A combination of ammeter and voltmeter can also measure resistance. In a circuit containing a power supply, connect the ammeter in series with and the voltmeter
in parallel to the component and apply the formula A Ohm’s Law LAWS AND PRINCIPLES
Fig 18.1: Circuit diagram illustrating how a voltmeter and ammeter can be used to measure resistance
OHM’S LAW
R resistance1 V IR
V voltage I current
SAMPLE PROBLEM 18A
Calculate the resistance of a metal wire when a voltage of 6 V across it causes a current of 0.5 A to flow through it.
SAMPLE ANSWER 18A The metal wire has a resistance of
R V I 6
12 . Æ
Power Power (P) is the energy produced or consumed per second. Power is usually written in the form when performing calculations in electricity. Electric power can also be referred to as wattage, as it is measured in watts.
P IV
GEORG SIMON OHM, 1787–1854 (GERMAN) He discovered the relationship between voltage and current now
called Ohm’s law.The SI unit of electri- cal resistance is named in his honour.
P IV POWER
P power V voltage I current
Combining Joule’s law and Ohm’s law and the definition of power The formula
P W t I2 Rt
P IV is more useful when doing calculations in electricity than
which we used in mechanics. This formula can be derived from Joule’s law and Ohm’s law and the definition of power:
P W t
t I2 R (IR)I VI Combining Joule’s law and Ohm’s law
It is often necessary to combine laws when performing numeric calculations. By combining Joule’s law and Ohm’s law we can derive the relationship between energy output for a given time with the voltage across and current through a circuit.
1
This law is very similar to the definition of resistance, the difference being that the definition of resistance holds true for any resistor, regardless of whether it is ohmic or not.
306 INVESTIGATING PHYSICS ,
Ohm’s Law Under constant physical conditions, the potential difference across a conductor is proportional to the current flowing through it.
