4.1 Charge and Current Flashcards
How is electric current defined?
Electric current is the rate of flow of charge/charged particles.
What equation defines electric current?
Current (A) = Change in Charge (C)/Change in Time (s)
What is 1 Ampere the same as?
1 Coulomb Second (1Cs)
What is electric charge?
Electric charge is a physical property, either positive or negative and is measured in coulombs or relative charge.
Define the coulomb.
The electric charge flowing past a point in one second when there is electric current of 1 Ampere.
What is the elementary charge?
1.60 x 10⁻¹⁹C
What is the relative charge of a proton?
+1
How does an object become charged?
Through the loss or gain of electrons.
What is meant by quantised charge?
A charge that can only have certain values, such as being an integer multiple of e.
What is Millikan’s oil drop experiment?
It was an experiment used to investigate the motion of electrically charged droplets between parallel plates. It was involved in the discovery of quantisation of charge. Depending on the charges, some drops sometimes became stationary as they were balanced by gravity and the upward attractive electrostatic force.
What are the charge carriers in a metal?
Charge carriers in metals are the electrons.
What is the structure of a metal?
Metal structures are types of giant lattice structures, with positive metal ions surrounded by a number of delocalised electrons. Cations are not free to move but can vibrate in a fixed position.
How can you produce an electric current in a wire?
Make one end of the wire positive, and the other negative, all electrons will be attracted to the positive end, moving as an electric current.
What is conventional current?
Conventional current is a current from a positive terminal towards a negative one, regardless of electron flow.
What is the name given to liquids that conduct electricity?
Electrolytes.
What are charge carriers in liquids?
Ions.
What are cations and anions?
Cations - positively charged ions.
Anions - negatively charged ions.
What instrument is used to measure current and how does it work?
An ammeter is used to measure current at any point in a circuit.
It should be connected in series and places with lowest resistance to reduce its effect on the current.
What is meant by conservation of charge?
A conservation law that states that electric charge can neither be created nor destroyed, the total charge in the universe is constant.
What is Kirchhoff’s first law?
In an electrical circuit, at any point, the sum of currents into a point is equal to the sum of currents out of that point, charge is conserved.
ΣI in = ΣI out
What is number density?
The number density is the number of free electrons per cubic meter of material - the higher number density = better electrical conductor.
What are the units of number density?
Cubic metre.
What are approximate values of number density for conductors, semi-conductors, and insulators?
Conductor - 10²⁸
Semi-conductor - 10¹⁶
Insulator - close to 0
How do electrons move in a wire as they travel towards the positive terminal?
They move slowly and repeatedly collide with positive metal ions as they drift.
Describe the motion of electrons in an unconnected wire.
There is a rapid, random motion, like vibrations, due to kinetic energy.
What is the equation relating electric current and number density?
I = Anev
I - current (A)
A - cross sectional area (m2)
n - number density (m3)
e - elementary charge
v - drift velocity (ms-1)
What is mean drift velocity?
The average velocity of electrons as they move through a wire.
What happens to the mean drift velocity of charge carriers when the cross sectional area reduces?
The narrower the wire, the greater the drift velocity so that current remains the same. To maintain current, electrons must travel faster, and so mean drift velocity is inversely proportional to the cross sectional area of the wire.
