Unit 1: Electric Current and Direct Current Circuits Flashcards
What is electric current?
The rate of flow of charge.
What needs to happen in order to pass an electric current around a circuit?
The circuit must be complete and there must be a source of potential difference.
What is the current due to?
The passage of charge carriers.
What happens when a current passes through a metal?
The charge carriers are electrons and they collide with each other and the fixed positive ions in the metal.
What is conventional current?
When the direction of current goes from the positive terminal to the negative terminal. It is opposite to the flow of electrons.
What is the unit for current?
Ampere
What is the unit for charge?
Coulomb
What is one coulomb equal to?
The charge flow in one second when the current is one ampere.
What is the symbol for charge?
Q
Why does an insulator not conduct electricity?
Each electron is attached to an atom and cannot move away from it.
What happens to the number of charge carriers in a semiconductor as temperature increases?
The number of charge carriers also increases. The resistance therefore decreases.
What happens in an intrinsic semiconductor?
Electrons break free from the atoms of the semiconductor.
What do electrons deliver from the battery to the components?
Energy
What does the battery supply each electron with?
Electrical potential energy
What is the work done by an electron equal to?
The loss of potential energy.
What is potential difference/voltage?
The work done per unit charge.
What is the emf of a source?
The electrical energy produced per unit charge passing through the source.
What is electrical energy produced equal to?
Charge x emf
What may happen due to a source’s internal resistance?
Some energy may be dissipated
What effect does an electric current passing through a component have?
A heating effect and a magnetic effect.
What is power equal to?
Energy/time
How do you calculate power?
IV
What is the unit for power?
Watt
What is resistance?
A measure of the difficulty of making current pass through the component.
What is resistance caused by?
Repeated collisions between the charge carriers and the fixed positive ions in the metal.
How can you calculate resistance?
V/I
What is the unit of resistance?
Ohm
How do you connect an ammeter?
In series
How do you connect a voltmeter?
In parallel
Why must no current pass through the voltmeter?
So that the ammeter can accurately record the current passing through the component.
What is the gradient of a pd/current graph?
Resistance
What is the gradient of a graph when current is plotted on the y axis and pd on the x axis?
1/R
What is ohm’s law?
The pd across a metallic conductor is proportional to the current through it, provided the physical conditions do not change.
What is resistance equal to in terms of resistivity?
PL/A where p is resistivity (a constant)
What is the unit of resistivity?
Ohm metre
What is a superconductor?
A wire or device made of material that has zero resistivity at and below a critical temperature that depends on the material.
Why does the current have no heating effect when it passes through a superconductor?
There is no resistance therefore there is no potential difference.
When does a superconductor lose its superconductivity?
If it’s temperature is raised above its critical temperature.
What are superconductors used to make?
High-power electromagnets that generate very strong magnetic fields and power cables that transfer electrical energy without wasting energy.
What is a battery?
A combination of cells.
What is a cell?
A source of electrical energy.
What does a diode do?
It allows current to flow in one direction only.
What happens when a light-emitting diode conducts?
It produces light.
What is referred to as the forward direction?
The direction in which the diode conducts.
What is referred to as the reverse direction?
The opposite direction to which the diode conducts.
When is a diode used?
To protect dc circuits.
What happens to the resistance of a thermistor with increasing heat?
It decreases.
What happens to the resistance of a light dependent resistance with increasing light?
It decreases.
What can you use to measure the variation of current with pd?
A potential divider to vary the pd from zero or a variable resistor to vary the current to a minimum.
For a current and pd graph for a thermistor, which temperatures would provide a steeper gradient?
High temperatures.
Why does the graph for a filament bulb have a decreasing gradient?
It’s resistance increases as it becomes hotter.
When does a silicon diode start to conduct?
0.6V
Why does resistance increase when temperature does?
The positive ions in the metal vibrate more when the temperature is increased. The charge carriers therefore cannot pass through as easily as before, it is said to have a positive temperature coefficient.
What type of temperature coefficient does an intrinsic semiconductor have?
Negative
What is critical temperature also know as?
Transition temperature.
What is the case at any junction in a circuit?
The total current leaving the junction is equal to the total current entering the junction.
Do components use up current?
No
How is current distributed amongst more than two components in a series circuit?
The current reaching each of the components is the same.
What happens if he charge carriers lose energy?
The potential difference is a potential drop
What happens if the charge carriers gain energy?
There is a potential rise
What is the rule for pd in series circuits across the components?
The total pd of the circuit is equal to the sum of the pd of each component.
What is the sum of the emfs in a loop equal to?
The sum of the pds.
In a series circuit what is the total resistance equal to?
The sum of the individual resistances.
What is the total resistance in a parallel circuit given by?
1/R=1/R1+1/R2 …
What happens when a charge carrier loses energy due to a collision?
The force of the pd causes it to accelerate until it collides with another positive ion.
What are other ways of expressing power?
I^2R or V^2/R
Does power supplied depend on the direction of current?
No
How can you calculate energy transferred?
I^2Rt
Why is the terminal pd less than the emf?
Internal resistance
What is internal resistance?
The loss of potential difference per unit current in the source when the current passes through the source.
How can you calculate emf according to internal resistance?
E=Ir+IR
What is the power supplied by the cell equal to?
IE=I^2R+I^2r
When is maximum power delivered to a load?
When load resistance is equal to the internal resistance.
What is the gradient of a terminal pd/current graph equal to?
Internal resistance
What is the y intercept of a terminal pd/current graph equal to?
The emf
What happens to the terminal pd as the current increases.
It decreases
What is terminal pd?
Emf at zero current
How can you calculate terminal pd?
E-Ir
What does a potential divider consist of?
Two resistors in series with a source of fixed potential difference.
What is the ratio of pds across each resistor equal to?
The resistance ratio of the two resistors.
What can a variable potential divider be used for?
To change the loudness of a sound from a loud speaker or to vary the brightness of a light bulb.
