Physics 184
Experiment 2
OHM'S LAW AND POWER
OBJECT:
To verify Ohm's Law, to study the transfer of power from a voltage source to a load resistor, to find
the emf and the internal resistance of a battery.
APPARATUS:
Voltage supply, ammeter, voltmeter, switch, resistance box, connecting wires and a 10 mm to 1
cm linear graph paper.
THEORY:
Ohm's Law states that the voltage across a conductor is proportional to the current in the
conductor:
V=IR,
where V is the voltage in volts, I is the current in am peres, and R is the
proportionality constant which is known as the resistance of the conductor whose unit is in
ohms. The power dissipated in the conductor is the product of the current in the conductor and
the voltage across it:
P = I V,
where P is the power in watts, I is in amperes, and V is in volts.
A voltage source may be considered as consisting of a perfect battery in series with a resistor. A
perfect battery supplies the same voltage no matter what current is supplied, but the current
in the series resistor produces a voltage drop (Ohm's Law). The voltage from the perfect
battery is known as the Emf (for Electro-motive force). In practice, any voltage source will
have some internal resistance and its voltage may be represented by
Vsource =- I Rint
where  is the Emf, I is the current in amperes, and Rint is the resistance in ohms. It should be
noted that IRint is the voltage drop caused by the internal resistance.
PROCEDURE:
1. Connect the apparatus as indicated in the diagram on the next page. In this experiment, the
resistance box serves as the external load of the circuit. Never close the switch at zero-ohm
on the resistance box.
2. Set the resistance box at 20-ohm and dose the switch. Measure the current and the
voltage and calculate the power dissipated in the resistor. Do not hold the switch closed
for long periods of time as that may overheat the resistances and the energy from the battery
will be overdrawn.
3. Repeat step 2 using resistances of 30, 40, 50, 70, 100, 200, and 300 ohms, respectively. Record your
data on the data page.
DATA ANALYSIS
1. From you data for the current and the voltage, plot the voltage across the resistance box versus
the current in the circuit. Draw the best straight line through your data points.
2. From your graph, determine the emf and the internal resistance of the voltage source. The emf will
be the intercept on the voltage axis and the internal resistance is absolute value of the slope of the
line.
3. From you data for the current and the voltage calculate the power delivered by the power
supply for different values of the load resistor.
4. Plot the power in the load resistor versus the load resistance and find the maximum power.
The resistance read at the maximum power is R int ,the internal resistance.
5. Compare the internal resistance values obtained in step 2 and in step 4.