Basics of electricity
Metals are conductors, as they have delocalized electrons. when these electrons are given energy, they are free to move and act as charge carriers. conduction electrons are attracted toward the positive terminal and repel the negative terminal, so move around the circuit.
in insulating materials, electrons are attached to atoms, and are not free to move. therefore when a voltage is applied. no current will pass.
Semiconductors– A material in which number of the delocalized electron increases with temperature therefore resistance decreases with temperature.
Current I- rate of flow of charge around the circuit – Amps A
- measured using an ammeter (in series)
Voltage V (Potential difference) – Work done or energy transferred per unit charge – Volts V
measured using a voltmeter
work done / energy = W/E – energy transferred = Joule J
Charge Q – Charge of electrons in time frame- Coulombs C
Q = I*t
Resistance R – the difficulty of making the current pass through. – ohms Ω
R= V/I
Power P- rate of work done – Watts W
Current Voltage Characteristics
Ohm’s Law states that p.d across the metallic conductor is propotional to the current through iy provided the physical conditions don’t change.
Resistivity
Resistance is caused by the repeated collision between the charge carrier in the materila, with each other and the positive ion in the fixed lattice of the material.
Resistance therefore cause increase in temperature, as the collision give off the energy in form of heat, this can be used for heating effect in kettle and heaters.
Resistance also increases with increase in temperature . this is because when the positive ions get warmer, they have much kinetic energy and therefore vibrate more. the charge carrier therefore collide more often and its harder for them to pass through the metals therefore they have positive temperature coefficient because its resistance increases with temperature.
The resistance of a intrinsic semi conductor decreases with increase in temperature. this is because, the number of charge carrier increases with temperature. it is therefore has a negative temperature coefficient.
Superconductor – A material that have zero electrical resistivity at and below a critical temperature.
- no heating effect
- used to make high power electromagnet in devices like MRI scanners, Particle accelerators
- reduce energy loss as heat
Resistivity
Every material has different resistance, as it is made of different ions in a different structure. this is referred to as the resistivity of the material.
3 things effects resistance
- The length of the wire : The longer the wire, the furture the electrons to travel and therefore an increase in the resistance.
- Cross sectional area : the wider the wire, the more space the electrons have to flow and therefore easir easier it is to flow and therefore a decrease in the resistance.
- The resistivity of the material
From this we can create an equation for resistance
Circuits
Potential devices
Potential devices – Two resistors in series in which the potential difference across them is shared in a particular ratio.
Voltage is shared between two resistors.
The ratio of potential difference of the resistor is equap to the ratio of the resistance
if there are multiple components, you can add them together to make it easier.
