Electricity and Magnetism

Topic 5

Charge

In a simplified atomic model, electrons orbit about a central nucleus:

As long as the number of electrons equals the number of protons, an atom is neutral.

If an electron is removed from an atom, the atom has a net (+) charge and becomes a positive ion.

If an electron is added to an atom, the atom has a net (-) charge and is called a negative ion.

Elementary Charge

• Typically the charge of an electron is -1 and the charge of a proton +1

• The actual charge of each is given in terms of the elementary charge e– Because your lives are not miserable enough

Charge Law

• Like charges repel and opposite charges repel• We use (+) and (-) to represent these

properties only because they are convenient and familiar to us. But there is another charge, color charge! (Topic 7)

Conservation of Charge

• Charge can be neither created nor destroyed but can be transferred

• Conservation of charge is never violated

Materials can be divided into three categories:

(1) Conductors - which easily transport electrons without trying to capture or impede them,

(2) Nonconductors or insulators - which capture or impede electrons

(3) Semiconductors - which lie between conductors and insulators.

Roughly speaking, metals are good conductors, nonmetals are good insulators, and metalloids are good semiconductors.

Electric Current

• is the time rate ∆t at which charge ∆q moves past a particular point in a circuit

• current is measured in Coulombs per second (C s-1) which is called an Ampere (A)

• When you cut a wire why does current stop?

Coulomb’s Law

• F between two point charges q1 and q2 separated by distance r

• Only Use this one when talking about permittivity of free space – different medium

Electric Field Force

• the force per unit charge acting on q due to the presence of Q

• units are Newtons per Coulomb (N C-1).

Drawing Electric Fields

• Rays always point towards the negative charge and away from the positive charge

Monopole

• Electric ray diagram with single charge

Dipole

• Two opposite charges that are close enough their electric fields will interact

Potential Difference

Potential Difference, V, between two points A and B as the amount of work W done per unit charge in moving a point charge from A to B.

Because measuring in volts is just not enough…

• Electronvolt, eV, as the work done when an elementary charge e is moved through a potential difference V

Coulomb

• SI unit of electric charge• 1 coulomb = 1 ampere – second• an electric current of A represents 1 C of unit electric

charge carriers flowing past a specific point in 1 s• 1 C = charge of 6.25 x 1018 electrons• Take the inverse, you get the charge of an electron, or

the elementary charge• Discovered during experiments finding the force

between charged particles• Force between particles was found before charge of

each particle

Drift Velocity

• charge carriers in a conductor are free to move, if a conductor is suddenly accelerated, the electrons would “pool” at the trailing side due to inertia, and a potential difference measured by a voltmeter would be set up between the ends.

Drift Velocity• Speed at which ions move along the

conductor• Smaller the current, the smaller the speed

• If we know the number n of free charges q per unit volume in a conductor, known as the number density, and the cross sectional area A of the conductor, and the drift velocity v of the charges, the current I is given above.

Drift Velocity

• Suppose the drift velocity is 0.0025 ms-1 for your house wiring. If the wire between your light switch and your light bulb is 6.5 meters, how long does it take an electron to travel from the switch to the bulb?

Practice

Suppose the current in a 2.5 mm diameter copper wire is 1.5 A and the number density of the free electrons is 5.01026 m-3. Find the drift velocity.

Magnetic Force

• Pole Law– Like poles repel–Opposites attract

Magnetic Field

• Created at a point if a force acts on a pole

• A bar magnet is a piece of ferrous metal which has a north and a south pole. Looking at the field about such a magnet, determine the north and the south poles

Field Lines

• Drawn from North-seeking pole to the south seeking pole

• Represent direction in which a north seeking pole would move at that point

• Strength shown by density of lines– Closer lines = stronger field

• Field lines never cross• Tend to be as short as possible

Magnetic vs. Electric Dipole

• A bar magnet is a magnetic dipole because it has two poles, N and S.

• Compare the field lines of the magnetic dipole with the electric dipole, which also has two poles (+) and (-).

• Externally, they are identical. How do they differ internally?

Electric Dipole

• Can be split into 2 electric monopoles

• Has a charge• Can you split a

magnetic dipole?

Current and Magnetic Field

• When current is flowing through a wire, a circular magnetic field is created around the wire

• What happens when you change the direction of the current?

• What is the right hand rule?

Drawing the Fields

• What would the magnetic field look like for each?

Solenoid

• Series of loops current moves through– Magnetic field created through center– Right hand rule to find North

Example

• Which is north? South?

Force in a Magnetic Field

• Motor Effect – current is placed into a magnetic field

Finding the Direction of the Force

• Fleming’s Left Hand Rule:

Practice Questions

• http://www.quia.com/quiz/2091837.html