Currents in Electrical Circuits Flashcards
What is a conductor?
A material that an electrical charge can easily pass through. Metals are good conductors thanks to their delocalised electron.
What is an insulator?
A material that electrical charges cannot easily move through. Plastics and rubber are good insulators.
What is a static charge?
An electric charge that cannot move.
Where are static charges usually found?
In insulators
How do insulators gain a static charge?
When some insulating materials are rubbed together, electrons are scraped off one onto the other. The one that loses the electrons gains a positive static charge and the one that gains electrons gains a negative static charge.
What does it mean if a circuit is complete?
The wire gird from one end of the cell/battery to the other.
How must voltmeters be connected to a circuit?
In parallel across the component you are measuring.
How must an ammeter be connected to a circuit?
In series
What is an electric current?
A flow of electric charge.
What side of a cell/battery is positive and which is negative?
The big end is positive and the small end is negative.
What way does a charge travel through a circuit?
From positive to negative
What is the size of a current? What is it measured in?
The rate of flow of charge. It’s measured in amperes, A.
What is the formula for current?
Q
I = —-
t
I = Current (A)
Q = Charge (C - coulombs)
t = Time (secs)
What is potential difference?
The driving force that pushes the current round the circuit. It’s the work done per coulomb of charge.
What is the formula for potential difference?
W
V = ——
Q
V = Potential difference (V)
W = Work done (J)
Q = Charge (C)
What is resistance?
Anything in a circuit that reduces the flow of current.
What is resistance measured in?
Ohms, Ω.
What is Ohm’s law?
V = I x R
V = Potential difference (V)
I = Current (A)
R = Resistance (Ω)
What does a variable resistor do?
You can change its resistance.
How would one get negative readings for potential difference?
Switch the way round the wires are connected to the power source. This means that the current will flow in the opposite direction.
What does a current-potential difference graph show?
How current varies with potential difference.
What do current-potential difference graphs look like for resistors?
Straight line graphs - the current through a resistor is directly proportional to potential difference.
What does the current-potential difference graph look like for a filament lamp?
S- shaped. This is because as more current flows through the filament it heats up which makes it more difficult for the current to get through.
How does temperature affect resistance?
When a component gets hotter, its ions begin to move which makes it harder for the charge to make it through as resistance increases.
Why do most components have a limit to the amount of current they can carry?
Because current increases the temperature of the component meaning eventually its resistance will increase to the point where the current can no longer fit any more charge through.
What is the difference between a cell and a battery?
A battery contains multiple cells.
What does the symbol for an ammeter look like?
A circle within A in it.
What does the symbol for a voltmeter look like?
A circle with a V in it.
What does the symbol for a resistor look like?
A hollow rectangle.
What does the symbol for a variable resistor look like?
A rectangle with an arrow going through it.
What is a diode?
A component that only lets current move freely in one direction through it. This because there’s a very high resistance in the other direction.
What does the symbol for a diode look like?
A circle with an arrow in it indicating the direction of current flow through it.
What does the I-V graph for a diode look like?
It is at 0 for negative values and then goes up very steeply on the positive side.
What do LEDs do?
Light Emitting Diodes emit light when a current flows through it in the forward direction. They can be used to indicate the presence of a current in a circuit.
What does the symbol for an LED look like?
A circle with an arrow in it indicating the flow of current through it. There are two arrows pointing away from it indicating that it emits light.
What three advantages do LEDs have over filament bulbs?
- LEDs have a much longer lifetime
- LEDs are much more energy efficient as they don’t produce as much waste heat energy
- LEDs require a much smaller current so they are cheaper to run
What are two disadvantages of LEDs?
- They are much more expensive than filament bulbs
* You often need multiple LEDs to produce the same amount of light as a filament bulb
What is the symbol for a filament bulb?
A circle with a cross through it
What is an LDR?
A light dependent resistor is a resistor that varies its resistance with light intensity:
- In bright light, resistance falls
- In darkness, resistance is very high
What I the symbol for an LDR?
A circle with a hollow rectangle inside it to represent the resistor. There are two arrows pointing toward the circle to represent the light going in.
What are two used of LDRs?
- Streetlights
* Burglar alarms
What is an NTC Thermistor?
A resistor whose resistance decreases as temperature increases.
What is the symbol for a thermistor?
A resistor with a line going through it. The line is flat on one end.
What is a use of thermistors?
Thermostats
What is a series circuit?
A circuit in which all the components are connected in line. If one were to be removed the circuit would be broken and it would stop working.
How does one work out the potential difference in a series circuit?
Add up the potential differences of all the cells OR add up the potential difference across each component.
What is current like in a series circuit?
The same in all parts of the circuit.
How does one work out the total resistance of a series circuit?
Add up the individual resistances of each component.
What is a parallel circuit?
A circuit in which components have their own branch meaning that if you were to disconnect one of the components it would have no effect on the others.
What is potential difference like in parallel circuits?
The same across every branch as each branches potential difference is the same as that of the source.
What is the symbol for an open switch?
Two dots with a diagonal line not quite connecting them.
What is the symbol for a closed switch?
Two dots connected by a straight line.
What is current like in a parallel circuit?
It is split across the branches. To understand the total current you have to add together the current on each branch.
What is the symbol for a fuse?
A straight line going through a rectangle.
What is the symbol for a motor?
A circle with an M in it.
