Physics Midterm part 4 Flashcards
Electrodynamics
The science of electrical charges in motion or current electricity.
Electric current can exist in…
- In a vacuum - Electrons jump from one charged electrode to another (such as in an x-ray tube), since there is no air to get in the way.
- ionized gas - Some gases (such as neon) will promote the drift of electrons from cathode to anode. Positive charge moves towards cathode, negative charge towards anode.
- Ionic solution - Some solutions can cause electrons to migrate to cathodes and anodes during electrolysis, when subjected to an electric current.
- Solid conductor - Such as copper wire.
A conductor is a material whose outermost or valance electrons are free to move about when forced.
Electrolysis
Electrolysis occurs when a material such as salt (Na+Cl-) is dissolved in water.
Valence energy band
determines chemical activity and conductivity. The further the valence electron from nucleus, the better conductivity it has.
Gold is more conductive than silver, and silver is more conductive than copper. Copper has higher melting point.
Potential difference
A difference in electrical potential between two points in an electric circuit. The greater the potential difference, the more force or strength of the electron flow.
Voltage
Electric potential difference between 2 points on a conductor. It is defined as the number of Joules/Coulomb, or E/Q.
Volt is the SI unit that measures potential difference.
Voltmeter
An electrical device that measures voltage
Current or intensity
The amount or quantity of electrical charge that is flowing per second or charge/time (Q/t).
SI unit: ampere
Ampere
the number of Coulombs of charge flowing per second (C/s).
Resistance
The property of the circuit that opposes or hinders the flow of current. It is analogous to friction in mechanics.
SI unit: ohms
4 factors of resistance
- Conducting Material – some materials are better conductors of electricity than others (gold, silver, copper as explained earlier). Tungsten has higher resistance
- Length of the conductor – the longer the conductor, the greater the resistance (they are directly proportional to each other). A longer length means that more electrons have to be moved from their energy bands
- Cross-sectional area or thickness – the larger the cross-sectional area, the lower the resistance (inversely proportional).
- Temperature of the conductor – the greater the temperature of a conductor, the greater the resistance.
What happens when conductors are cooled to absolute zero?
electrical resistance will virtually disappear. Opposite effect in semiconductors and insulators
Ohms law
When a steady direct current is flowing in a circuit, there is a definite relationship between the voltage, amperage and resistance.
Direction of flow
Electron flow is the movement of electrons from negative to positive. Conventional current flow is the movement of the electrical field from positive to negative.
Direct current
The electrons always travel in the same direction through the circuit. EX: battery
Alternating current
Produced by an electric generator.
The direction of flow continually changes direction along with the amount of voltage, amperage and resistance.
Alternating current characteristics
- Frequency – the number of cycles per second. SI unit: Hertz. US operates with 60 hz, rest of world is 50 hz.
- Voltage - as the sine wave graph indicates, the voltage varies between zero and a peak value while electricity is produced.
- Amperage - the amperage varies the same as the voltage
- Resistance - since voltage and amperage are not constant, neither is resistance.
Series circuit
an electric circuit whose component parts are arranged end to end so that the current must pass consecutively through each part. Voltage cumulative, amperage constant
Parallel circuit
an electric circuit whose component parts are connected side by side, so that the current is divided among the branches. Voltage constant, amperage cumulative.
Electrical power
the amount of work that the current can do per second. Measured in watts
Capacitance
the ability to store electrical charges. Done by capacitor or condenser, which is simply two metal plates separated by an insulator.
SI unit: Farad
3 Factors that electrons stored in a capacitor depend on
- Area of plates – a larger area can store more charge.
- Distance between the plates – a greater distance can store more charge
- Insulator used between the plates – a stronger insulator can store more charge.
The greater the resistance, the longer the time of discharge.
Heat in electric circuits
When amperage or resistance become too great, heat is produced in the circuit.
Fuse
contains a wire that will melt inside a sealed container when a certain amperage is exceeded.
Circuit breaker
Contains a switch that will shut off the current when the rated amperage is exceeded.
2 types of circuit breakers
- Bi-metallic strip – 2 dissimilar metals are placed together.
- Electromagnetic - Electromagnet will open the switch (turn it off) when the rated number of amperes is exceeded
