Energy, power and resistance Flashcards
What is the volt?
1V=1JC^-1. One volt is the p.d. across a component when one joule of energy is transferred per unit charge across the component.
What is potential difference?
The transfer of energy done by charge carriers from electrical energy to other forms like heat or light per unit charge.
What is the equation for p.d.?
V=W/Q
What is e.m.f.?
Electromotive force is when work is done on the charge carriers. This is the transfer of energy from chemical energy from the cell or battery to electrical energy.
What is the equation for emf?
Epsilon=W/Q
What is resistance?
Defined as the ratio between V and I of the component.
What is the ohm?
The ohm is defined as the resistance of a component when a pd of 1V is produced per ampere of current.
What is Ohm’s law?
For a metallic conductor kept at constant temperature, the current in the wire is directly proportional to the pd across its ends.
What happens to the resistance when temperature increases?
The resistance increases because the positive ions have more energy to vibrate at greater amplitudes about their mean positions. The frequency of the collisions between the free electrons and the positive ions increases, the charge carriers do more work and transfer more energy. The energy transferred from the electrons causes the positive ions to vibrate more.
What is the IV characteristic for a resistor?
Directly proportional and linear
Obeys Ohms law, so ohmic conductor
Resistance is constant
Behaves the same way regardless of the polarity
What is the IV characteristic for a filament lamp?
Not directly proportional
Non ohmic as it doesnt obey ohms law
Resistance is not constant
Behaves in the same way regardless of the polarity
What is a diode?
A component that only allows current to flow in one direction.
What is the IV characteristic for a diode?
Not directly proportional
Resistance is not constant
Non ohmic
Behaviour depends on polarity
What is the threshold pd?
The pd of a diode where the resistance gradually starts to drop. After this value, the resistance drops rapidly.
What is resistance affected by?
The material of the wire
The length of the wire
The cross sectional area
What is resistivity?
The electrical property of a material.
Is R directly proportional to L?
Yes
Is R inversely propertional to A?
Yes
What is the equation for resisitivity?
rho=RA/L
What happens to the resistivity as the material gets hotter?
Increases
How is resistivity determined from a graph of R against L?
gradient*A
What is a negative temperature coefficient?
When the resistance drops as the temperature increases.
Do thermistors have a negative temperature coefficient?
Yes
What is the IV characteristic for a thermistor?
Opposite to the filament lamp
What is an LDR?
Made from a semiconductor in which the number density of charge carriers changes depending on the intensity of the incident light. In the dark, R is very high and vice versa.
Power equations?
P=VI
P=I^2R
P=V^2/R
P=W/t
P=VIt
What does the energy transferred to an electrical device and the running cost depend on?
The power of the device
How long the device is used for
Energy transferred equation?
E=Wt
