03 electric circuits Flashcards
Define current
A rate of flow of charge.
What is the gradient on a charge-time graph?
Current (I = Q/t).
How do you work out charge in a current-time graph?
Area under the graph (Q = It).
An Ammeter must…
Have a low resistance and must be placed in series.
A voltmeter must…
Have an infinitely high resistance and must be placed in parallel.
Another word for voltage is…
emf / potential difference.
Potential difference is…
A measure of the amount of energy or unit of charge transferred between 2 points in a circuit.
What is Kirchhoff’s first law?
The total current flowing into a point is equal to the current flowing out of that point. Conservation of Charge.
What is Kirchhoff’s second law?
Around any closed loop in a circuit, the sum of the pd across all the components is the pd of the supply. Conservation of energy.
Power (watts) is…
Rate of transferring energy/ rate of doing work.
1 watt of power means…
1 joule of energy is used every second.
Ohm’s law:
V = IR.
In a series circuit the current is…
The same across all the components.
In a parallel circuit the current…
Total current = sum of the current in all the branches.
Potential difference in a series circuit…
Total = sum of pd in all appliances.
Potential in parallel circuits is…
The same across each branch.
Resistance in a series circuit…
Total resistance = the sum of the resistance in all the components.
Resistance in a parallel circuit…
Reciprocal of Resistance = sum of all the reciprocals of the resistance of all the components.
Is a resistor a conductor?
Yes, it’s a conductor so that current can still flow around a circuit, however, resistors let less current flow than other conductors.
What affects the resistance of a resistor?
Cross-sectional area of the wire, length of the wire, material of the wire, temperature of the wire.
What is resistance proportional to?
Length.
What is resistance inversely proportional to?
Cross-Sectional Area.
What is the Resistivity?
The resistance of a 1m² 1m long sample (the resistance per unit cube). It’s a property of a material.
The resistivity is constant providing…
The temperature is constant.
What is the conductivity?
1/ resistivity.
On a resistance-Length graph, what does the line look like?
Positive straight gradient.
On a resistance-length graph, what does the gradient of the line tell you?
Resistivity/ Area.
What does a Resistance-Area graph look like?
A curve getting less steep as it goes down (negative gradient). It’s a 1/X curve.
What does a resistance 1/cross-sectional area graph look like?
Positive straight gradient.
What does the gradient of a resistance 1/cross-sectional area graph mean?
Resistivity multiplied by length.
Prove 1/R1 + 1/R2 + 1/R3 = 1/R:
Itotal = i1 + i2 + i3; i = v/r, hence v/Rtotal = v/R1 + v/R2 + v/R3; v is constant and is the same across all branches, therefore 1/R1 + 1/R2 + 1/R3 = 1 / R.
What is a potentiometer?
A device that provides a potential difference ranging from 0V to Vs (voltage of supply).
Length needed =
Length (of wire) * (voltage out / total voltage).
What do the letters stand for in I = nAvq?
I = Current (A)
, v = Drift velocity (m/s),
A = Cross-sectional area of the conductor (m²)
, n = Charge density (m⁻³),
q = Charge on each charge carrier (C).
What is the charge density?
The number of charge carriers that can move per m³.
A poor conductor has ___ charge carriers:
Few.
A GOOD conductor has ___ charge carriers:
Many.
4V means 4J per…
Coulomb of charge.
5Ω resistance means 5V is needed for…
1A of current.
Vout / Vsupply =
R2 / (R1 + R2).
Prove Vout / Vsupply = R2 / (R1 + R2):
For the whole circuit
Vsupply = I(R1 + R2)
; Vout = IR2, I = Vout / R2;
Vsupply = Vout / R2 x (R1 x R2)
R2 / (R1 + R2).
Define emf:
Energy supplied per unit of charge by the supply.
Define terminal pd:
Energy transferred per unit of charge to the load.
Define lost volts:
Energy transferred per unit of charge as the electrons flow through the cell.
What is the difference between R and r?
R denotes the load resistance and r denotes the internal resistance.
On a graph of current against voltage, what does the y-intercept and gradient show?
Y-intercept shows the emf, gradient shows the internal resistance.
What happens to the graph of current against voltage if there are two cells?
The gradient is twice as steep and the intercept is twice as far up.
Define thermistor:
Resistor whose resistance varies with temperature.
Define LDR:
A light-dependent resistor is a resistor whose resistance varies with light intensity.
LDR relationship with incident light:
As more light is incident, the resistance decreases.
What are the two types of thermistors?
Negative temperature coefficient (NTC) and positive temperature coefficient (PTC).
What is an NTC?
A thermistor where as the temperature increases, the resistance decreases.
What is a PTC?
A thermistor where as the temperature increases, the resistance increases.
how does an increase in temp lead to increase in resistance?
- Increased temperature means increased lattice vibrations, which leads to more electron collisions. This reduces v in I = nAqv and reduces the current (i.e., increases resistance).
describe how thermistors / ldrs work
- Energy absorbed (light or heat) results in the release of conduction electrons (this increases n in I = nAqv so increased current, therefore reduced resistance).
How does current flow?
Free electrons (already in the conductor) are repelled by the negative terminal and attracted by the positive terminal. The positive terminal is the long side of a cell. The arrows in a circuit are drawn the opposite way around.
Insulators:
Have a low number of electrons per m³ and so do not conduct well.
Number of electrons =
Total charge / charge of an electron.
1 coulomb:
The amount of charge that passes a point where a current of 1 amp flows for 1 second.
1 volt of p.d.:
A joule of electrical energy transferred for each coulomb of charge.
Basic points of I = nAvq proof:
Wire volume = Ad (area * length); number of electrons in volume of wire = nAd;
total charge of all electrons = nAdq
I = Q/t, so I = nAdq/t;
V = d/t, so I = nAvq.
define resistance
The ratio of the p.d. applied across it to the current passing through it.
What affects the resistance of a resistor/wire?
• Length (longer means more resistance), • Cross-sectional area (smaller means more resistance), • Temperature (in metals, a hot wire means more resistance), • Type of material.
define Resistivity ρ:
The resistance of a 1m² 1m long sample, measured in ohmmeters.
Filament lamp current voltage graph:
The resistance increases as the current increases. As the wire gets hotter, the current doesn’t increase as much.
LED/diode current voltage graph:
Semiconductor; therefore, the reverse diode has very high resistance, and the forward diode has very low resistance. They only conduct when they are forward biased.
As temperature increases, resistance increases because (in metals):
The ions in the metal lattice vibrate faster, with greater amplitude. This means electrons collide more with ions, opposing the flow and increasing resistance.
As temperature increases, resistance decreases because (in semiconductors):
At room temperature, there are few free electrons available for conduction. At high temperatures, some electrons have enough energy to ‘escape’ from their atoms and be able to conduct.
what is a Superconductors
If a wire is cooled to a low enough temperature, it loses all its resistance. Electrons flow through them without any transfer of energy.
what is a Fuse
A thin wire that melts if the current gets too high.
Kilo-watt hour:
The electrical energy transferred by a 1kW device in 1 hour.
Variable resistor uses:
- Control current (rheostat)
, * To control voltage (potentiometer).
how to work out the Root mean square:
Square all the values to make them positive. Then find the mean, then square root this mean.
Root mean square uses in electricity:
Finding an average voltage or current when it’s alternating. (A regular average would just be zero).
what is the Potential:
The amount of energy available at a point in a circuit.
How does the potential along a wire vary with distance?
As distance increases, potential decreases.
What is current?
A flow of charged particles which are already in the conductors. Free electrons are repelled by the negative terminal and attracted by the positive terminal.
Which way does current flow?
Short side of the cell to the long side of the cell (negative to positive, it’s drawn the other way around as conventional current).
Precautions to obtain accurate values for temperature when making a thermistor:
• Stir the water / make sure the thermometer and coil are in the same part of the beaker, • Check for zero error, • Read temperature at eye level to avoid parallax, • Switch on/off between readings to avoid the wire heating up, • Use a small current to avoid the wire heating up.
Explain why resistance increases using the structure of the metal:
• As temperature increases, the lattice ion vibrations increase. Therefore, electrons will collide more frequently with the vibrating ions. • More energy dissipated by collisions means greater V required. • Since V increases and R = V/I, R will increase with temperature.
Explain, in terms of energy, the difference between potential difference (p.d.) and electromotive force (e.m.f.):
• p.d. is electrical energy transferred between two points in a circuit, • emf is the energy supplied to the circuit.
What happens inside a cell as load resistance increases?
• Total resistance increases. • If emf remains constant, then the current inside the cell must increase.
Why may large wires be used?
• Prevent energy dissipation, • Allows large currents, • Lower cable resistance.
Vout/Vsupply = for a potentiometer:
Given length / full length.
Vout/Vsupply = for a potential divider:
R2 / (R1 + R2); R2 is the resistor with the voltmeter in parallel over it.
If there is a parallel loop over part of a series circuit and a component blows…
The resistance of the parallel increases, therefore it gets more voltage.
What does increasing r do?
Reduces the terminal pd, V due to the lost volts, so less energy/power output to load.
Frequency of mains supply:
50Hz.
