Physics Subject Area Test

ELECTRICITY & MAGNETISM

*Electrons have a negative electrical charge.

*Protons have a positive electrical charge.

*These charges interact to create an electrical force.*Like charges produce repulsive forces

–they repel each other*Unlike charges produce attractive forces

–they attract each other

Electric Charge and Electrical Forces:

* A very highly simplified model of an atom has most of the mass in a small, dense center called the nucleus.

The nucleus has positively charged protons and neutral neutrons.

Negatively charged electrons move around the nucleus at much greater distance.

Ordinary atoms are neutral because there is a balance between the number of positively charged protons and negatively charged electrons.

Electrostatic Charge:

*Electrons move from atom to atom to create ions.

*positively charge ions result from the loss of electrons and are called cations.

*Negatively charge ions result from the gain of electrons and are called anions.

(A)A neutral atom has no net charge because the numbers of electrons and protons are balanced.

(B) Removing an electron produces a net positive charge; the charged atom is called a positive ion (cation).

(C) The addition of an electron produces a net negative charge and a negative ion (anion).

The charge on an ion is called an electrostatic charge.

*An object becomes electrostatically charged by*Friction, which transfers electrons between two objects in contact,*Contact with a charged body which results in the transfer of electrons,*Induction which produces a charge redistribution of electrons in a material.

Electrical conductors are materials that can move electrons easily.*Good conductors include metals. Copper is the best

electrical conductor.

Electrical Conductors and Insulators

Electrical nonconductors (insulators) are materials that do not move electrons easily.Examples are wood, rubber etc.

Semiconductors are materials that sometimes behave as conductors and sometimes behave as insulators. Examples are silicon, arsenic, germanium.

fundamental charge - the electrical charge on an electron

-has a magnitude of 1.6021892 X 10-19 C (measured in coulombs).

coulomb - the charge resulting from the transfer of 6.24 x 1018 of the charge carried by an electron.

magnitude of an electrical charge (Q) is dependent upon how many electrons (n) have been moved to it or away from it.Mathematically,

Q = n e-

where e- is the fundamental charge.

*Coulomb’s law: Electrical force is proportional to the product of the electrical charge and inversely proportional to the square of the distance.

This is known as Coulomb’s law.

where, *F is the force,*k is a constant and has the value of 9.00 x 109 Newtonmeters2/coulomb2 (9.00 x 10 9 Nm2/C2),*q1 represents the electrical charge of object 1 and q2 represents the electrical charge of object 2, and*d is the distance between the two objects.

221

dqq

kF

Force Fields: The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge.

Lines of force diagrams for

(A) a negative charge and

(B) a positive charge when the charges have the same magnitude as the test charge.

The potential difference (V) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt.

*A volt is a measure of the potential difference between two points,*electric potential = work done,

charge Or, V=W Q*The voltage of an electrical charge is the energy

transfer per coulomb.

*The energy transfer can be measured by the work that is done to move the charge or by the work that the charge can do because of the position of the field.

The falling water can do work in turning the water wheel only as long as the pump maintains the potential difference between the upper and lower reservoirs.

Electric Current ( I )

Electric current means a flow of charge in the same way that a water current flows.

• It is the charge that flows, and the current is defined as the flow of the charge.

A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to follow.

Voltage is a measure of the potential difference between two places in a circuit.

*Voltage is measured in joules/coloumb.

*The rate at which an electrical current (I) flows is the charge (Q) that moves through a cross section of a conductor in a give unit of time (t),

I = Q/t.

*the units of current are coulombs/second.

*A coulomb/second is an ampere (amp).

A simple electric circuit carrying a current of 1.00 coulomb per second through a cross section of a conductor has a current of 1.00 amp.

Electrical Resistance:*Electrical resistance is the resistance to

movement of electrons being accelerated with an energy loss.

*Materials have the property of reducing a current and that is electrical resistance (R).

*Resistance is a ratio between the potential difference (V) between two points and the resulting current (I).R = V/I

*The ratio of volts/amp is called an ohm ().

The relationship between voltage, current, and resistance is:

V =I R

This is known as Ohms Law.

The magnitude of the electrical resistance of a conductor depends on four variables:

*The length of the conductor.

*The cross-sectional area of the conductor.

*The material the conductor is made of.

*The temperature of the conductor.

Resistors in Series*Resistors can be connected in series; that is, the

current flows through them one after another. The circuit in Figure 1 shows three resistors connected in series, and the direction of current is indicated by the arrow.

Note that since there is only one path for the current to travel, the current through each of the resistors is the same.

I1= I2 = I3

Also, the voltage drops across the resistors must add up to the total voltage supplied by the battery:

V total = V1+V2+V3

Series Circuit

Resistors in Series

*resistance for resistors connected in series.

*R equivalent = R1 + R2 + R3

Resistors in Parallel Resistors can be connected such that they branch out from a single point (known as a node), and join up again somewhere else in the circuit. This is known as a parallel connection. Each of the three resistors in Figure 1 is another path for current to travel between points A and B.

At A the potential must be the same for each resistor. Similarly, at B the potential must also be the same for each resistor. So, between points A and B, the potential difference is the same. That is, each of the three resistors in the parallel circuit must have the same voltage. V1 =V2 = V3

Parallel Circuit

Also, the current splits as it travels from A to B. So, the sum of the currents through the three branches is the same as the current at A and at B (where the currents from the branch reunite).

I = I1 +I 2 + I3

Resistors in Parallel

I = I1 +I2 + I3By Ohm's Law, this is equivalent to:

vRequivalent

=v1R1

+v 2R2

+v3R3

Resistors in Parallel

we see that all the voltages are equal. So the V's cancel out, and we are left with

1Requivalent

=1R1

+1R2

+1R3

Electrical Power and Electrical Work:All electrical circuits have three parts in common.

*A voltage source.

*An electrical device

*Conducting wires.

The work done (W) by a voltage source is equal to the work done by the electrical field in an electrical device,

Work = Power x Time.

electrical work is measured in joules

A joule/second is a unit of power called the watt.

Power = current x potential

Or, P = I V

Magnetism

Magnetic Fields:A magnet that is moved in space near a second magnet experiences a magnetic field.

A magnetic field can be represented by field lines.

The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.

Electric Currents and

Magnetism

Oersted discovered that a compass needle below a wire

(A) pointed north when there was not a current,

(B) moved at right angles when a current flowed one way, and

(C) moved at right angles in the opposite direction when the current was reversed.

A magnetic compass shows the presence and direction of the magnetic field around a straight length of current-carrying wire.

When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet.

Electric Meters:

*The strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet.

*A galvanometer measures electrical current by measuring the magnetic field.

*A galvanometer can measure current, potential difference, and resistance.

Electric Motors:*An electrical motor is an electromagnetic device that converts electrical energy into mechanical energy.

*A motor has two working parts - a stationary magnet called a field magnet and a cylindrical, movable electromagnet called an armature.

*The armature is on an axle and rotates in the magnetic field of the field magnet.

*The axle is used to do work.