# G482 OCR Physics A Revision Notes - Unit 2 (Resistance)

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Potential difference (p.d) is the electrical energy transferred per unit charge when electrical energy is converted from one form to another. An example is a filament bulb transfering electrical energy into light and heat energy.

Electromotive Force (e.m.f.) is the energy transferred per unit charge when a type of energy is being converted into electrical energy. An example is chemical energy in a battery being coverted into electrical energy.

The volt (V) is the unit of p.d and e.m.f. 1 volt = 1 joule of energy/1couloumb of charge (1V=1J/1C).

A voltmeter is a device used to measure p.d and emf, and measures the difference in electrical potential energy when connected in parallel over a component in a circuit.

Resistance is a property of a component which regulates the electrical current that flows through it. The unit of resistance is an Ohm (Ω). 1 ohm is equal to 1 volt/1 amp (1Ω=1V/1A)

Ohm's Law states that the current through a conductor is proportional to the p.d across it provided physical conditions such as temperature remain constant. (IαV)

The I-V characteristics of different conductors: A resistor is seen as an ohmic conductor as it obeys ohm's law (its current is proportional to p.d).

A filament lamp is seen as a non-ohmic conductor as it does not obey ohm's law (its current is not proportional to its p.d as the filament heats up when current passes through it).

A diode is also an non-ohmic conductor as it's resistance is seen as infinite until 2V of voltage passes through the component. The current is not proportional to the voltage.

The IV characteristics of a material (such as a long thin wire) can be experiementally found by putting the long thin wire ina…