threshold value for p.d is reached.
The resistance rapidly decreases as the PD is increased above the threshold value, because the density of charge carriers increases.
