Module 4.1 - Electricity ✓ Flashcards
Define the Coulomb (C)
One coulomb (C) is the amount of charge that passes a point in 1 second when the current is 1 Ampere
What is Current?
Current is the rate of flow of charge
What is the equation linking charge, time and current?
I = Q/t current = charge/time
Define the Volt (V)
One Volt (V) is the potential difference across a component when 1 joule of work is done moving 1 coulomb of charge through it
What does charge being ‘quantised’ mean?
The elementary charge is the smallest unit that charge comes in, the net charge on any object will always be a multiple of e, so charge is quantised
What is potential difference?
Potential difference is the work done per unit charge moved
What is the equation linking pd, charge and work done
V = W/Q
How do you get the electronvolt equation?
When a charged particle is accelerated by a pd, the energy transferred to it = the work done on it. W = VQ
The energy transferred to it is equal to the change in it’s kinetic energy, so:
1/2mv^2 = eV
What is the equation for current in a wire? And what does each part mean?
I = Anqv A = cross sectional area n = number density in m^-3 q = charge on charge carrier v = mean drift velocity
What is ‘mean drift velocity’?
The mean drift velocity is the average velocity of the electrons in a wire. They move about randomly but tend to ‘drift’ in one direction.
What is the difference between conventional current and electron flow?
With conventional current, the direction of all electric currents is treated as positive to negative, however electron flow moves from negative to positive.
Explain why when the two electrodes are placed in the sodium chloride solution, the bulb lights up?
The salt separates into positively charged sodium and negatively charged chloride ions when dissolved in water. These ions are attracted towards the electrodes, since opposite charges attract. The Na+ ions move towards the cathode and Cl- towards the anode, this movement of ions sets up a flow of electric charge, a current. When Na+ ions reach the electrode they accept an electron, When Cl- ions reach the anode they donate and electron so electrons can flow through the metal part of the circuit, creating a flow of charge, a current which causes the bulb to light up.
State two differences/similarities between current in metals and current in electrolytes?
- While currents in metals are caused by the flow of electrons, in electrolytes they are caused by the flow of ions
- While in metals, the charge carriers are negatively charged, in electrolytes they can be positive or negative
- While in metals electrons flow in the opposite direction to conventional current, in electrolytes they can flow in the same or the opposite way
Why do semiconductors only conduct past certain temperatures?
Semiconductors have few free charge carriers at low temperatures, so can only conduct a small current. When heated up, the electrons in the atoms have thermal energy transferred to them. If the electrons in the outermost shell gain enough energy to escape and become free electrons, the semiconductor gains charge carriers so can conduct a higher current.
What is an electrolyte?
An electrolyte is a substance containing ions that can conduct electricity
Define the ohm (Ω)?
A component has a resistance of 1 ohm if a potential difference of 1 volt makes a current of 1 amp flow through it.
What is resistance?
A measure of how difficult it is to get a current to flow through a component
What is the equation for resistivity? What is it’s unit?
p = RA/L
Measured in ohm metres (Ωm)
Define resistivity
The resistivity of a material is the resistance of a 1 metre length with a 1 m^2 cross sectional area.
State Ohm’s law?
Provided temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it (V=IR).
What are three factors affecting the resistivity of a wire?
- The length of the wire
- The cross sectional area of the wire
- The temperature
- The metal the wire is made of
