Time Constant = CR

Coulomb's law can be used to calculate the force of repulsion or attraction (remember force is a vector quantity) acting between two point charges.

What is an electric field?

An electric field describes an area where an electric charge would experience a force. It is defined as the force per unit positive charge (in English; what force would 1 unit of a positive charge experience?).

How is an electric field created?

Electric fields are created by electric charges. An example of an electrically charged particle would be an electron, which has a negative charged, and so repels positively charged particles.

How do we illustrate Electric fields?

Similarly to magnetic fields, we use field lines to represent electric fields. Field lines are drawn radiating away from positive charges and towards negative charges. Where the field lines are closest together is where the electric field is strongest (where a positive charge will experience the greatest force).

Faraday's Law states that the induced electromotive force (e.m.f.) in any closed loop is equal to the negative rate of change of magnetic flux linkage through the circuit. (induced e.m.f.= -rate of change of magnetic flux linkage)

What does this mean?

If you look at some capacitors, you will see that they are marked with the value of their capacitance. The greater the capacitance, the greater the charge stored by the capacitor for a given potential difference across it. The capacitance 'C' of a capacitor is defined by:

Capacitance=Charge/Potential Difference

Or

C=Q/V, where Q is the charge stored by the capacitor and V is the potential difference across it.

The capacitance of a capacitor is the charge stored per unit of potential difference across it.

The charge on a capacitor may be calculated using the equation Q=VC, where V is the potential difference across the capacitor and C is it's capacitance.

What is a capacitor?

Capacitors are components used in many electrical and electronic circuits. They store charge (and energy), and this means that they have many valuable applications.

All capacitors have two leads, connected to two metal plates where the charge is stored. Between the plates is an insulating material called the dielectric.

Charging Capacitors

To charge a capacitor, it must be connected to a voltage supply. The negative terminal of the supply pushes (negatively charged) electrons onto one plate, making it negatively charged. Electrons are repelled from the other plate, making it positively charged. The current in the circuit will eventually stop when the capacitor is fully charged. When this happens, the potential difference (p.d.) across the capacitor is equal to the electromotive force (e.m.f.) of the supply. Remember conventional current is concerned with positive charge, and so is opposite in direction to the flow of electrons.

Stored Charge

Fleming's left-hand rule can be used to determine the force exerted on a current carrying wire in a magnetic field.

For this rule you need to hold your thumb, index finger and third finger at right angles to one another.

-Your thumb represents the force (F)

-Your index finger represents the magnetic field (B)

-Your third finger represents the conventional current flow in the wire (I)

If two magnetic fields combine, a force is exerted. One magnet exerts a force (attractive or repulsive) on the other.

The electromagnetic field shown in the image above is the magnetic field due to a flow of charge carriers (electrons in a wire in this case).

The permanent magnetic field is created between a north and a south magnetic monopole.

When the wire is placed in the permanent magnetic field the strong field behind the wire pushes it upwards.

This effect allows us to turn electrical energy into kinetic energy. We call it the motor effect.

The size of the force can be increased by:

Increasing the magnetic field strength;

Increasing the length of the wire in the field;

Increasing the current in the wire.

F = B I L

Where:

F = Force on the wire (N)

I = current (A)

B = magnetic field strength (T)

L = length of the wire (m)

Note: The angle between the current and the magnetic field must be greater than zero for a force to be produced. The greatest effect is when the angle is 90º. So if the wire and the conductor are not perpendicular to each other we must use:

F = B I L sin θ

Where there are moving charge carriers there will be a magnetic field. The 'right-hand grip rule' can be used to determine the nature of the magnetic field surrounding a flow of charge carriers (electrons in the case of a metal wire). Your thumb is used to represent the direction of conventional current flow (remember conventional current represents the flow of positive to negative), and the gripped remaining fingers show the magnetic field as a result of the moving charge carriers.

Image:

A dynamo consists of a magnet which is rotated close to a coil. The repeated motion of the magnet's poles past the ends of the coil induces an alternating voltage across the coil.

When dealing with magnetism and electromagnetism we often consider how much of a magnetic field has been 'cut'. To quantify this a new quantity is defined; magnetic flux (often denoted Φ or ΦB).

To calculate magnetic flux we can consider the equation Φ=BA cosθ

B, is the magnitude or 'strength' of the magnetic field (the magnetic flux density). The unit for magnetic flux density is the Tesla or Weber per square Meter (Wb/m2)

Wikipedia-

'A solenoid (from the French solénoïde, derived in turn from the Greek solen "pipe, channel" + combining form of Greek eidos "form, shape"[1]) is a coil wound into a tightly packed helix. The term was invented by French physicist André-Marie Ampère to designate a helical coil.[2]

Like other fields, the magnetic field is a way of describing a region of space where other magnets will experience a force. It can be represented by field lines that show both the size and direction of the force. Field lines are drawn from north to south.

How is the field strength represented?

- By the spacing of field lines (where the field lines are closest together represents where the field strength is greatest).

How is its direction shown?

The image above shows a simple generator.  A voltage (and current if connected to a complete circuit) can be induced in the wire if the wire 'cuts' through a magnetic field. The wire above is between a north and south end of two magnets, and therefore in a magnetic field. The magnetic field can be represented by lines from the North to the South. When the wire rotates it 'cuts' through the magnetic field and so electricity is generated.

-A current is produced when either the wire or the magnet move (the wire must cut the magnetic field lines).

-The faster the relative movement of the magnet or the wire the bigger the current.

-Changing the direction of the movement changes the direction of the current (hence the sinusoidal voltage output in the image above, which represents an AC generator).

-The stronger the magnet, the larger the current.

-The more coils of wire, the larger the current.

Image: