Component 2: Electricity and Light Flashcards
What is charge? What is the unit of electric charge? Which symbol represents it?
Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field.
Coulomb (C)
(C) = Ampere-Second (As)
Charge is represented by the symbol Q.
What is the charge of a proton? What is the charge of an electron?
The charge of a proton is represented by e.
The charge of an electron is represented by -e.
e = 1.6 x 10^(-19)C
(A small fraction of a Coulomb)
Which materials does charge flow through?
Electric charge flows through materials which are conductors. They do not flow through insulators.
What is the law of conservation of charge?
Providing that charges cannot enter or leave a system, the net charge in a system will remain constant.
What is electric current? What is the unit of electric current? Which symbol represents it?
Electric current through a conductor is the rate of flow of charge. The charge passing per unit time through a cross-section of the conductor.
Ampere (A)
(A) = (Cs^(-1))
Current is represented by the symbol I.
In a circuit, which way does charge flow?
In circuits, electrons will flow from the negative terminal of a cell to the positive terminal.
Before the discovery of protons and electrons, scientists made an agreement to say that charge was positive. This conventional current flows in the opposite direction to the flow of electrons. Diagrams with arrows denoting current still show the direction of positive charge.
What is the equation for current (In terms of charge and time)?
I = ΔQ / Δt
Why do metals conduct?
In metal conductors, the positive nucleus of each atom is surrounded by most of the atom’s electrons, making a positive ion. The ions vibrate randomly about a fixed position in regular crystal lattice. There will then be some electrons which are free to flow in the conductor. The free electrons is what makes metals conduct.
What is drift velocity?
Drift velocity is the flow velocity that an electron attains in a magnetic field.
What is the equation for current (In terms of charge per electron, drift velocity, free electron concentration, and cross-sectional area)? How can this equation be derived?
I = nAve
I = Current n = Free electron concentration (the number of free electrons per unit of volume of the conductor) A = Cross-sectional area of the wire v = Drift velocity e = The charge of an electron
Derivation -
If the drift velocity is v, then the electrons will travel a length vΔt in the time Δt. The volume is the calculated by multiplying vΔt by the cross-sectional area A, leaving you with AvΔt. The number of free electrons in this volume is therefore nAvΔt, n representing the number of free electrons per unit of volume.
As the charge of an electron is -e, charge passing through a point in time Δt is ΔQ = -nAvΔte.
I = ΔQ / Δt, can be substituted in and you are left with I = nAve. (you drop the - sign).
How is current measured in a circuit?
An ammeter is an instrument used to measure current. It is placed in series, not in parallel.
The ammeter can be placed anywhere in a circuit (as long as it is in series) and it will read the same result, as the current is the same throughout.
What is potential difference?
The potential difference, V, between two points on both sides of a component is the work done (by charge), that is the loss of electrical potential energy, per unit of charge passing between the two points.
What is the unit of potential difference and how is it measured?
The symbol for potential difference is V (sometimes p.d.).
It is measured in volts (V)
(V) = (JC^(-1))
To measure the p.d. between two points (usually across a component) a voltmeter is placed in parallel with the circuit, connected to the two points where you want to measure the p.d.
Example…
If a voltmeter reads 5V, then for every coulomb passing between the two points, 5J of energy is done, resulting in 5J of energy changing from electrical potential energy to another form.
What is the equation for potential difference?
V = W / Q (JC^(-1)) = (J) / (C) (V) = (JC^(-1))
What is the equation for work done by charge?
Work done = Electric potential energy lost per unit of charge * Charge passing
Work = VIΔt (not provided in exam)
What is power? What is the symbol for power? What is the unit of power?
Power is the rate of doing work, or the rate of transfer of energy.
Power is represented by P.
It is measured in Watts.
What is the equation for power in terms of current and voltage? How is it derived?
P = IV
(Js^(-1)) = (Cs^(-1))(JC^(-1))
(W) = (Js^(-1))
(Provided in exam)
Power = Work done / Time taken
P = VIΔt / Δt P = VI
What is Ohm’s Law? What is resistance?
V = IR (Provided in exam) (R = V/I) (VA^(-1)) = (V)/(A) (VA^(-1)) = (Ω)
The current through a conductor is proportional to the pd across it when at a constant temperature.
What is a I-V graph and what do they show?
An I-V graph is a graph with current (I) through a conductor plotted on the y-axis and the pd (V) across it on the x-axis. The gradient of this line is the resistance (Ω) of the material.
What are the characteristics of an I-V graph for a filament lamp? For a metal wire at a constant temperature?
A filament lamp is an example of an non-ohmic conductor. The graph starts with a steep incline but then begins to level out.
The metal wire at a constant temperature is ohmic and the graph increases proportionally.
What equations can be derived from P =IV and R = V/I
P = IV P = I^(2)R P= V^(2)/R
(These derived equations are not provided in the exam).
What is conductance? What is conductance in terms of resistance?
Conductance, G = I/V
(AV^(-1)) = (A)/(V^(-1) (AV^(-1)) = Siemens (S)
G = 1/R
Not in specification.
How does resistance arise in a metal?
A metal will have finite resistance - due, essentially to collisions between free electrons and vibrating ions. Electrical resistance increases with temperature.
What is resistivity?
The resistance, R, of a wire of length, l and cross-sectional area, A, is given by…
R = ρl/A
in which ρ is a constant for the material of the wire at a given temperature, called its resistivity.
(Ω) = (Ωm)(m)/(m^2)
The lower the resistivity, ρ, the better a material conducts electricity. At room temperature (20°C), most relativities measure between 1x10^(-8)Ωm to 10x10^(-8)Ωm.
What is conductivity?
The conductivity, σ, of a material at a particular temperature is the reciprocal of its resistivity.
σ = 1/ρ
G = σA/l
Not in specification.
How does the resistance of a material depend on temperature?
A metal wire’s resistance increases with temperature. It is mainly due to the change in resistivity, ρ, of the material as thermal expansion makes very small changes to l and A. When temperature is plotted against resistivity there is a positive correlation. Different materials will have different gradients.
Free electron explanation:
The higher the temperature, the greater vibration amplitude of the ions, creating more collisions. This decreases the drift velocity. As the drift velocity decreases, current decreases and therefore resistance increases.
What is a superconductor?
A superconductor is a material that, below a certain temperature (the superconducting transition/critical temperature), loses all of its resistance.
How do superconductors work?
Once the conductors reach a certain sub-zero temperature (critical temperature), the amplitude of the vibrations of the lattice ions decreases to a negligible amount, meaning that resistance becomes exactly zero. To reach this critical temperature, conventional conductors are cooled with liquid helium whereas some cuprate superconductors can be cooled using liquid nitrogen. Nitrogen gas is more abundant then helium in the atmosphere so superconductors which can reach their transition temperature using liquid nitrogen are more sustainable.
Also at these low temperatures, Cooper pairs are formed within the material. These occur when electrons pair together within the conductor, and become less scattered, allowing them to move more freely, with no collisions.
What temperatures do different materials superconduct?
Many metals have a transition temperature within a few degrees of absolute zero (-273°C). The critical temperature of magnesium diboride (MgB2), a conventional superconductor, is -234°C. This is the highest known critical temperature amongst conventional superconductors.
Certain ceramic materials have a transition temperature slightly above -196°C, the boiling point of liquid nitrogen.
Some mercury-based cuprate superconductors have been discovered to have critical temperatures in excess of -143°C.
What are some uses of superconductors?
Superconducting magnets are extremely powerful. They are used in Magnetic Resonance Imaging (MRI) scanners in order to force protons in the body to align with the magnetic field. The process is usually to identify disease in the body. So the magnets are very useful in saving lives and identifying health risks. They are also very important in other aspects of science including particle accelerators.
Future applications may include generators, motors, transformers, magnetic levitation devices and magnetic refrigeration. In the future they may become commonplace in most circuits and household items to increase efficiency of products.
What temperature is named ‘absolute zero’?
Absolute zero = -273.15°C = 0°K
How can the I-V characteristics of the filament of a lamp and a metal wire at constant temperature be investigated?
- A variable voltage supply, an ammeter (series), the conductor/filament lamp, and voltmeter (parallel) are placed in a circuit.
- The voltage supply is increased gradually, and readings are taken from the ammeter and voltmeter. (10+ readings for the filament bulb, less for the metal wire.)
- The results of each are plotted on an I-V graph, with current, I, on the y-axis, and voltage, V, on the x-axis.
- If the trend of the graph is not clear, more readings can be taken to have an accurate graph.
- I-V characteristics will then be clear.
How can the resistivity of the metal of a wire be determined?
- In order to calculate the resistivity of the wire, the resistance, length and cross-sectional area must be found.
- Resistance can be calculated by using a ohmmeter. (Alternatively, it can be calculated from using a battery, ammeter and voltmeter.)
- Length can be found using a meter rule.
- Cross-sectional area can be calculated by measuring the diameter, d, using an electronic caliper or micrometer screw gauge. Taking multiple measurements to find an average increases accuracy. (πd^(2))/4 will calculate the area.
- Multiple readings for R at varying lengths, l,can be plotted on a graph. The resistivity can be calculated by multiplying the gradient by the cross-sectional area of the wire.
How can the dependence of the resistance of a metal wire on temperature be investigated?
Equipment:
- Glass beaker (with water)
- Tripod
- Bunsen burner
- Coil of thin, insulated, waterproof copper wire.
- Ohmmeter
- Thermometer
- Clamp
Method:
- Set up the apparatus so that the beaker of water is placed on the tripod, above the Bunsen burner. With the ohmmeter attached to either end of the coil. The coil and thermometer are both held in place in the water by the clamp.
- Turn on the Bunsen burner and increase the temperature gradually, measuring the resistance on the ohmmeter every time the temperature rises 10°C.
- Resistance can then be plotted against temperature. The gradient should be positive and the line straight.
What happens to current through a series circuit?
Current remains the same throughout a series circuit.
What happens to current when components are added in parallel to a circuit?
The current in both branches of the parallel section add to the current in the series section.
What is Kirchhoff’s first law?
The sum of currents coming into a point in a circuit equals the sum of the currents going out from it.
What is the pd across different components in parallel?
When components are in parallel, the pd is the same across all of them: there is only one pd.
