4.1 Current and Charge
Electric current is rate of flow of charge, due to the passage of charged particles (charge carriers)
In metals, charge carriers are conduction electrons.
In solutions, charge carriers are ions.
Current, I, is measured in ampere (A)
Charge, Q, is measured in coulomb (C)
Current travels from positive to negative.
Charge of electron=1.6x10^-19C
- In an insulator, electron's are attached to atoms and can't move, so no current passes through as no electrons can move.
- In metallic conductor, most electrons are attached but some aren't, so they are the charge carriers.
- In semiconductors, number of charge carriers increases with temperature, so resistance decreases as temperature rises.
4.2 Potential difference and power
Potential difference is work done per unit charge.
The emf of a source of electricity is the electrical energy produced per unit charge passing through the source. (V)
In devices with resistance, work done on device is transferred as thermal energy.
In motors work done is transferred as kinetic energy, due to the motor's magnetic field.
Work done W (Joules) W=IVΔt
Electrical Power, P (Watts) P=IV
Resistance of any component is the pd across the component/The current through it. (Ω)
Resistors are designed to have a certain resistance, regardless of current through it. So, ther graph of a resistor, current against pd, gives a straight line going through the origin.
Ohm's law states that the pd across a metallic conductor is proportional to the current through it, provided the physical conditions don't change.
For conductor length L, cross-sectional area A, resistance R, resistivity is:
Resistivity, ρ (Ωm) = RA/L
Superconductor has zero conductivity at and below a critical temperature.
When a current goes through it, there's no pd as resistance is zero, so no heating effect.
It loses it's superconductivity if it's temperature rises above the critical temperature, and they're used for high-power electromagnets for very strong magnetic fields, and power cables to transfer energy without loss.
4.4 Components and their characteristics
Diode Light-emitting diode
Thermistor Variable resistor
Light-dependent resistor Heater
On a graph of pd against current:
Wire gives straight line with gradient equal to 1/Resistance
Filament bulb gives curve with decreasing gradient, as filament increases with temperature.
Thermistor at constant temp. gives straight line. The higher the temp, the higher the grad.