Current Electricity and Electrical Quantities : Gate Academy

Electric current is the flow of charged particles
Charge at a point is the product of the current at that point and the time for which the current flows
- Q = It
Coulomb is the charge flowing per second past a point at which the current is one ampere
Charge is Quantized: values of charge are not continuous, they are discrete
All charges are multiples of 1e: 1.6 × 10^-19 C
Potential Difference (p.d.): Two points are a potential difference of 1 V if the work required to move 1 C of charge between them is 1 Joule
Volt: is joule per colulomb
P = VI
W = VQ
P = I²R
P = ^V²/_R

Electrons move in a certain direction when p.d. is applied across a conductor causing current
Deriving a formula for current:
- I = ^Q/_t
- t = ^L/_v
- Volume of container = LA
- Number of free electrons = nLA
- Total charge = Q = nLAq
- ∴ I = ^nLAq/_{^L/_v}
- ∴ I = Anvq
- where:
  - L = length of conductor
  - A = cross-sectional area of conductor
  - n = number of free electrons per unit volume
  - q = charge on 1 electron
  - v = average electron drift velocity

Ohm’s Law states that the current in a component is proportional to the potential difference across it, provided all physical conditions (e.g. temperature) stay constant

Resistance is the ratio of potential difference to the current
Ohm is volt per ampere
- V = IR
Resistivity is the resistance of a material of unit cross-sectional area and unit length
- R = ^ρL/_A

You can detect an electrostatic charge using a leaf electroscope
- If a charged object is placed near the cap, charges are induced
- The metal cap gets one type of charge (positive or negative) and the metal stem and gold leaf get the other type of charge so they repel each other
There are 2 types of charges: positive and negative
Unlike charges attract and like charges repel
Electric Field is the region in which an electric charge experiences a force
Conductors are materials that let electrons pass through them
- Metals are the best electrical conductors as they have free electrons
Insulators are materials that hardly conduct at all
- Their electrons are tightly held to their atoms and hardly move, bu they can be transferred by rubbing
The SI unit of charge is the Coulomb (C)

An induced charge is a charge that ‘appears’ on an uncharged object because of a charged object nearby
For example, if a positively charged rod is brought near a small piece of aluminum foil, electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge
The attraction is stronger than repulsion because the attracting charges are closer than the repelling ones

Current is the flow of charge, its SI unit is the Ampere (A)
An ammeter measures the current in a circuit and is connected in series
Current is a rate of flow of charge
- Charge (C) = Current (A) × Time (s)
- Q = It
Current follows the path of least resistance
The conventional current is the opposite of what actually happens:
- red = conventional current
- green = actual current
1 ē = 1.6 × 10^-19
1 C = 6.25 × 10¹⁸ē

The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts
When a current is being supplied, the voltage is lower because of the energy wastage inside the cell
A cell produces its maximum p.d. when not in a circuit and not supplying current

Potential difference is also known as voltage
Voltage is the amount of energy the cell gives the electrons it pushes out.
Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel). If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (^J/_C)
Voltage = ^Energy/_Charge
V = ^E/_C

Ohm’s Law states that the voltage across a resistor is directly proportional to the current through it
This is only true if the temperature of the resistor remains constant