PHY115 – Sault College – Bazlurslide 1 Electricity

PHY115 – Sault College – Bazlur slide 1

Electricity


Electric potential causes flow of chargeElectric potential, measured in volts.

Voltage acts like an “electrical pressure” that can produce a flow of charge, or current.

Current is measured in amperes (or, simply, amps, abbreviated A).

We'll also see that the resistance that restrains this flow of charge is measured in ohms (Ω).

When the flow is in only one direction we'll call it direct current (dc), and for back-and-forth flow we'll call it alternating current (ac).

Electric current can deliver electric power, measured, just like mechanical power, in watts (W) or in thousands of watts, kilowatts (kW).


Flow of Charge when the ends of an electrical conductor are at different

electric potentials - when there is a potential difference - charge flows from one end to the other.

The flow of charge persists for as long as there is a potential difference.


Electric Current Electric current is simply the flow of electric

charge.

In circuits of metal wires, electrons make up the flow of charge.

Protons do not move because they are bound inside the nuclei of atoms.


AmperesThe rate of electrical flow is measured in

amperes.

One ampere is a rate of flow equal to 1 coulomb of charge per second.

1 coulomb, the standard unit of charge, is the electric charge of 6.25 billion billion electrons.


Voltage Sources Charges flow only when they are “pushed” or “driven.”

A sustained current requires a suitable pumping device to provide a difference in electric potential - a voltage.

An “electrical pump” is some sort of voltage source.

Generators or chemical batteries are sources of energy.


Sources of energyBatteries and electric generators do work to pull

negative charges away from positive ones.

In chemical batteries, this work is done by the chemical disintegration of zinc or lead in acid, and the energy stored in the chemical bonds is converted to electric potential energy.

Generators such as alternators in automobiles separate charge by electromagnetic induction.


Electrical Resistance A battery or generator is the prime

mover of charges in an electric circuit.

How much current there is depends not only on the voltage but also on the electrical resistance the conductor offers to the flow of charge.

This is similar to the rate of water flow in a pipe, which depends not only on the pressure difference between the ends of the pipe but also on the resistance offered by the pipe itself.


ResistanceThe resistance of a wire depends on the thickness and

length of the wire, and on its particular conductivity. – Thick wires have less resistance than thin wires. – Longer wires have more resistance than short wires. – Copper wire has less resistance than the same size steel

wire. – Electrical resistance also depends on temperature.


OhmsElectrical resistance is measured in units called ohms.

The Greek letter omega, Ω, is commonly used as the symbol for the ohm.

This unit is named after George Simon Ohm, a German physicist who in 1826 discovered a simple and very important relationship among voltage, current, and resistance.

Ohm's law:


Ohm’s Law The current in a circuit is

proportional to the applied voltage.


Ohm's lawOhm's law provides the mathematical relationship

between current, voltage and resistance current.

I V

I = V / RWhere, I is the current in Amps

V is the voltage in Volts (often referred to as E)

R is the resistance in Ohms

If voltage is doubled then current will double.

If resistance is doubled then current will be reduced by half.


Ohm's Law


Ohm's Lawhttp://www.walter-fendt.de/ph14e/ohmslaw.htm


Ohm's Law


Ohm's Law


Ohm's LawHow much current will flow through a lamp that has a

resistance of 60 Ω when 12 V are impressed across it?


Ohm's Law1/5 A.

This is calculated from Ohm's law.

I = V / R

= 12 v / 60 = 0.2 A


Ohm's LawWhat is the resistance of an electric frying pan that

draws 12 A when connected to a 120-V circuit?


Ohm's Law10 Ω.

Rearrange Ohm's law (I = V/R) to read

R = V/I

= 120v / 12A

= 10 Ω


Ohm's LawAt a resistance of 100,000 Ω, what will be the current in

your body if you touch the terminals of a 12-volt battery?


Ohm's Law1/5 A.

This is calculated from Ohm's law.

I = V / R

= 12 v / 100,000 Ω

= 0.00012 A


Ohm's LawIf your skin is very moist - so that your resistance is only

1000 Ω - and you touch the terminals of a 12-volt battery, how much current do you receive?


Ohm's Law10 Ω.

Ohm's law (I = V/R)

I = V/R

= 12v / 1000 Ω

= 0.012 A


Electric shockWhat causes electric shock, current or voltage?

Electric shock occurs when current is produced in the body, which is caused by an impressed voltage.

So the initial cause is the voltage, but the current does the damage.


Electric shockThe round prong connects the body of the appliance

directly to ground (the earth).

Any charge that builds up on an appliance is therefore conducted to the ground - preventing accidental shock.


Electric shockElectric shock can overheat tissues in the body and

disrupt normal nerve functions.

It can upset the rhythmic electrical patterns that maintain proper heart beating, and can upset the nerve center that controls breathing.

In rescuing shock victims, the first thing to do is find and turn off the power source.

Then do CPR until help arrives.

For heart-attack victims, on the other hand, electric shock can sometimes be beneficial in getting the heartbeat started again.


Direct Current and Alternating Current Electric current may be dc or ac.

By dc, we mean direct current, which refers to the flowing of charges in one direction.

A battery produces direct current in a circuit because the terminals of the battery always have the same sign.

Electrons move from the repelling negative terminal toward the attracting positive terminal, always moving through the circuit in the same direction.

Even if the current occurs in unsteady pulses, so long as electrons move in one direction only, it is dc.


Direct Current and Alternating CurrentAlternating current (ac) acts as the name implies.

Electrons in the circuit are moved first in one direction and then in the opposite direction, alternating to and fro about relatively fixed positions.

This is accomplished by alternating the polarity of voltage at the generator or other voltage source.

Nearly all commercial ac circuits in North America involve voltages and currents that alternate back and forth at a frequency of 60 cycles per second.

This is 60-hertz current.

Officially 120 volts.


Ohm's LawTime graphs of dc and ac.


Converting ac to dcHousehold current is ac.

The current in a battery-operated device such as a pocket calculator is dc.

You can operate these devices on ac instead of batteries with an ac/dc converter.

In addition to a transformer to lower the voltage, the converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in only one direction.


Converting ac to dcSince alternating current changes its direction each half cycle,

current passes through a diode only half of each period.

The output is a rough dc, off half the time.

To maintain continuous current while smoothing the bumps, a capacitor is used


Speed and Source of Electrons in a Circuit The solid lines suggest a random path of an electron bounding

about in an atomic lattice at an average speed of about 1/200 the speed of light.

The dashed lines suggest an exaggerated and idealized view of how this path would be altered when an electric field is applied.

The electron drifts toward the right with a drift velocity much slower than a snail's pace.


Energy and Power

Work and energy are both measured in joules

Power = work/time;

Units for mechanical power and electrical power

1 J/s = 1 Watt

P = I x V


Electric PowerUnless it is in a superconductor, a charge moving in a

circuit expends energy.

This may result in heating the circuit or in turning a motor.

The rate at which electric energy is converted into another form such as mechanical energy, heat, or light is called electric power.


Electric PowerElectric power is equal to the product of current and

voltage.

If the voltage is expressed in volts and the current in amperes, then the power is expressed in watts.

So, in units form,

If a lamp rated at 120 watts operates on a 120-volt line, you can see that it will draw a current of 1 ampere (120 watts = 1 ampere × 120 volts).


Electric Energy A kilowatt is 1000 watts, and a kilowatt-hour represents

the amount of energy consumed in 1 hour at the rate of 1 kilowatt.

Power = Energy / Time

Energy = Power x Time

The unit can be written as joules or kw-hour

Therefore, in a locality where electric energy costs 5 cents per kilowatt-hour, a 100-watt electric light bulb can be run for 10 hours at a cost of 5 cents.

A toaster or iron, which draws much more current and therefore much more energy, costs about ten times as much to operate.


Electric Current If a 120-V line to a socket is limited to 15 A by a safety

fuse, will it operate a 1200-W hair dryer?

At 10¢/kWh, what does it cost to operate the 1200-W hair dryer for 1 h?


Electric Fuse From the expression watts = amperes × volts, we see

that

current = watts / volts

= 1200 W/120 V

= 10 A,

so the hair dryer will operate when connected to the circuit.

But two hair dryers on the same circuit will blow the fuse.


Electric Energy At 10¢/kWh, what does it cost to operate the 1200-W

hair dryer for 1 h?


Electric Energy 12¢

Power = 1200 W = 1.2 kW;

Energy = Power x Time

= 1.2 kW × 1 h

= 1.2 kWh

Total Cost = Energy x cost per unit energy

= 1.2 kWh x 10¢/kWh

= 12¢


Electric Circuits Any path along which electrons can flow is a circuit.

Electrical devices are commonly connected in a circuit in one of two ways,

– series or – parallel.


Series CircuitsA simple series circuit is shown in Figure.

Three lamps are connected in series with a battery.

The same current exists almost immediately in all three lamps when the switch is closed.

The charge does not “pile up” in any lamp but flows through each lamp.

Electrons in all parts of the circuit begin to move at once.

Some electrons move away from the negative terminal of the battery, some move toward the positive terminal, some move through the filament of each lamp.


Series CircuitsEventually the electrons move all the way around the circuit (the

same amount of current passes through the battery).

This is the only path of the electrons through the circuit.

A break anywhere in the path results in an open circuit, and the flow of electrons ceases.

Burning out one of the lamp filaments or simply opening the switch could cause such a break.


Electric Circuits A simple series circuit.

The 6-V battery provides 2 V across each lamp.


Parallel CircuitsA simple parallel circuit is shown in Figure.

Three lamps are connected to the same two points A and B.

Electrical devices connected to the same two points of an electrical circuit are said to be connected in parallel.

The pathway for current from one terminal of the battery to the other is completed if only one lamp is lit.

In this illustration, the circuit branches into three separate pathways from A to B.

A break in any one path does not

interrupt the flow of charge in the

other paths.

Each device operates independently

of the other devices.


Parallel CircuitsA simple parallel circuit.

A 6-V battery provides 6 V across each lamp.


Parallel Circuits and OverloadingLights and wall outlets are connected in parallel, so all are

impressed with the same voltage, usually about 110–120 volts.

As more devices are plugged in and turned on, more pathways for current result in lowering of the combined resistance of each circuit.

Therefore, a greater amount of current occurs in the circuits.

The sum of these currents equals the line current, which may be more than is safe.

The circuit is then said to be overloaded.

The heat generated by an overloaded circuit may start a fire.


Safety FusesTo prevent overloading in circuits, fuses are connected

in series along the supply line.

If the fuse is rated at 20 amperes, it will pass 20 amperes, but no more.

A current above 20 amperes will melt the fuse, which “blows out” and breaks the circuit.

Before a blown fuse is replaced, the cause of overloading should be determined and remedied.


Short circuitOften, insulation that separates the wires in a circuit

wears away and allows the wires to touch.

This greatly reduces the resistance in the circuit, effectively shortening the circuit path, and is called a short circuit.


Summary

• Potential difference

• Electric current • Electrical resistance • Superconductor • Ohm's law • Direct current (dc) • Alternating current (ac) • Electric power • Series circuit • Parallel circuit


Summary •

Potential difference (synonymous with voltage difference) The difference in electric potential between

two points, measured in volts. It can be compared to the difference in water pressure between two containers of water: When connected by a pipe, water flows from the container with the higher pressure to the one with the lower pressure—until the two pressures are equalized. Similarly, when two points having different electric potential are connected by a conductor, charge flows so long as a potential difference exists.

• Electric current The flow of electric charge that transports energy from one place to another. Measured in amperes, where 1 A is the flow of 6.25 × 1018 electrons per second, or 1 coulomb per second.

• Electrical resistance The property of a material that resists electric current. Measured in ohms (Ω).

• Superconductor A material in which the electrical resistance to electric current drops to zero under special circumstances that include low temperatures.

• Ohm's law The statement that the current in a circuit varies in direct proportion to the potential difference or voltage across the circuit and inversely with the circuit's resistance. A potential difference of 1 V across a resistance of 1 Ω produces a current of 1 A.

• Direct current (dc) Electrically charged particles flowing in one direction only.

• Alternating current (ac) Electrically charged particles that repeatedly reverse direction, vibrating about relatively fixed positions. In the United States the vibrational rate is 60 Hz.

• Electric power The rate of energy transfer, or the rate of doing work; the amount of energy per unit time, which electrically can be measured by the product of current and voltage. Measured in watts (or kilowatts), where 1A × 1V = 1W.

• Series circuit An electric circuit in which electrical devices are connected in such a way that the same electric current exists in all of them.

• Parallel circuit An electric circuit in which electrical devices are connected in such a way that the same voltage acts across each one and any single one completes the circuit independently of all the others.

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PHY115 – Sault College – Bazlurslide 1 Electricity