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Physics Paper 2 (Electricity)
Electricity is the flow of electrons from the negative to the positive poles of a power source. It cannot be seen, but the effects can be: magnetic
field, chemical changes (electrolysis) or increased temperature.
Voltage is a measure of the energy per unit charge i.e. Joules per coulomb. Therefore:
V =E Q
Current is a measure of charge in a given time i.e. coulombs per second and is given as:
Power is the rate at which energy is transferred to an element. It is also known as power dissipation.
P = IV = V R = I R
The electrical energy in a circuit is:
E = V It
Electromotive force (EMF) is the energy source of a circuit to make an electrical current flow this could be mains or a cell.
Resistance is the resistance to the flow of electrons in a circuit. It is a ratio between V and I:
Ohms law states that, provided the temperature and other physical factors remain constant, the current through a wire is directly proportional to
the potential difference across its ends.
Ohmic resistors are where current and voltage are always proportional and thus the lines are linear. Nonohmic conductors are not proportional
and thus the curves and +/ exponential.
The resistivity of a wire can be calculated as:
Where is the resistivity of a material.
A superconductor is a material that has no resistivity and thus becomes a perfect conductor e.g. mercury cooled to 41.5K. A superconductor
will continue to carry the current, as long as it is kept below the critical temperature, Tc.
In a series circuit, the current is the same at all locations. In a parallel circuit it splits according to the resistance on each branch.
The potential difference of the circuitry elements is equal to the EMF. The p.d. on each element will depend on its resistance. The potential
difference in a parallel circuit always remains the same.
In a series circuit, the resistors can simply be added to calculate the new resistance i.e.
RT = R1 + R2 + R3
The following shows how to calculate the resistance in a parallel circuit.
V , I = V , I = V and I = V then:
If I1 = R
1 2 R2 3 R3 T RT
V = V V + V and thus,
RT R1 2 R3
1 = 1 1 + 1
RT R1 2 R3
In a potential divider is made of two resistors with the effect of splitting the potential difference across them. The split is of the same ratio to
the difference of the two resistors.
V V EMF ,
If I = R11 , I = R22 and I = R1+R2
V2 = R2
V 1 = ( R1+1R2 )EMF
A potentiometer uses a potential divider to very the voltage supplied. A slider is used to vary the resistance on each side and thus varies
voltage. This is used on volume controllers.
Charge carriers do not necessarily flow directly on the way we understand electron flow. They move very quickly in all directions but have an
average velocity opposite to conventional current. This is much much slower than their actual velocity. This is known as drift velocity.
I = nAvQ
Where I=current, n=number of free electrons, A=cross sectional area, v=drift velocity and Q=charge of charge carrier.
A source of EMF always has some form of internal resistance. It results in the source being less that 100% efficient as it uses up energy to
power itself and the voltage across the terminals drops as current is drawn from it (as resistance increases).
Open circuit voltage is the p.d. of a source with no current being drawn.
The internal resistance is thought of as a resistor in series with the nominal EMF. The voltage across the source is:
V = E - iR
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