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
For an IV graph on which axis is which variable?
y-axis: Current (A)
x-axis: Potential Difference (V)
What is the equation linking current, resistance and pd?
V = I*R
What are ohmic conductors?
Ohmic conductors are conductors that obey ohm’s law
Why does resistivity (of a metal) increase with increasing temperature?
Charge is carried through metals by free electrons in a lattice of positive ions. The ions in the metal vibrate more when heated, making electrons collide with them more often, losing energy to other forms making it harder for them to move about.
Explain the I/V graph of a filament lamp
1- The curve for a filament bulb starts out steep as it has a low resistance
2- Current flowing through a wire increases its temperature causing the bulb to heat up. This increases its resistance making the gradient shallower
What does the resistance against temperature graph and I-V graph look like for an NTC thermistor?
Ω
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| \ _ _
‘ ─ ─ ─ *C
I-V:
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/
─
/
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What does the resistance against light intensity graph look like for an LDR?
Ω
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| \ _ _
‘ ─ ─ ─ Lux
How does an NTC thermistor work?
It uses semiconductors - Warming the termistor gives more electrons enough energy to escape from their atoms, meaning more charge carriers are available so resistance is lower.
How does an LDR work?
It uses semiconductors. Light provides energy to the outer shell electrons of atoms allowing them to escape. More free charge carriers available means resistance is lower.
What is the threshold voltage for most diodes?
0.6V
What is ‘forwards bias’?
Forwards bias is the direction in which a diode allows current to flow, the direction the triangle points in.
Define the watt (W)
1 watt is equivalent to one joule of work done per second
What is the formula for power in electric circuits?
P = V*I
What are the three formulas for power in circuits?
P = V*I P = V^2/R P = I^2*R
How do you calculate total energy transfer for a circuit?
W = VIt
How do you calculate kWh?
W = P*t (kWh = kW * h)
What is internal resistance?
Internal resistance is the resistance of a cell in a circuit
What is load/external resistance?
Load resistance is the total resistance of all the components in an electric circuit
What is emf, electromotive force?
Electromotive force is the total amount of work a battery does on each coulomb of charge.
What are ‘lost volts’?
The energy wasted per coulomb overcoming internal resistance is called lost volts
What is the terminal p.d. of a circuit?
The terminal p.d. is the work done per coulomb of charge flowing through the load resistance
What are the equations needed for internal resistance calculations?
e = V + v e = IR + Ir
How do you calculate the total emf of cells in:
- Series
- Parallel
Series: The total emf is the sum of all emfs
Parallel: The emf for identical cells in parallel is that of one of those cells.
This is because the current splits equally between identical cells, so each charge only gets the emf of one of the cells
State Kirchhoff’s first law
At any point in a circuit the total current entering that point is equal to the total current leaving it.
State Kirchhoff’s second law
The total emf around a series circuit = the sum of the p.d s across each component
How do you get Kirchhoff’s first law?
The conservation of charge states that as charge flows around a circuit it is not used up. This means as charge flows into a junction it will flow out again. Current is the rate of flow of charge so current is conserved.
How are resistors in series added?
Rt = R1 + R2 + R3
How are resistors in parallel added?
1/Rt = 1/R1 + 1/R2 + 1/R3
What is a potential divider, what does it do?
A potential divider is a circuit with a voltage source and a couple of resistors in series. The p.d. of the voltage source is divided in the ratio of the resistances, so you can choose the resistances to get the voltage you want across one of them.
What is the potential divider equation?
V-out = R2/(R1+R2) * V-in
(Vout of r2)
or
V1/V2 = R1/R2
What is a potentiometer? What does it do?
A potentiometer is a potential divider with a single variable resistor replacing R1 and R2.
By adjusting the relative sizes of the R1 and R2 sections you can vary the output voltage. This allows for a voltage to be changed continuously
What would be necessary for a perfect insulator?
A perfect insulator would have no charge carriers
What is the equation for emf?
e=W/Q
