REQUIRED PRACTICAL 4: current/PD characteristics Flashcards
we are looking at how current through a component depends on the pd across the component
we are going to start by looking at the resistor.
we use a circuit with a battery connected by wires to a resistor.
the resistor is in series with an ammeter and a variable resistor
we have a voltmeter in parallel across the resistor.
-first we use a voltmeter to read the pd across the resistor
-we then use the ammeter to read the current through the resistor
-we record these values in a table
-adjust the variable resistor and record the new readings on the voltmeter and ammeter.
-we do this a few times to get a range of readings
-next we switch the direction of the battery.
-both the voltmeter and ammeter should now have negative values
-we continue taking a few readings of pd and current
-finally we plot a graph of the current against the pd, we get a straight line passing through zero
-this tells us that the current through a resistor is directly proportional to the pd
(the resistor is an ohmic conductor)
why did we switch the direction of the battery
what happens when when we do this then plot the graph?
so that the direction of the pd
reverses
we will still get a straight line passing through zero
what two things are directly proportional through a resistor
making a resistor an ohmic conductor
the current is directly proportional to the pd
resistor is an ohmic conductor under one condition
if the temperature of the resistor stays constant
what happens to the graph results if the resistor gets hot?
it will not have a straight line passing through zero
what increases the temperature of the resistor?
leaving the circuit connected for too long
we are looking at how current through a component depends on the pd across the component
now we are looking at a filament lamp
the filament lamp is in series with an ammeter and a variable resistor, what do we do to carry out this experiment?
we adjust the variable resistor and read both the potential difference and the current,
we do this for a range of values with the battery in the forward and reverse direction.
in the case of a filament lamp out graph looks different
how does the graph of an experiment with the filament lamp look like?
why ?
diagonal S
passes through zero
the current is not directly proportional to the pd so we do NOT have a straight line
why is the pd of a filament lamp not directly proportional to the current
because as we increase the current, the temperature of the filament lamp increases.
this causes the resistance to increase
what shape will we get if we change the direction of the pd of the filament lamp circuit?
the same shape
stays a diagonal S
we are looking at how current through a component depends on the pd across the component
now we are looking at a diode in the circuit
describe how to carry out this experiment with an ammeter, variable resistor and an extra resistor in series.
- we adjust the variable resistor a number of times to record the pd and current
- we do this with a battery in the forward and reverse direction
- we get a graph shape like __| along three x axis
- in the case of a diode, we only get a current when the potential difference is around 0.6V-0.7V.
- As the pd increases beyond this point, the current rises sharply.
why do we have an extra resistor in the diode circuit?
what does the extra resistor do?
bio sed are easily damaged by a high current, the extra resistor will keep the current relatively low and protect the diode
the extra resistor keeps the current low, what will we need?
a milliammeter
we need a very sensitive ammeter this is called a milliammeter
what happens as the pd increases beyond 0.6V?
the current rises sharply
with a diode, if the pd is reversed, explain the state of the current
explain why
we get NO current if the pd is reversed
in the reverse direction, a diode has a high resistance
