Electric Current Flashcards
(48 cards)
Define electrical current
The rate of flow if charge in a wire or component
State the charge carriers in:
i) a metal wire
ii) a salt solution
i) electrons
ii) ions
State and define the unit for current (I)
Unit : Ampere (A)
The force between two parallel wires when they carry the same current
State and define the unit for charge (Q)
Unit : Coulomb (C)
Equal to the charge flow in 1 second when the current is 1 ampere
Give the equation for charge flow (ΔQ)
ΔQ = IΔt
Define an insulator and explain its properties
In an insulator, each electron is attached to an atom and cannot move from the atom.
When a voltage is applied, no current passes through the insulator because no electrons can move through the insulator
Define a metallic conductor and explain its properties
In a metallic conductor, most electrons are attached to atoms but some are not; these are the charge carriers in the metal.
When a voltage is applied across the metal, these conduction electrons are attracted towards the positive terminal
Define a semiconductor and explain its properties
In a semiconductor, the number of charge carriers increases with an increase of temperature. The resistance of the semiconductor therefore decreases as its temperature is raised
Define potential difference
The work done per unit charge across a component
V = W / Q
Define emf
The electrical energy produced per unit charge passing through the source
Describe the transfer of energy from a charge carrier to heat
If a component has resistance, the work done on the component is transferred as thermal energy because the charge carriers collide with the atoms in the component more, transferring energy to them, so the atoms vibrate more and the resistor becomes hotter
Describe the transfer of energy from a charge carrier to kinetic energy
In an electric motor, the work done on the motor is transferred as kinetic energy of the motor because the charge carriers are electrons that need to be forced through the wires of the spinning motor coil, against the opposing force on the electrons due to the motor’s magnetic field
Describe the transfer of energy from a charge carrier to sound energy
In a loudspeaker, the work done on the loudspeaker is transferred as sound energy because the electrons need to be forced through the wires of the vibrating loudspeaker coil, against the force on them due to the loudspeaker magnet
Give the equation for work done in terms of current and potential difference
W = IVΔt = ΔE
Derive the equation for electrical power from work done
Power = energy / time P = IVΔt / Δt = IV P = IV
Give the unit for power
Watts (W)
Define resistance
The resistance of a component in a circuit is a measure of the difficulty of making current pass through the component
R = V / I
What causes resistance?
The repeated collisions between the charge carriers in the material with each other and with the fixed positive ions of the material
Give the units for resistance
Ohm (Ω)
Give the order of the following prefixes from lowest to highest and give their symbols and values:
- micro
- kilo
- mega
- giga
- nano
- mili
nano - n - x10⁻⁹ micro - μ - x10⁻⁶ mili - m - x10⁻³ kilo - k - x10³ mega - M - x10⁶ giga - G - x10⁹
Define a resistor
A component designed to have a certain resistance, which remains the same regardless of the current through it
State Ohm’s law
The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change
Describe an experiment to find the resistance of a resistor
A circuit is set up containing a variable resistor, an ammeter and a resistor with a single voltmeter connected to either side of the fixed resistor
The variable resistor is used to adjust the current and potential different of the circuit.
Plotting a graph of pd vs current gives a straight line with a gradient equal to the resistance of the fixed resistor
What is resistivity (ρ) and what is its units?
Resistivity is a constant for a material:
resistivity, ρ = RA / L
where R is the resistance, A is the cross sectional area and L is the length
Units : Ohm metre (Ωm)
