"One ampere is the current in a conductor when a charge of one coulomb passes through a cross section of the conductor each second."
Note that this is consistent with the formal SI definition of the ampere, namely,
"The ampere is the constant current which, when flowing in each of two parallel conductors of infinite length and 1m apart in a vacuum, produces between them a force of 2 x 10-7 N per metre of length".
Current is measured using an instrument called an ammeter.
Ammeters are connected in series with the part of the circuit through which one wishes to measure the current, I, and they have negligible resistance.
Potential difference:
Current flows in a circuit as a result of a difference in potential between two points in the circuit.
The potential difference between two points in a conductor is the work per unit charge done by the charge in moving from a point of higher potential to a point of lower potential.
The unit of potential difference is called the volt, V. One volt of potential difference exists between two points if one joule of work is done by each coulomb of charge in moving between them. Potential difference is measured by an instrument called a voltmeter.
Voltmeters are connected in parallel to the component across which one wishes to measure the potential difference. They have a resistance which is several orders of magnitude higher than the resistance of the component.
The current which flows through a voltmeter is negligible.
Resistance:
Georg Ohm discovered that for any conductor, the ratio of the potential difference across the conductor and the current flowing through it, is constant. This constant is called the resistance of the conductor, R. We can write:
The unit of resistance is the OHM, .
"A conductor has a resistance of one ohm if on application of a potential difference of one volt, a current of one ampere flows through it."
Resistance is a property of a particular conductor and depends on:
1. The material of which the conductor is made.
2. The length, L, of the conductor (R L).
3. The cross-sectional area, A, of the conductor (R A-1)
4. The temperature of the conductor. (Resistance increases non-linearly with temperature).
Electric power:
Remember that power is defined as the rate at which work is done:
By substituting from W = VIt, we obtain the formula for the power dissipated in an electric circuit, as follows:
This formula gives the power which is dissipated when a current I moves through a conductor across which there is a potential difference V.
From Ohm's law we may also write
The unit of power is the WATT, W which is equivalent to one joule per second, J.s-1.
