Chapter 12 - Electric current Flashcards
Definition of current
The rate of flow of charge in the wire or component
What two things do we need to make an electric current pass through a circuit?
- Circuit must be complete
- There must be a source of potential difference (like a battery) in the circuit.
In metals, what are the charge carriers?
Conduction electrons, move about inside the metal, repeatedly colliding with each other and the fixed metal ions in the metal.
What is the magnitude of the charge of an electron?
1.6x10^-19 C
What is an insulator and what gives it these properties?
Each electron is attached to an atom and can’t move away from the atom. When a voltage is applied across an insulator, no current passes through and no electrons can move through the insulator.
What is a metallic conductor and what gives it these properties?
Most electrons are attached to atoms but some are delocalised (the charge carriers). When a voltage is applied across the metal, these conduction electrons are attracted towards the positive terminal of the metal.
What is an semiconductor and what gives it these properties?
The number of charge carriers increases with an increase in temperature. The resistance of a semiconductor decreases as its temperature is raised. A pure semiconducting material is referred to as an intrinsic semiconductor as conduction is due to electrons that break free from the atoms of the semiconductor.
What is the definition of potential difference?
The work done per unit charge
What is the work done by an electron equal to?
Its loss of energy
What is the definition of the Emf of a substance?
The electrical energy produced per unit charge passing through the source
In a device that has resistance what is the work done on the device transferred as and why?
It is transferred as thermal energy as the charge carriers repeatedly collide with atoms in the device and transfer energy to them, so the atoms vibrate more and the resistor becomes hotter.
In an electric motor turning at a constant speed, what is the work done on the motor equal to?
Its equal to the energy is transferred to the load and surroundings by the motor, so the kinetic energy of the motor remains constant.
For a loudspeaker what is the work done on the loudspeaker transferred as and why?
It is transferred as sound energy as the electrons need to be forces through the wires of the vibrating loudspeaker coil against the force on the due to the loudspeaker magnet.
What is the resistance of any component defined as?
The potential difference across the component/ the current through it
What is resistance caused by?
The repeated collisions between the charge carriers in the material with each other and the fixed positive ions of the material.
What is giga compared to standard units?
10^9
What is mega compared to standard units?
10^6
What is nano compared to standard units?
10^-9
What is micro compared to standard units?
10^-6
What is the graph for a resistor on a current - voltage graph?
A straight line through the origin. (resistance is the same regardless of the current)
What does ohms law state?
The potential difference across a metallic conductor is proportional to the current through it, providing the physical conditions don’t change.
How to we determine resistivity from a resistance- length graph?
The graphs gradient x Area
What is a superconductor?
A wire or a device made of a material that has zero resistivity at and below a critical temperature that depends on the material.
When does a superconductor material lose its superconductivity?
If its temperature is raised above its critical temperature
What are superconductors used to make?
High power electromagnets that generate a very strong magnetic field in devices such as MRI scanners and particle accelerators.
What is a cell a source of?
Electrical energy
What happens to the resistance of a light dependent resistor as light intensity increases?
The resistance decreases.
