C8-9. DC Electricity I Flashcards
How do we define electrical current?
The rate of flow of electrical charge. Represented by the symbol I, and measured in amperes.
What equation links current, charge and time?
I = Q/t. We can think of this as ‘current is the number of coulombs of charge passing a point over a given time’.
What are the base and derived units of electrical charge?
Coulombs (Q) and, from the equation I = Q/t, ampere-seconds (A s).
What types of particles can be considered charge carriers?
Electrons and charged ions.
What is the value of the elementary charge e, and what does it represent?
This is given as 1.6 * 10^-19 C, and this represents the charge on one proton, from which all relative charges such as that of an electron, or a charged 1+/ 2-/etc. ion, are derived from.
Why do we describe the charge on an object as quantised?
Charges must always be multiples of 1.6 * 10^-19 (the elementary charge) so there is a limited set of values charge can have. Therefore, the value of charge is ‘quantised’.
How can we express the net charge on an object?
This must always be some multiple of e, and we calculate it using the equation Q=ne, where n is the number of electrons and Q is the charge passing through the given point per second.
How do most objects obtain a net charge?
- Most of the time, a net charge on an object arises due to the gain or loss of electrons by the object.
- If the object starts in a neutral (uncharged) state, if it gains (negatively charged) electrons, it will gain an overall negative charge.
- The opposite happens if electrons are lost.
What are the charge carriers when current flows in the solid state?
Electrons are usually the charge carriers in the solid state.
What are the charge carriers when current flows in the liquid state?
Charged ions are usually the charge carriers in the liquid state.
How might we model the flow of charge through a (metallic) wire?
- The wire itself has a metallic lattice (crystalline) structure, with a pool of delocalised electrons surrounding positive metal ions.
- If the wire becomes charged on opposite ends, the electrons will be drawn towards the oppositely-charged positive end, hence the charge carriers move and current flows.
What factors might cause an increase in current?
We can think about this using the equation I=Q/t ir we need to:
- A greater number of electrons per second flowing increases the current (this might be caused by increased cross-section)
- The same number of electrons moving past a point more quickly increases the current
What is the difference between conventional current and electron flow?
- Conventional current flows between positive - negative termini.
- Electron flow runs in the reverse direction - i.e. from negative to positive terminals of a source of emf.
What do we call a liquid that can carry an electrical current?
These are called electrolytes, and are used in the process of electrolysis, wherein an ionic solution is split into positive and negatively charged ions which are attracted to oppositely-charged electrodes.
How does electrolysis result in the flow of an electric current?
- The ionic solution (salt) splits into +ve metal ions and -ve non-metal ions.
- These are attracted towards -ve and +ve electrodes respectively.
- This is the movement of charge carriers, so a current flows through the electrolyte.
- The +ve metal ion accepts an electron, which is donated by the -ve non-metal ion when it reaches an electrode. This flow of electrons causes a current to flow in the metal circuit.
Ideally, should ammeters have a high or low resistance?
They should have a low resistance. They are connected in series to the rest of the circuit, so a lower resistance would reduce the effect they have on the size of the current.
What discovery did Millikan’s experiment help him to make?
It proved that electrical charge is a quantised value, through investigation the motion of droplets between oppositely-charged metal plates.
How did Millikan’s experiment work?
- When the oil droplets were sprayed they experienced air resistance, upthrust and gravitational force.
- Some were held stationary as the force of gravity = the electrostatic force of the charged plates
- Some drifted slowly through the induced electric field of the plates.
Describe the law of conservation of charge.
Electrical charge can neither be created nor destroyed, so the total amount of charge in the universe is constant.
What is Kirchhoff’s 1st Law?
For any point/junction in an electrical circuit, the sum of currents into that point is equal to the sum of currents flowing out of the point.
How do we classify conductivity of a material?
Conductivity is based on a material’s number density. The higher the number density, the greater number of free electrons per unit volume, so the electrical conductivity increases.
What is the definition of number density?
This is the number of free charge carriers per unit volume in a material.
What is the (functional) difference between a conductor and a semiconductor?
- Semiconductors have a much lower number density than conductors.
- This means that, in order to carry the same current, more charge must flow per unit time, so the electrons must be moving more quickly for a constant cross-section.
- This incidentally causes the temperature of a semiconductor to increase.
Do charge carriers move at a high or low velocity?
Although the effects of charge flowing seem instant, charge carriers actually move at a relatively slow velocity. This is because free electrons collide frequently with metal ions, so their path is very indirect and therefore their velocity is very low overall.
