why bulb is used as the resistance in ohms law circuit diagram
Answers
Answer:
Three quantities which are fundamental to electric circuits are current, voltage (potential difference) and resistance. To recap:
Electrical current, I, is defined as the rate of flow of charge through a circuit.
Potential difference or voltage, V, is the amount of energy per unit charge needed to move that charge between two points in a circuit.
Resistance, R, is a measure of how `hard' it is to push current through a circuit element.
We will now look at how these three quantities are related to each other in electric circuits.
An important relationship between the current, voltage and resistance in a circuit was discovered by Georg Simon Ohm and it is called Ohm's Law.
Ohm's Law
The amount of electric current through a metal conductor, at a constant temperature, in a circuit is proportional to the voltage across the conductor and can be described by
I=
V
R
where I is the current through the conductor, V is the voltage across the conductor and R is the resistance of the conductor. In other words, at constant temperature, the resistance of the conductor is constant, independent of the voltage applied across it or current passed through it.
Ohm's Law tells us that if a conductor is at a constant temperature, the current flowing through the conductor is directly proportional to the voltage across it. This means that if we plot voltage on the x-axis of a graph and current on the y-axis of the graph, we will get a straight-line.
ff9eaad1559c2750f07e8fc8ef30a0d1.png
The gradient of the straight-line graph is related to the resistance of the conductor as
I
V
=
1
R
.
This can be rearranged in terms of the constant resistance as:
R=
V
I
.
Ohm's Law
Aim
To determine the relationship between the current going through a resistor and the potential difference (voltage) across the same resistor.
Apparatus
4 cells, 4 resistors, an ammeter, a voltmeter, connecting wires
b6abf134019a08713ec8fabf409457aa.png
Method
This experiment has two parts. In the first part we will vary the applied voltage across the resistor and measure the resulting current through the circuit. In the second part we will vary the current in the circuit and measure the resulting voltage across the resistor. After obtaining both sets of measurements, we will examine the relationship between the current and the voltage across the resistor.
Varying the voltage:
Set up the circuit according to circuit diagram 1), starting with just one cell.
Draw the following table in your lab book.
Number of cells Voltage, V (V) Current, I (A)
1
2
3
4
Get your teacher to check the circuit before turning the power on.
Measure the voltage across the resistor using the voltmeter, and the current in the circuit using the ammeter.
Add one more 1,5 V cell to the circuit and repeat your measurements.
Repeat until you have four cells and you have completed your table.
Varying the current:
Set up the circuit according to circuit diagram 2), starting with only 1 resistor in the circuit.
Draw the following table in your lab book.
Voltage, V (V) Current, I (A)
Get your teacher to check your circuit before turning the power on.
Measure the current and measure the voltage across the single resistor.
Now add another resistor in series in the circuit and measure the current and the voltage across only the original resistor again. Continue adding resistors until you have four in series, but remember to only measure the voltage across the original resistor each time. Enter the values you measure into the table.
Analysis and results
Using the data you recorded in the first table, draw a graph of current versus voltage. Since the voltage is the variable which we are directly varying, it is the independent variable and will be plotted on the x-axis. The current is the dependent variable and must be plotted on the y-axis.
Using the data you recorded in the second table, draw a graph of voltage vs. current. In this case the independent variable is the current which must be plotted on the x-axis, and the voltage is the dependent variable and must be plotted on the y-axis.
Conclusions
Examine the graph you made from the first table. What happens to the current through the resistor when the voltage across it is increased? i.e. Does it increase or decrease?
Examine the graph you made from the second table. What happens to the voltage across the resistor when the current increases through the resistor? i.e. Does it increase or decrease?
Do your experimental results verify Ohm's Law? Explain.
Questions and discussion
For each of your graphs, calculate the gradient and from this determine the resistance of the original resistor. Do you get the same value when you calculate it for each of your graphs?
How would you go about finding the resistance of an unknown resistor using only a power supply, a voltmeter and a known resistor R0?