Current of Electricity

Electric charge Q passing a point is defined as the product of the (steady) current at that point and the time for which the current flows.

Q = It

Q = ne

I is the current

t is the time taken

When charges flow, there is electric current I, therefore,

Electric current is the rate of flow of electric *charge*

I = ΔQ/Δt

The conventional direction of current is that of the +ve charge i.e current moves from the positive terminal to the negative terminal

One coulomb: is defined as the charge flowing per second past a point at which the current is one ampere.

**Potential difference:** Potential difference is defined as the energy transferred from electrical energy to other forms of energy e.g heat, light, sound e.t.c, when unit charge passes through an electrical device.

W = QV

W is the workdone

Q is the electric charge

v is the potential difference

V = W/ Q, the unit is JC-1 or volt

The volt: is defined as the potential difference between two pts in a circuit in which one joule of energy is converted from electrical to non-electrical energy when one coulomb passes from one point to the other, i.e 1 volt = 1 JC-1.

Resistance: is defined as the *ratio* of the potential difference across a component to the current flowing through it, provided temperature( and other physical conditions like resistivity, area of cross section, and length of wire remains constant.)

The Ohm: is the resistance of a resistor if there is a current of 1 A flowing through it when the p.d across it is 1 V, i.e,

1 Ω = One volt per ampere

R = V/I

Power is the rate of energy expended

P = E / t

E = Pt

V = W/Q

Substitute for E

V = Pt / Q

Q = It

t = Q / I

Substitute for t

V = PQ/IQ

V = P /I

P = IV ……….i

Note

V = IR

substitute for V

P = I^2 R

**sketch and explain the I-V characteristics of a metallic conductor at constant temperature, a semiconductor diode and a filament lamp**

For Metallic conductor

For filament lamp

For semiconductor

**Ohm’s Law**: The current flowing through a piece of metal is proportional to the potential difference across it providing the temperature remains constant

* *From the ohm’ law equation for ohmic conductors,

(1) as R increases, the p.d drawn by the load increases, and vice versa, but R is inversely proportional to I, so current decreases because, as resistance increases, current decreases and vice versa.

(2)for a resistor and/(variable) resistor, as the R of one decreases, the p.d of the other increases, its current also increases, because, the

(3)V/I is always a constant value at the same temperature

Resistivity

Resistivity is defined as the resistance of a material of unit cross-sectional area and unit length.

R = Resistance

r = Resistivity of material

L = Length of conductor

A = Area

E.m.f and P.d

Electromotive force Ɛ is defined as the(total) energy transferred / converted from non-electrical forms into electrical energy when unit charge is moved round a complete circuit.

**distinguish between e.m.f. and p.d. in terms of energy considerations**

**e.m.f. = **(energy converted from other forms to electrical) / charge

**p.d. = **(energy converted from electrical to other forms) / charge

Internal resistance

Internal resistance is the resistance to current flow within the power source. It reduces the *potential difference* (not the emf) across the terminal of the power supply *when it is delivering a current*.

Consider the circuit below

E.m.f = I(R +r)

E.m.f = IR +Ir

IR is the terminal P.d

Ir is the lost volt

The greater the internal resistance, then the greater percentage loss in energy per unit charge, and the lower the terminal p.d V( = IR), and vice versa. So reducing the internal resistance of a battery increases the effective/useful energy delivered per unit charge across the external load.

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