Resistance (Unit 2) Flashcards
What is Potential Difference (PD)?
energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other.
What is Potential Difference (PD) measured in?
Volts = JC-1
In a Current (I) Voltage (V) Graph, when the temperature remains constant, what are the characteristics of a resistor?
-voltage and current are directly proportional
-resistance remains constant
In a Current (I) Voltage (V) Graph, when the temperature remains constant, what are the characteristics of a FILAMENT LAMP?
-As Voltage increases current increases at a decreasing rate
-resistance increases
What is Ohms Law?
-The current in a metal wire at constant temperature is proportional to the pd across it.
- V=IR
Definition of Resistance?
- opposition to flow of charge,
- the voltage per unit current
What is Resistance measured in?
Ohms = VA-1
When do we use P= IV
any device, all forms of energy
When do we use P= I^2 R = V^2/R
-when ALL electrical energy is transferred into heat energy
- only for pure resistor
What creates electrical resistance?
collisions between free electrons and ions
What is the relationship between resistance and temperature?
electrical resistance increases with temperature
Unit for resistivity?
Ohm Metre
What is the relationship between resistance and temperature in a metal?
-The resistance of a PURE metal increases linearly with increasing temperature
How does when resistance increases, temperature also increases?
-At HIGHER temperatures the positive ions vibrate with greater amplitude
- the electron-ion collisions are more frequent, so the flow of electrons is slowed down (current is reduced) resistance increases
What is Superconductivity?
a material when cooled down below its transition temperature has a resistance of zero
What is transition temperature?
The temperature below which a material must be cooled in order
to have zero resistance and become superconducting
Why are HIGH-TEMPERATURE SUPERCONDUCTORS?
-having transition temperature above the boiling point of nitrogen (–196 °C)
-we can use liquid nitrogen to turn materials into superconductors, 10 times cheaper than using liquid helium
-more PRACTICAL and COST EFFECTIVE
Describe some uses of superconductivity.
- magnets to make vehicles float.
- MRI scanners use superconducting magnets to
expose the human body to a strong magnetic field. - Superconducting magnets are often used in particle
accelerators to alter the paths of subatomic
particles.
What is EMF?
energy converted from some other form to electrical potential energy per coulomb of charge flowing through the source
What is Potential Divider Equation?
V Out = R2 / R1+R2 x V In
In a circuit what happens when a THERMISTOR is in low temperature?
-At low temperature
-High Voltage and Resistance at VARIABLE resistor
-Low Voltage AT FIXED RESISTOR
In a circuit what happens when a THERMISTOR is in HIGH temperature?
-At high temperature
-High Voltage and Resistance at FIXED resistor
-Low Voltage AT VARIABLE RESISTOR
What is Kirchoff’s first law?
-At a junction in a circuit, the sum of the currents entering the junction is equal to the sum of the currents leaving the junction
What is Kirchoff’s Second Law?
In any closed loop in a circuit, the sum of the emf’s is equal to the sum of the pd’s around the loop
How does LDR (Light Dependent Resistor) work?
- When light intensity increases, the resistance of LDR DECREASES
What happens to LDR when dark?
High Resistance, High voltage
-Fixed Resistor has Low Voltage
What happens to LDR when bright (day)?
-Low Resistance, Low Voltage
-Fixed Resistor has High Voltage
Test to measure Ohm’s law? (how to draw circuit)
Circuit with variable resistor
-ammeter (connected in series)
-voltmeter connected to metal wire/ filament lamp (in parallel)
Test to determine resistivity of a metal?
-circuit with ammeter, voltmeter, crocodile clips, nichrome wire, metre ruler vernier , vernier callipers
- record voltage and current at increasing 10 cm intervals using crocodile clip on wire
-Calculate the value of R for each length.
-Convert the values for length from cm to m.
-plot a graph of R (vertical axis) against L (horizontal axis).
-Calculate the cross-sectional area of the wire, A. using vernier callipers to record radius
-Calculate the gradient of your graph.
-Calculate resistivity, ρ, using this equation:
ρ=gradient×A
Test to determine variation of resistance with temperature for a metal wire?
- water bath with heating element, ice , thermometer, stirrer, ohmmeter, copper coil
- put copper coil in boiling tube filled with oil and connect to ohmmeter
- increase water bath heat till reaches 100 degrees, stir continuously , record readings at 10 degree intervals and measure resistance, record readings again when cooling down to ensure accuracy and reliability
- use ice and water mix to reach 0 degrees
- plot resistance against temp, graph should be straight line (shows resistance increases LINEARLY with temperature)
Test to determine speed of sound using stationary waves?
-1 litre measuring cylinder , glass, tube ,ruler ,set of tuning forks ,retort stand, boss and clamp large rubber bung
- strike tuning fork on rubber bung
- hold over tube
- move tube up and down until resonance position is obtained
- measure length of tube above water
- repeat with other tuning forks of known frequency
- measure each length of the tube above water until open end
- use Lander = 4L to work out wavelength and Wave speed = frequency x wavelength to work out speed of sound
Conservation of charge?
-Electric charge cannot be created or destroyed, (though positive and negative charges can neutralise each other).Charge cannot pile up at a point in a circuit.
