Physics Unit 1 Electric Current

1) Current and Charge

2) P.d. and power

3) Resistance

4) Components and their characteristics

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Electrical Conduction

To make an electric current, the circuit must be complete and there must be a source of p.d. The electric current is the rate of flow of charge in the wire or component. The current is due to the passage of charged particles.

The unit of current is amp, I

The unit of charge is coulomb, Q

For a current, the charge flow in time is given by:

Q=It

The equation shoes that a current of 1 A is due to a flow of charge of 1 coulomb per second.

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Charge Carriers

In an insulator, each electon is attached to an atom. When a voltage is applied no current passes through as the electrons can move away from the atom.

In a metallic conductor, most electrons are attached to atoms but some are charge carriers of the metal as they are not. When a voltage is applied the electrons are attracted to the positive terminal of the metal.

In a semi-conductor, the number of charge carriers increases with an increase of temperature. The resistance will then decrease as the temperature is raised.

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Energy and P.d.

Potential Difference is defined as the work done per unit charge. The unit of pd is the volt which is equal to 1 joule per coulomb.

V = W/Q

The emf of a source of electricity is defiined as the electrical energy produced per unit charge passing through the source. The unit of emf is the volt.

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Energy transfer in different devices

An electric current has a heating effect when it passes through a component with resistance.

1) In a device that has resistance, the work done is transferred as thermal energy. This is because the charge carriers constantly collide with atoms so as they vibrate more, the resistance becomes hotter

2) In an electric motor, the work done on the motor is transferred as kinetic energy of the motor.

3) In a loudspeaker, the work done on the loudspeaker is transferred as sound energy.

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Electric power and current

Q = IΔ t  

W = QV = ( IΔ t) V = IVΔ t  

Therefore Work done  = IV Δ t

The energy transfer Δ E is equal to the work done W.

Because power = energy/time, the electrical power is:

(IVΔ t)/ Δ t = IV

Electrical power P= IV

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Resistance

The resistance of a component in a circuit is a measure of the difficulty of making current pass through it. It is caused by repeated collisions of the charge carriers and the fixed ions in the material.

Ohms law - V=IR,   unit is ohm which is equal to 1 volt per amp.

Resistance and temperature

The resistance of a metal increases with temperature. This is because the ions in the conductor vibrate more with a higher temperature. The charge carriers can therefore not pass through as easily.

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Prefixes

Prefix           Nano          Micro          Milli          Kilo          Mega          Giga

Symbol        n                 u                 m            k               M                G

Value          10^-9          10^-6          10^-3       10^3         10^6            10^9

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Measure of Resistance

The ammeter is used to measure current through the resistor. The ammeter must be in series.

The voltmeter is used to measure the pd across the resistor. The voltmeter must be in parallel. No current should pass through the voltmeter otherwise the ammeter will not record the exact current through the resistor.

The variable resistor is used to adjust the current and pd.

The graph for  a resistor is a straight line through the origin. This means that the gradient is equal to the resistor.

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Resistivity and Superconductivity

p=(RA)/L

where p is the resistivity of the material. L is the length ad A is the cross sectional area, R is the resistance

The unit of resistivity is ohm metre

A superconductor is a wire or a device made of a material that has zero resistivity at and below a critical temperature that depends on the material. This means that when a current passes through it, there is no pd across it because its resistance is zero so the current has no heating effect.

The material will lose its superconductivity if its temperature is raised above its critical temperature.

Superconductors are used to make high power electromagnets that generate strong magnetic fields and power cables that transger electrical energy without wasting it

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Circuit Diagrams

Symbols

Resistor

Diode

Cell

Thermistor

Lamp

LDR

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Investigating the Characteristic of different comp

To measure the variation of current with pd use

1) a potential divider to vary the pd from zero

2) a variable resistor to vary the current

In both circuits an ammeter and a voltmeter connected to a data logger could be used to obtain data.

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Current v Pd graphs for different components

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