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COE
Electric current at a particular point in a circuit is the rate of flow of positive charges past that point
I = Q/t (scalar)
- Units: A, ampere or ?? −1
- Conventional flow of current = opp flow of electrons = flow of positive charges
- Electron has charge of 1.6 x 10^-19 C
Electric charge, which flows past a point in time when there is a constant current, is the product of current and time.
One coulomb is the quantity of electric charge passing a point in the circuit when there is a constan

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COE
Electric current at a particular point in a circuit is the rate of flow of positive charges past that point I = Q/t (scalar) - Units: A, ampere or
−
- Conventional flow of current = opp flow of electrons = flow of positive charges - Electron has charge of 1.6 x 10^-19 C Electric charge, which flows past a point in time when there is a constant current, is the product of current and time. One coulomb is the quantity of electric charge passing a point in the circuit when there is a constant current of one ampere for a time of 1 second. Q=It (assuming current is constant) For inconsistent current,
= ∫
= area under current-time graph Unit: Coulomb, C = As Magnitude of electric current is affected by: (worked eg 2.4) 1) Mean drift speed of charged particles - Drift speed refers to the average distance traveled by a charge carrier per unit of time. 2) Physical dimension of conductor 3) Type of material Potential difference between two points in a circuit is the amount of electric energy that is converted to other forms of energy when a unit charge passes from one point to another. One volt is the potential difference between two points in a circuit in which one joule of energy is converted when one coulomb of charge passes from one point to the other. Apply PD, electric field created
electric field exerts forces on the conduction electrons, causing them to move in the wire, thus creating a net movement of electric charges
flow of electrons As positive charge moves from positive to negative terminal, its electric potential energy decreases due to loss of energy to heat/light/etc energy V=W/Q W = energy converted from electrical to other forms = Pt Q=It V = W/Q = Pt/It = P/I P = rate at which energy is converted/power The resistance of a conductor is the ratio of the p.d. across it to the current through it. One ohm is the resistance of a conductor when a current of one ampere passes through it when the p.d. across it is one volt. R = V/I
Units: Ω or
−
Ohm’s law states that:
Under constant physical conditions, the steady current flowing through a metallic conductor is directly proportional to the pd between its ends *assumption: constant physical conditions (constant temp, pressure etc)
- no. of free electrons is fixed, rate of atomic vibration is constant
constant resistance
I/V is constant
I
V
straight line graph passing thru srcin for current pd graph - Gradient is resistance -
Conductors tt follow ohm’s law are ohmic conductors
Filament lamps - non-ohmic - Metal contains a lot of FREE electrons
as electrons move thru metal lattice, they collide with vibrating metal ions
collisions oppose flow of electrons, causing metal to have resistance - Current increases
metal filament becomes hotter
ions vibrate faster with greater amplitude
more difficult for electrons to pass thru metal lattice
greater opposition of flow of electrons
higher resistance
Graph becomes less and less steep as current increases from zero (V/I increases) * caused more by increased vibration than increase in no. of charge carriers Semiconductor diode - Forward bias diode (A)
–
very low resistance, when PD applied, large current that increases very quickly when pd > 0.7V - Reverse bias diode (B)
–
very high resistance, when PD applied, very small current Thermistor (a kind of semiconductor) - Has negative temperature coefficient of resistance
Temp increase, resistance decreases - in semiconductors, no. of free electrons is small compared to metals
low temperatures, poor conductor - as temp rises, more and more electrons break free from atoms
more free electrons
better conductor * caused more by increase in no. of charge carriers than increased vibration Resistivity R =
A is cross sect area L is length of conductor/wire Units
High resistivity
higher R
poorer conductor
From V=P/I earlier, P = VI = power supplied by source = rate of energy conversion in electrical device when there is pd across it and current passing thru it Power Rating The rated power is the rate at which energy is used (power consumption) by a device when the device is operating at the rated potential difference across it. i.e. For an electrical device rated 100 W, 220 V, it means that when a potential difference of 220 V is applied across the terminals of the device, the device dissipates 100 W of power. *brightness of bulb depends on power Kilowatt-hour = kWh can be used instead of joule 1kWh = 1 x 1000 x 60 x 60 = 3.6 x
10
6
J One electronvolt is the energy transferred when an electron is moved through a potential difference of 1 volt. 1eV = 1.6 x
10
−9
J Electromotive Force (e.m.f.) of a source is the amount of energy converted from other forms to electrical energy when the source drives a unit charge round a complete circuit. E = emf = W/Q Unit: Volt, V Difference between EMF and pd - EMF is energy from other forms being converted to electrical energy - PD is electrical energy being converted to other forms - PD is btwn 2 points, EMF is whole circuit **Battery has internal resistance (r) Some electrical energy is converted to heat within battery due to internal resistance Terminal pd = PD across battery terminal and the internal resistance < EMF V=IR IR + Ir = EMF -
When there’s no current
no pd across internal r = no heat loss due to internal resist
ance Terminal PD = EMF
y-intercept -
When there’s current, terminal pd = EMF –
Ir, by math (y=mx+c) , r = gradient, BY COE, Rate of energy supplied by source = rate of heat loss by resistance of circuit and internal resistance
== ∗=
+
EMF = I(R+r) MAXIMUM POWER THEOREM Determining the value of R that results in max. power delivered to output From output Power =
2
and EMF = I(R+r), I = EMF/(R+r) = E/(R+r) (P =
) P =
2
(
+
)
2
Since we want max power, dP/dR = 0 Resistance of load should be the same as internal resistance for max power delivered to a load Read eg 4.2

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