Electricity Flashcards
What is Ohm’s Law?
The current and voltage of a component are directly proportional if the temperature is constant
Define resistance
Voltage across a component divided by the current through a component
Define current
The rate of charge flowing through a component
Define voltage
The energy transferred per unit of charge
Sketch the circuit you would use to produce a current voltage characteristic graph
(cell, ammeter, bulb, variable resistor, voltmeter in parallel)
Explain the current voltage characteristic for a diode
Resistance is very high at low voltage (until 0.6V), diodes only let current flow in one direction
Explain the shape of the filament bulb graph
- Initially, I is proportional to V,
- As the current flows, electrons collide with the metal particles
- As this happens the temperature of the metal increases
- this happens as particles vibrate more rapidly - so more resistance.
- As a result, the gradient of the graph decreases - a greater change in the potential difference is required to cause a change in the current
What experiment do you do to determine the resistivity of a wire
-Measure the thickness of the wire with a micrometre, 5 measurements, get an average
-Set up circuit
-Tape wire to ruler to ensure it is straight and taught
-Attach crocodile clips so wire length is 10cm
-Record current and voltage
-Turn off power supply, increase length by 10cm
-Record current and voltage
-Repeat
What are superconductors
Superconductors are a group of certain metals that have 0 resistance when they are below a certain temperature.
They can be used to produce strong magnetic fields and there is no energy loss
Current in a series circuit…
Is the same at any point
Current flowing into a junction in a parallel circuit…
Is equal to the current flowing out of a junction
Potential difference across the cell of a series circuit…
Is the sum of the potential difference across all of the components
Potential difference in a parallel circuit…
Is the same across each branch
Potential difference in a closed loop
Is the same as in a series circuit
What are the 3 equations for power?
P=IV P=I2R P=V2/R
Define electromotive force, emf
the energy provided to a unit charge passing through a battery (energy in)
Which way does convectional current flow?
from the positive terminal to the negative terminal (clockwise) - opposite to electron flow
Why is the potential difference across the load resistance always less than the EMF of the cell?
There is always a pd across the internal resistance, which reduces the pd across the output
terminals of the cell.
As temperature decreases, thermistor’s resistance ……….. which affects the ………..
decreases / current
Ideal voltmeters have …..
infinite resistance
Ideal ammeters have…..
zero resistance
When EMFs are in parallel
do not add them
How can cells be added to a circuit to make the bulbs light at normal brightness for a longer time?
-cells added in parralell
-so less internal resistance
-so less power wasted
Why is a resistor needed in the circuit with a variable resistor?
-to limit current when variable resistor is set to 0
-so components aren’t damaged