Nature of Electricity

Nature of Current Electricity To make things work  Need source 1.5 V cell has a limited life ( use it then throw it away) 240 Hydro Electric Cell virtually limitless

Nature of Current Electricity To make things work  Need Thing  A device that converts energy into a different form of energy  Heat, light, sound, motion, etc

Nature of Current Electricity To make things work  Need Switch  Interrupts the circuit  Wire  Carries the electrons to and from the thing

Why do we need two wires? the motion of electrons from high potential energy to low potential energy = One wire carries electrons to the globe or CD player from the source, the other returns them to the source whereupon they are boosted back up to continue their circulation

Law of Repulsion and Attraction  Like charges repel and unlike charges attract.  Negative electrons are pushed away from the negative terminal and are attracted to the positive terminal In a battery how do charges flow?

Measuring Current  Current is represented by the symbol (I)  Current is measured in Amps  Current is measured using an Ammeter which measures the number of electrons passing through a circuit at a given point

Current One amp is equal to the flow of one coulomb* of charge per second. COPY DIAGRAMS

1.A current of 2 A flows for 30 seconds through a lamp. How much charge has moved? 2.A current of 12 A flows for 20 minutes into an electric cooker. How much charge has the cooker used? 3.If C of charge flows through an air conditioning unit every hour, what current does it draw? 4.A current of 3 A flows into a television set. For how long would it take 1500 C of charge to flow through it?

What is VOLTAGE?  In order for current to flow something has to make it flow =  Voltage  Voltage measures the energy available to drive the flow of current.  Provided by an energy source like a battery  Measured in volts (by a voltmeter)

Potential Difference  The Voltage available to drive a flow of current is called Potential Difference

There is a direct relationship between voltage and current. We can demonstrate this with the circuit below which connects a cell and a lamp. The cell has an output of 2 volts. The lamp glows moderately.

If we replace the 2-volt cell with a 1-volt cell, the lamp will glow dimly. This is because a 1-volt cell provides half as much energy as a 2-volt cell, so the lamp converts less energy into heat and light. But how was the current affected?

To find out how the current was affected we repeat the experiment, adding an ammeter. As before, the cell has an output of 2 volts and the lamp glows moderately. But this time we know that the current is 1 amp. As before, we replace the 2-volt cell with a 1-volt cell and the lamp glows dimly. But what does the ammeter read?

The ammeter reads 0·5 amps. The current is halved because a 1-volt cell provides half as much energy as a 2-volt cell.

This time we connect a cell with an output of 4 volts. The ammeter reads 2 amps and the lamp glows brilliantly. Now we replace the 4-volt cell with a 2-volt cell and the lamp glows moderately. The ammeter reads 1 amp. The current is halved because a 2-volt cell provides half as much energy as a 4-volt cell.

Voltage & Current Summary  Voltage is the energy that drives the flow of current.   Voltage is measured in volts with a voltmeter:   Voltage is represented by the symbol, V.   One volt is the energy required to drive a current of one amp through a circuit with a resistance of one ohm.   An increase in voltage means an increase in energy and therefore an increase in current.   A decrease in voltage means a decrease in energy and therefore a decrease in current.

Resistance & Resistors  Resistance & Resistors  When walking, we don't normally notice we are moving through air as it offers such little resistance to our motion.  But compare moving through air with moving through water which offers significant resistance to our motion.  It's the same for a current moving through a conductor...

Conductors  All conductors offer resistance to the flow of current. This resistance is determined by the conductor's atoms.  For example:  Silver and copper atoms offer negligible resistance to an electric current because a significant proportion of their electrons are free to move from atom to atom. Silver and copper, having negligible resistance, are commonly in used as conductors. On the other hand...

Insulators  Glass and Plastic offer very considerable resistance to an electric current because a significant proportion of their electrons are not free to move from atom to atom. Glass and synthetic polymers, having very considerable resistance, are commonly used as insulators.  A conductor's length, temperature, and cross- sectional area also affects its resistance.

Ohm's Law  In Voltage & Current we explained the relationship between voltage and current.  In Resistance & Resistors we explained the relationship between resistance and current.  Here we explain the relationship between all three.

 The relationship between voltage, resistance and current is expressed in Ohm's Law which is named after the physicist who discovered it.  Ohm's Law most simply translates to the equation, V = IR, or:  Voltage = Current × Resistance  This is all, however, at a constant temperature

We can demonstrate the relationship between voltage, current and resistance in the circuit below which connects a cell, an ammeter and a resistor. The cell provides a voltage of 1 volt. The resistor has a value of 1 ohm. The ammeter reads a current of 1 amp. If we replace the 1 ohm resistor with a 2 ohm resistor, what will the ammeter read?

A 2 ohm resistor is twice as resistant as a 1 ohm resistor, so it is twice as hard for the current to flow. The resistor allows half of the 1 amp current to pass. By Ohm's Law: V = IR Solving for I: Now replacing the symbols with the values given: 0.5 = I I = 0.5 amps

Summary  The relationship between voltage, resistance and current is expressed in Ohm's Law which most simply translates to: V = IR or Voltage = Current × Resistance  Which is true only at a constant temperature.