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Wednesday, March 24, 2010

What is a magnet?

The magnet is a very important object that have many applications in physics. The most important ones being the compass, motors and generators.

So what is a magnet?

It is simply a material that has a magnetic field around it. In physics a field is a region around an object that enable that object to exert a force at a distance. As a result due to its magnetic field, the magnet is able to exert a magnetic force on metallic objects as shown in fig 1 below, on other magnets or on electromagnets.

Fig 2

There are two main characteristics that a  that a magnet must have.

1. Firstly the magnet must be made up of a magnetic material. A magnetic material must be a metal and it will be attracted to a magnet. However some metals are not attracted to a magnet and as a result are not magnetic materials. Aluminium, copper or gold objects for example cannot be turned into magnet and  they are not attracted to a magnet. Magnetic material can be made into a magnet using an appropriate methods that we are going to see in a future post.

2. The magnet must have at least two poles. Fig 2 below shows a magnet with two poles. For the time being there has not been a magnet with one pole.

Fig  2 A magnet

As you can see in fig 2 the two sides of the two-pole magnet is labeled “N” and “S”. So how do you determine which side is the “N”  and which side is the “S”.

In order to do that we suspend the magnet such that it is able to move freely as shown in fig 3 below.

Fig 3

A magnet that is suspended like that will always come to rest along a north-south axis. One part of the magnet will always point towards the north. Hence this side of the magnet is called the north-seeking pole or simply the north pole (N) . The other side of the magnet will always point towards the south. Hence this side of the magnet is called the south-seeking pole or simply the south pole(S).

Hence if any material has the two characteristics then  it will be be a magnet.

What is the potential difference?

As we have seen it  a previous post a power supply has an electromotive force (e.m.f). The e.m.f. is the energy that the power supply will provide to 1 C of charge to make the 1 C of charge move around the circuit.

Now in a circuit there may be circuit components. Fig 1 below shows a circuit where there is a power supply and a bulb.

 Fig 1

Now the 1 C is moving from the positive terminal and is moving around the circuit. While passing through the bulb it will move form the point A to the point B. Now the 1 C of charge has electrical energy. When it will pass through the bulb it will lose the electrical energy which will be transformed into another form.We say that the energy is dissipated.

Hence we can define the potential difference between point two points is the energy dissipated when 1 C of charge move from one of the point to the other.

Hence if the potential difference across the bulb is 3 V it means that when 1 C of charge flows from point A to point B 3 J of energy is dissipated in the bulb.

Hence if the potential difference across a circuit element is V and Q charge flows in the circuit element then the energy dissipated W can be calculated using the equation

W = VQ

Friday, March 19, 2010

What is electro-motive force (e.m.f.)?

As you would remember from this post on complete circuit a power supply is needed in order to move charges in the circuit.

Thus if we consider that an electric current is a flow of positive charges according to the conventional definition of current, for the positive charges to move from the positive terminal to the negative terminal of the power supply they need energy. It is this energy that is supplied by the power supply to the positive charges. The positive charges can thus use this energy to move around the circuit. It is this energy that the positive charges possesses that is called electrical energy.

Fig 1

Fig 1  shows a closed circuit. If the circuit is closed then an electric current will flow as indicated from the positive terminal of the power supply to the negative terminal of the power supply.

Fig 2

In fig 2 you can see 1 c of charge leaving the positive terminal of the power supply. Now for this 1 C of charge the power supply will provide a certain amount of energy.

Suppose that for every one coulomb of charge that leaves the power supply the power supply provides 10 J of energy. Then the electromotive force (e.m.f.)  of the power supply will be 10 Volt (V).

Hence if the e.m.f. of a dry cell is 1.5 V it means that when the dry cell is in a circuit for every 1 C of charge then the power supply will provide 1.5 J of energy.

Hence we can define the electromotive force as the energy that the power supply provide to a unit charge to move it around the circuit.

Hence if the a charge Q is moved around a circuit and the power supply supplied W amount of energy, the the e.m.f. E can be calculated as shown:

E  = W/Q

Example

If 4 C of charge  needs 10 J of energy is provided by the power supply. Calculate the e.m.f. of the power supply?

E = W/Q

= 10 /4

2.5 J

Saturday, March 13, 2010

What is the conventional current?

We have seen in a previous post that when you have a complete circuit an electric current will flow. We have also see that an electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal of the power supply.

Fig 1 below shows how the electron flows from the negative terminal to the positive terminal of the power supply.

Fig 1

However in a circuit the direction of the electric current is not defined as moving from the negative terminal to the positive terminal just as the flow of electrons. It is defined as moving from the positive terminal to the negative terminal of the power supply as shown in fig 2 below.

Fig 2

So why is it that despite knowing that the electric current is due to a flow of electrons that moves from the negative terminal to the positive terminal as shown in fig 1we set the direction of the electric current as being from the positive terminal to the negative terminal as shown in fig 2.

The answer lies in convention. A convention is like an assumption. When electricity was first  discovered it was assumed that electricity is a flow of positive charges that the positive terminal to the negative terminal. Hence in all physics books at the time the electric current was taken as moving from the positive terminal to the negative terminal.

However it was discovered later on that in a circuit only electrons move. they move from the negative terminal to the positive terminal of the power supply. However the concept of conventional current was so ingrained that it could not be altered.

Hence as from now we will say that electric current is a flow of positive charges even though we know that it is in fact due to the flow of electrons.

Thursday, March 11, 2010

what is an electric current?

As we have seen in an earlier post, an electric current will only flow if their is a complete circuit. You have also seen that an electric current is also due to the flow of electrons.

Fig 1 below show a complete circuit. As you can see from the circuit electrons will flow through the bulb on its way to the positive terminal of the power supply.

Fig 1

Now depending on the circuit a certain number of electrons will flow through the bulb every second.

Hence if we know the number of electrons that is flowing through the bulb every second, then it means that we cal calculate the amount of charge that flow through the bulb in one second.

Example 1

If the number of electrons that flows through the bulb in 10 s is 3.0x1022 and the charge of one electron is 1.6 x 10-19 C,

Calculate (i) the amount of charge that flows through the bulb in 10 s.

(ii) the amount of charge that flows through the bulb in 1 s.

Ans

(i) The amount of charge flowing though the bulb in 10 s is

Q = 3.0 x 1022 *1.6x10-19

= 4800 C

(ii) The amount of charge flowing through the bulb in 1s is

Q = 4800/10 =480 C

Now the quantity electric current is defined as the rate of flow of electric charge.

The unit of electric current is the Ampere (A) .

Thus if we are able to determine the rate of flow of electric charge or the amount of charge that flows in a circuit element like the bulb every second it means that we have determined the electric current.

If the amount of charge  flowing through the bulb every second is 480 C then

The electric current = 480 C /s or 480 A.

Thus we can conclude that the electric current is merely an indication of the amount of charge flowing per second in a circuit element.  The more charge flowing per second the greater the electric current flowing through the circuit element.

Wednesday, March 10, 2010

What is a complete circuit?

As you know in order to have electricity you will need a complete. So in order to have a complete circuit there are several features that must be present.

The power supply

A Power supply may comes in different forms such as:

1. The power pack that you have in your school’s physics lab. An example if in fig 1 below.

Fig 1

2. A dry cell such as the one in your portable radio. An example is shown in fig 2 below.

Fig 2

3. A battery such as the one that you have in your car.

The purpose of the power supply is to give energy to the electrons such that they can move around the whole circuit and come back to the power supply.

The symbol of the power supply is shown in fig 3.

Fig 3

As you know all power supplies have a positive terminal usually red in colour while the negative terminal is black in colour. Hence in the symbol the long vertical bar is the positive terminal while the short vertical bar is the negative terminal.

The switch

A circuit must also contain a switch. The purpose of the switch is to allow to to stop the flow of current in the circuit by opening it or to allow the flow of current by closing it. An example of a switch is shown in fig 4.

fig 4

The symbol of the switch is as follows:

A closed switch –current flowing

An open switch- no current flowing

A load is a circuit component that will use the energy supplied by the power supply and will thus convert the electrical energy. Fig 5 below show a bulb that convert electrical energy to light energy.

Fig 5

Each load has a symbol. The symbol for the bulb is as shown below:

The complete circuit

Now let us combine all this together. In order to have a complete circuit we need the power supply the switch and the bulb to be connected together with wires as shown in fig 6.

Fig 6

Let us try to understand what happens when the switch is closed and a current is flowing on the circuit.

Fig 7

As you know an electric current is due to a flow of electrons. When the switch is closed electrons can leave the negative terminal of the power supply as shown in fig 7 and move in a clockwise direction. The electrons will move through the switch, through the bulb and finally will reach the positive terminal of the power supply. The movement of the electrons are represented on fig 7 using arrows.The electron will lose energy to the  bulb and this energy will be lost in the form of light. Should the switch be open then the electron will not be able to move from the negative terminal to the positive terminal.

If the circuit is represented using symbols then the circuit will be as shown below in fig 8.

 Fig 8

what is the gravitational force?

The gravitational force is an attractive force that a mass exerts on another mass when the former is placed in the latter’s gravitational field.

Fig 1

In fig 1 above we have two masses m1 and m2 that are situated at a distance r apart. As you know these two masses will create gravitational fields around themselves such that mass M1 will exert a gravitational force F2 on mass M2 while the mass M2 will exert a gravitational force F1 on mass M1. That is each mass will exert a gravitational force on the other mass.

And from the third law of motion we know that “For every action there is an equal and opposite reaction.

Hence the gravitational force F2 will be equal to the gravitational force F1 .

F1  = F2

If the two forces are equal then they have the same magnitude and as a result

|F1 | = |F2 | = F

We can thus replace F1 and  F2  by F as shown in fig 2 below.

Fig 2

Law of gravitation

In order to calculate the magnitude of the force that each mass will exert on the other the law of gravitation must be used.

It merely states that the gravitational force that the two objects will exert on each other is directly proportional to the product of the two masses and inversely proportional to the square of their distance of separation.

Simply written in an equation it will be as shown below:

Removing the proportionality sign you will obtain the equation below:

Where G is the universal gravitational constant

G = 6.67 x 10-11N m2 kg-2

Example 1

The moon and the earth are separated by a distance of 3.8x 108 m. The mass of the moon is 6.4 x 1022 kg while that of the earth is 6.0x1024 kg. Calculate the gravitational force between the moon.