Physics Module 4: Section 1 - Resistance, Resistivity and Ohm's Law
Benefits of LEDs
Operate at low p.d. and low current Long working life More efficient - less energy wasted as heat Switch on instantly Very robust and versatile
What is the forward bias/direction of a diode?
The direction in which the current is allowed to flow
Units of resistivity
Ωm
Equation for resistivity
ρ = RA / L ρ is the resistivity in Ωm R is the resistance in Ω A is the cross-sectional area in m² L is the length in m
Uses of LEDs
Cycle / car lights Road traffic lights Appliance warning lights
What are diodes?
Diodes control the direction in which current can flow in a circuit, only allowing current through in one direction when a particular threshold voltage is applied. An example is an LED (light-emitting diode)
Define resistance in base units
1Ω = kgm²s⁻³A⁻²
What is an ohm?
1Ω is the resistance when there is a p.d. of 1V and a current of 1A. It can also be written as VA⁻¹ or volt per amp
What is an LDR?
A component that alters its resistance with light intensity
What is a thermistor?
A component that alters its resistance with temperature. The main type of thermistor is a Negative Temperature Coefficient thermistor or NTC
What is resistivity?
A property of a material that is a measure of how much that particular material resists the flow of current. It is defined as the resistance of a 1m length with a 1m² cross-sectional area
Why does increasing the temperature increase the resistance in metals?
As temperature increases, atoms in the metal lattice gain kinetic energy and vibrate faster This makes it harder for conduction electrons to pass through, as they collide more frequently with the metal lattice
Why don't filament lamps obey Ohm's law?
As the current increases, so does the temperature of the filament. This causes an increase in resistance, which leads to less current. This is why the gradient of the IV graph levels off at high currents
IV graph for an Ohmic conductor
Current is directly proportional to p.d. so the resistance is constant. The gradient will have a constant slope. Examples are metallic wires, fixed resistors and LDRs in constant conditions
Why does increasing the temperature decrease the resistance in semiconductors?
Increasing the temperature increases the kinetic energy of the atoms. When energy is supplied to a semiconductor, more charge carriers are released so the resistance decreases
State a use of LDRs
Light intensity sensors
What is resistance?
Resistance = p.d. / current A component's resistance is how hard it is for the current to flow through it
State Ohm's Law
The current through a component is directly proportional to the potential difference across it, given that physical conditions remain constant
What is the reverse bias/direction of a diode?
The direction in which the current cannot flow. In reverse bias, the resistance of the diode is very high and the current is very small
IV graph for a thermistor
The gradient gets steeper and the resistance decreases as the current increases
What is the gradient of an IV graph?
The gradient is the resistance. The shallower the gradient, the greater the resistance
Which common factors affect resistance?
The length of wire - resistance is proportional to length, so the longer the wire, the higher the resistance The thickness of wire / its cross-sectional area - resistance is inversely proportional to the area, so the thicker wire, the lower the resistance Temperature - for many metals, resistance is proportional to temperature. However, in semiconductors, resistance is inversely proportional to temperature
What is the threshold voltage of a diode?
The minimum voltage required for a diode to conduct current. It is about 0.6V
How does light intensity affect the resistance of an LDR?
The resistance decrease as the light intensity increases. This is because light provides the energy to release charge carriers and decrease the resistivity of the metal
How does temperature affect the resistance of an NTC thermistor?
The resistance decreases as the temperature increases. This is because increasing the temperature gives more electrons enough energy to escape from their atoms. As a result, there are more charge carriers, so the resistance decreases
IV graph for a filament lamp
The resistance increases as the current or p.d. increases. For low voltages, the curve is steep as the resistance is constant but gets shallower as the voltage rises.
IV graph for a diode
The resistance will be very high when the current is 0A / in the reverse direction. The resistance decreases when there is a current flowing / positive direction or bias
Factors affecting resistivity
The structure of the material Temperature - in metals, resistivity is proportional to temperature. In semiconductors, resistivity is inversely proportional to temperature Light intensity
Examples of semiconductors
Thermistors Diodes LDRs