Current Electricity (O Level Physics)

Posted on Monday, September 26, 2016 by Mysterious QUIZ!

CURRENT ELECTRICITY!

Introduction:

Welcome back everyone! Hope you are all good. Today we’ll learn about current electricity. We have studied in the tutorial on static electricity that insulators and conductors become charged when electrons are added or removed from them. We know that these electrons are stationary, but what will happen if these electrons are provided with a conducting path? The electrons will start to flow, and moving electrons produce electric current.

Electric Current:

As we just saw, electric current is produced when electrons flow. These electrons always flow from a negatively charged to a positively charged end. This is the electron flow.

Now coming to the convectional current, in the previous days it was assumed that current flows from positive to negative end and it is widely held today. This is called the convectional current flow. The diagram below makes it clearer.

Electric current (I) is a measure of the rate of flow of electric charge (Q) through a given cross section of a conductor.

Such that:

I = Q / t

The SI unit of current is ampere (A). An ammeter is used to measure current.

Now before moving on, you should all be aware of how a simple circuit looks like.

In the circuit shown above, it consists of:

1. a source of electromotive force that drives electric current (e.g. battery)

2. a load on which moving charges can do a useful job (e.g. a bulb)

3. conductors to connect the components together (e.g. copper wire)

4. switch to open or close a circuit.

Task: Google the symbols used in circuit diagram for different apparatuses.

Electromotive Force and Potential Difference:

Electromotive force (e.m.f) is the energy required to move a unit positive charge from one end of the circuit to another. Such that:

E = W / Q

where E is the e.m.f. of the power supply, W is the amount of electrical energy converted from electrical to non-electrical forms (work done) and Q is the amount of charge. The SI unit for e.m.f. is Joule per Coulomb or volt (V).

Remember that e.m.f. is the movement of charge through the entire circuit.

The diagram above shows the voltage calculated of the cell, and as cell is providing voltage to the entire circuit, it is hence e.m.f.

Now, potential difference (p.d.) is the amount of electrical energy consumed to move a unit positive charge from one point to another in an electrical circuit. Such that:

V = W / Q

where V is the p.d., W is the electrical energy converted to other forms and Q is the amount of charge. The SI unit for this is the same as that for e.m.f. and that is volt (V).

The diagram shows how p.d. can be calculated across the bulb (between two points).

Resistance:

Resistance, as the name suggests, is the measure of how difficult it is for an electric current to pass through a material, copper wire let’s say. So it is basically the restriction (resistance) of a material to the free moving electrons in the material. If you compare it with the friction in moving objects, it’s quite correct.

Now in more scientific terms, resistance R of a component is the ratio of the potential difference across it to the current I flowing through it, such that:

R = V / I

where R is resistance, V is the p.d. across the component (note that across a component, that is, between two points, it’s p.d. and not e.m.f.) and I is the current flowing through it.

The SI unit of resistance is ohm (Ω).

Resistance is measured using a conductor called resistor. Resistors are of two types: fixed and variable (rheostats). Now you can tell exactly from the name what these are, right? Fixed resistors have a fixed value while variable resistors can vary the resistance and are used in circuits to vary current.

Ohm’s Law:

Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the p.d. across the ends, provided the physical conditions (such as temperature) are constant.

Such that:

I α V

where I is current and V is p.d.

and this drives us to the formula which we have already learned, i.e.

V / I = constant = R

From this, we can make another conclusion that resistance of a metallic conductor remains constant under steady physical conditions, and such conductors which obey Ohm’s law are called ohmic conductors. For ohmic conductors, I-V graph has a constant gradient (i.e. inverse of resistance), as shown below:

Not all conductors obey Ohm’s Law, such conductors are non-ohmic conductors. The resistance of such conductors can vary, but how do we differentiate? The I-V graphs of different conductors can help us differentiate.

For example, for a filament lamp, when the p.d. across the lamp increases, the current does not increase proportionally. The graph below makes it clearer:

The deviation of I-V graph from straight line is due to increase in the resistance of the filament with temperature. The graph is straight line in initial stage because the increase in resistance of the filament with the temperature due to small current is not appreciable. As the current is further increased, the resistance of the filament continues to increase due to rise in temperature (Though the gradient is decreasing, how can we say that the resistance is increasing? That’s because slope is the inverse of gradient in this case). How is the temperature rising? It’s rising because as the bulb remains on for a long time, more energy is dissipated to heat energy.

Task: Google other non-ohmic conductors and find out how their resistance varies in an I-V graph.

Resistivity:

Apart from temperature, there are other factors as well on which R depends. As for temperature, the higher the temperature of metallic wire, the larger the resistance.

The resistance depends on

1. the length l of the wire,

2. the cross-sectional area A or thickness of the wire, and

3. the type of material.

To memorize how these factors affect the resistivity of the conductor, memorize the following formula:

R = p (l / A)

where R is the resistance, p (a constant) is the resistivity, l is the length and A is the cross-sectional area of the wire.

This shows that R α l and that R α 1 / A.

So now me have made it quite easy: as R is directly proportional to length, the longer the length of the wire, the greater is its resistance, and as R is inversely proportional to cross-sectional area of wire, the greater is its cross sectional area, the lower is its resistance.

Now for the type of material, every material has a different resistance. For example, the resistance of silver is 1.6 x 10^-8 Ω while that of graphite is 3000 x 10^-8 Ω.

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