Electric Current Flashcards
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)
Define superconductor
A wire or device made of a material that has 0 resistivity at and below a critical temperature (which depends on the material). This property is known as superconductivity
A material with a critical temperature above ______ is referred to as a ______-_______ ______________
A material with a critical temperature above 77K (-196°C) is referred to as a high-temperature superconductor
What is the highest critical temperature currently claimed and what elements does the compound contain?
150K (-123°C)
Contains Mercury, Barium, Calcium, Copper and Oxygen
Give 2 uses for superconductors?
To make high-power electromagnets that generate very strong magnetic fields
Power cables that transfer electrical energy without wasting energy
Describe the circuit symbol for an ammeter
A circle with an A in the middle
Describe the circuit symbol for a voltmeter
A circle with a V in the middle
Describe the circuit symbol for a cell
2 parallel lines, with the longer line representing the positive terminal
Describe the circuit symbol for an indicator or light source
A circle with a diagonal cross through it
Describe the circuit symbol for a diode
A triangle pointing in its forward direction (the direction in which a current can flow) with a line perpendicular to the line of the wire at the tip of the triangle
Describe the circuit symbol for a light-emitting diode
A triangle pointing in its forward direction (the direction in which a current can flow) with a line perpendicular to the line of the wire at the tip of the triangle. 2 small arrows are pointed away from the triangle
Describe how a diode and light-emitting diode work
A diode only allows current to flow one way (its forward direction)
A light-diode emits light when it conducts a current
Describe the circuit symbol for a fixed resistor
A rectangular box
Describe the circuit symbol for a variable resistor
A rectangular box with an arrow diagonally through the middle
Describe the circuit symbol for a thermistor
A rectangular box with a line diagonally through the middle, which has a small line parallel to the wire at the end
Describe the circuit symbol for a light-dependent resistor
A rectangular box with 2 small arrows pointing towards the box
Describe the circuit symbol for a heater
A rectangular box with 3 lines cutting through the box, perpendicular to the direction of the wire
Describe the circuit symbol for an electric motor
A circle containing M underlined with ┌―┘
Describe how the resistance of a thermistor changes with increasing temperature
The resistance of a thermistor decreases with increaing temperature (if the thermistor is an intrinsic semiconductor - such as silicon)
Describe how the resistance of a light-dependent resistor changes with increasing light intensity
The resistance of a light-dependent resistor decreases with increasing light-intensity
Describe the graph of current against pd for:
i) a wire
ii) a lamp
iii) a thermistor at a a) high temperature
b) low temperature
i) A straight line, with the gradient 1/R
ii) A curved line with decreasing gradient because its resistance increases as it becomes hotter
iii) a) a straight line with a large gradient
b) a straight line with a small gradient
Describe the graph of current against pd for a diode
The current is extremely small (and can only be measured with a miliammeter) before 0.6V (for a silicon diode). At around 0.6V, the diode conducts easily and the gradient of the line increases greatly
State and explain how the resistance of a metal changes with in increase in temperature and why
The resistance increases
Because the positive ions in the conductor vibrate more when its temperature is increased so the charge carriers cannot pass through the metal as easily
Define positive temperature coefficient and give an example
A material where its resistance increases with an increase in temperature
Example : metal
Define a negative temperature coefficient, explain why it exhibits this effect and give an example
A material where its resistance decreases with an increase in temperature
This happens because as the temperature increases, the number of charge carriers increases
Example : An intrinsic semiconductor
