Electricity Flashcards
State the potential divider equation.
Vout = Vin x (R1 / (R1+R2))
How does the resistance of a thermistor vary with temperature?
At low temperatures, the resistance is high, but as the temperature increases the resistance decreases
How could you reduce the charge flowing through a resistor, R, in time, t?
Add a second resistor in series as it increases the resistance and the rate of flow of charge decreases
A transmission line delivers 800MW of power at 132kV, losing 1% of transmitted power as heat. What is the resistance?
0.2 ohms
I = P / V = (8x10^6) / 132000 = 60.606 A P = V^2 / R R = (8x10^6) / 60.60 = 0.217 ohms = 0.2 ohms
A potential divider circuit has a cell connected across a thermistor and variable resistor in series. What would cause the potential difference across the thermistor to increase?
Add a resistor across the variable resistor
Describe what a V-I graph would look like for how V varies with I in a cell
A diagonal line downwards to a certain point. Y-intercept is the EMF of the cell, gradient is the negative internal resistance of the cell
What is electric current? State its units.
The rate of flow of charge, measured in amperes, A
What is potential difference?
The work done moving a unit of charge between two points in a circuit. V = W / Q
A cylindrical conductor of length l, diameter D, and resistivity p (rho), has a resistance R. What is the resistance of a cylindrical conductor of length l, diameter D/2, and resistivity p (rho)?
4R
What is resistance?
The ability of a component to restrict the flow of charge. A component has a resistance of 1 ohm if 1A flows through it when a potential difference of 1V is across it
What is meant by an ohmic conductor?
A conductor that obeys Ohm’s law, current is directly proportional to potential difference, providing physical conditions are constant
How can you measure the current in a circuit?
Measure current with an ammeter connected in series with the component
How do you measure potential difference across a component?
Using a voltmeter connected in parallel across the component being measured
What does the gradient of a I-V graph represent?
1/R
Two I-V graphs, one with a steep diagonal line going through the origin, the other with a far lower gradient. Which has a higher resistance?
The shallower gradient, as gradient = 1/R, a smaller value for the gradient equals a higher value for resistance
Why does the current increasing on a filament lamp cause an increase in the resistance?
As current flows through the lamp, electrical energy is converted to thermal energy so the metal ions vibrate with increased amplitude. Electrons collide with them more so resistance increases
What is a diode?
A diode is an appliance that allows current to flow in one direction, past the threshold voltage of around 0.6V. Current is not allowed to flow in the opposite direction unless the voltage is reversed and higher than the breakdown voltage of roughly -30V
Explain what is meant by a potential divider.
A circuit that uses resistors to supply a variable potential difference
State and explain why it is an advantage for a rechargeable battery to have a low internal resistance.
Small internal resistance gives a small amount of wasted energy so the battery charges more efficiently
Unless stated in the question, should you assume that voltmeters have zero resistance or infinite resistance?
Infinite resistance - current takes the path of least resistance so virtually no current will flow through the voltmeter and all of the current will flow through the component
Why should you assume that an ammeter has zero resistance, unless stated otherwise?
This assumption means that there would be 0V across the ammeter and no energy is lost across it therefore would not affect the current
What is a light dependant resistor?
LDR - a semiconductor that is sensitive to light. As the light intensity increases, its resistance decreases
How does a thermistor work?
Similar to an LDR, but as the temperature increases, the resistance decreases so have a negative temperature coefficient
What is resistivity?
The resistance of a 1m cylinder with a cross-sectional area of 1m^2. It is an inherent property of a material
What is a superconductor?
A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material, often around 10K
How would you find the total resistance in a series circuit?
Add the individual resistances of each component
R = R1 + R2 + R3
If 6 cells, each of voltage 5V, are arranged in parallel, what is the voltage in the circuit?
5V
If 6 cells, each of voltage 5V, are arranged in series, what is the voltage in the circuit?
5V x 6 = 30V
How does the current vary between each component of a series circuit?
The current is the same through all of the current so does not vary
Is the current in parallel components the same?
No - each branch of a parallel circuit can have different currents through them, according to Kirchhoff’s first law
State the equation for charge transferred.
Q = It
Q: charge, C
I: current, A
t: time, s
State Kirchhoff’s first law in the form of an equation.
I = I1 + I2 + I3 …
State the equation for energy transferred using p.d. or emf.
W = QV
W = QE
State the equation for calculating the resistance of two resistors in parallel.
R = (R1 x R2) / (R1 + R2)
What is Kirchhoff’s first law?
All of the current going into a junction is equal to the current leaving the junction
Give the equation for conductance.
G = I / V
G: Conductance, measured in amperes per volt, s
State the equation for potential difference.
Vout = Vin ( R2 / (R1 + R2))
State the equation for power using time.
P = W / t
Give the equation for finding energy transferred using power
E = P x t
State the four equations for finding EMF.
E = V + v E = V + Ir E = IR + Ir E = I (R + r)
State the relationship between EMFs of two similar cells in parallel.
E total = E1
How do you find the total internal resistance of similar cells in parallel?
r1 / n
where n is the number of cells
How would you calculate the EMF of cells in series?
E total = E1 + E2 + E3 …
How would you calculate the total internal resistance of cells in series?
r total = r1 + r2 + r3 …
What is Kirchhoff’s second law?
For any path of a circuit, the sum of all the potential differences must equal the total EMF of the circuit
In a series circuit, if two cells are connected negative too negative, would their EMF add up or cancel out?
They will cancel out as the total EMF would be equal to:
E total = E1 - E2
What is power?
The rate of transfer of energy
Give an equation for power in terms of current and voltage.
Power = Current x Voltage
P = IV
What is the purpose of a potential divider?
To provide variable potential difference or a constant specific potential divider
A circuit is set up with a cell providing a voltage of 12V to two resistors of 6 ohms and 7 ohms respectively, in series. What is the voltage across the 7 ohm resistor?
R total = 6 + 7 = 13 ohms
(7 / 13) x 12 = 6.5V
What is EMF?
Electromotive force: the electrical energy transferred by a power supply per unit charge
Rearrange the equation E = I (R + r) into the form of y = mx + c.
E = V + Ir
So for a graph of V against I: V = -rI + E
Gradient = -r
Y-intercept = E
What are two applications of superconductors?
Power cables, strong magnetic fields
What is the total resistance of a parallel arrangement of 3 ohm, 7 ohm and 9 ohm resistors?
1/R total = 1/3 + 1/7 + 1/9
1/R total = 37/63
R total = 63/37 = 1.7 ohms
State the equation for calculating charge or current.
I = Q / t
Q = I x t
State the equation for potential difference using energy transferred.
V = W / Q where W is the energy transferred
Define resistance and state its units.
The ability of a component to restrict the flow of current. Measured in ohms
What does Ohm’s law state?
For an ohmic conductor, current is directly proportional to the p.d. across it, given that physical conditions are kept constant
Describe what an IV graph or a VI graph will look like fir an ohmic conductor.
Straight, diagonal line through the origin
Describe an IV graph for a semiconductor diode.
Line is horizontal until the threshold voltage (0.6V) where it claims rapidly. It also falls off at approximately -30V knowns as the break down voltage
Why do IV graphs for filament lamp have their characteristic curve?
The component contains a length of wire, which heats up as current increases therefore resistance also increases
What is resistivity a measure of?
How easily a material conducts electricity and is an inherent property of the material
Define resistivity.
The product of resistance and cross-sectional area, divided by the length of the material
