Electricity Flashcards
Definition of convensional current
The rate of flow of positive charge
What is the definition of Drift Velocity?
The net movement of electrons parallel to current
What is the definition of potential difference?
Work done per unit charge
What is the definition of charge
Intrinsic property of matter that explains phenomenon
What is resistance?
Voltage divided by current caused by the same voltage. R = V/I
What is resistivity?
Resistivity is an intrinsic property of all materials
What is Kirchhoff’s first law?
Current is conserved at junctions
What is Kirchhoff’s 2nd Law?
In a closed loop, the sum of emf is equal to the sum of p.d.
What is the Electromotive force?
The amount of energy transferred from chemical to electrical energy. Joule transferred per Columb charge
What is Terminal potential difference ?
Potential Difference across terminals of a power supply when a current is flowing
What is electrical power?
Product of p.d. and current.
What is Ohm’s Law?
P.d. across a metallic conductor is proportional to the current through it, provided the physical conditions do not change
Why does an ammeter have 0 resistance?
In order to accurately measure the current through the components as any additional resistance will reduce current
Why does a voltmeter have infinite resistance?
So that an ammeter can accurately measure the current through a component as it is connected in parallel.
Why does resistance cause heating?
- Charge carriers repeatedly collide with the positive ions of the conducting material
- Net transfer of energy from the charge carriers to the positive ions
- Results in the positive ions gaining KE and vibrating more, increasing temperature
- Force due to p.d. across the material accelerates the charge carriers until they collide again
What is rate of heat transfer?
The power dissipated at the component.
What is a potential divider?
Consists of two or more resistors in series with each other and with a fixed source of potential difference which is split between the components
What are the uses of a potential divider?
- Supply a p.d. which is fixed between 0 and source p.d.
- Supply a variable p.d.
- Supply a p.d. that varies with physical conditions
How are the ratios of pd and resistance related for two resistors in series?
Ratio of pds of each resistor is equal to the resistance ratio of the two resistors.
What are two examples of a variable pd potential divider?
- Volume control, audio signal pd is supplied and variable output pd is supplied to loudspeaker
- Brightness control, vary the brightness of a light source from 0 to normal brightness
What are the advantages of a variable potential divider?
The output pd can be set to zero which is impossible when using a resistor and a variable resistor.
What is a sensing circuit?
A circuit that produces an output pd which changes as a result of a change in a physical variable (temperature or light intensity)
What is the purpose of a variable resistor in a sensing circuit?
The pd across the thermistor or LDR can be set to a desired value.
What is a semiconductor?
A material in which the number of delocalised electrons increases with the temperature. Its resistance therefore decreases with temperature.
What is a superconductor?
A wire or device made out of a material that has zero resistivity at and below a critical temperature.
What is considered a high temperature superconductor?
Any material with a critical temperature above 77K, the boiling point of liquid nitrogen
What are the uses of superconductors?
- High power electromagnets that generate very strong magnetic fields used in MRI machines and particle accelerators
- Power cables that transfer electrical energy without energy dissipation
Why does heating increase resistance?
- Positive ions in the conductor vibrate more
- Charge carriers cannot pass as easily when a pd is applied
- Resistance increases
What is a positive temperature coefficient?
When resistance increases with temperature
What is a negative temperature coefficient?
When the resistance decreases with temperature
Why does resistance of a semiconductor decrease with temperature?
- Electrons gain KE and brake away from the nuclei
- Number of charge carriers increases
- Resistance decreases
Why are thermistors used in temperature sensitive sensors?
Their % change of resistance per change in temperature is much greater than for a metallic conductor.
