Dasar Listrik OHM’S LAW

Teknologi dan Rekayasa Ohm’s Law Current through an ideal conductor is proportional to the applied voltage  Conductor is also known as a resistor  An ideal conductor is a material whose resistance does not change with temperature For an ohmic device, V = Voltage (Volts = V) I = Current (Amperes = A) R = Resistance (Ohms = Ω)

Teknologi dan Rekayasa Current and Voltage Defined Conventional Current : (the current in electrical circuits) Flow of current from positive terminal to the negative terminal. - has units of Amperes (A) and is measured using ammeters. Voltage: Energy required to move a charge from one point to another. - has units of Volts (V) and is measured using voltmeters. Think of voltage as what pushes the electrons along in the circuit, and current as a group of electrons that are constantly trying to reach a state of equilibrium.

Teknologi dan Rekayasa Ohmic Resistors  Metals obey Ohm’s Law linearly so long as their temperature is held constant  Their resistance values do not fluctuate with temperature i.e. the resistance for each resistor is a constant Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.

Teknologi dan Rekayasa Voltage and Current Relationship for Linear Resistors Voltage and current are linear when resistance is held constant.

Teknologi dan Rekayasa Ohm’s Law continued

Teknologi dan Rekayasa Ohm’s Law continued The total resistance of a circuit is dependant on the number of resistors in the circuit and their configuration Series Circuit Parallel Circuit

Teknologi dan Rekayasa Kirchhoff’s Current Law Current into junction = Current leaving junction The amount of current that enters a junction is equivalent to the amount of current that leaves the junction

Teknologi dan Rekayasa Kirchhoff’s Voltage Law Net Voltage for a circuit = 0 Sum of all voltage rises and voltage drops in a circuit (a closed loop) equals zero

Teknologi dan Rekayasa Series Circuit Current is constant  Why?  Only one path for the current to take

Teknologi dan Rekayasa Series Equivalent Circuit

Teknologi dan Rekayasa Parallel Circuit Voltage is constant Why? There are 3 closed loops in the circuit

Teknologi dan Rekayasa Parallel Equivalent Circuits

Teknologi dan Rekayasa  We’ve now looked at how basic electrical circuits work with resistors that obey Ohm’s Law linearly.  We understand quantitatively how these resistors work using the relationship V=IR, but lets see qualitatively using light bulbs.

Teknologi dan Rekayasa The Light Bulb and its Components  Has two metal contacts at the base which connect to the ends of an electrical circuit  The metal contacts are attached to two stiff wires, which are attached to a thin metal filament.  The filament is in the middle of the bulb, held up by a glass mount.  The wires and the filament are housed in a glass bulb, which is filled with an inert gas, such as argon.

Teknologi dan Rekayasa Light bulbs and Power  Power dissipated by a bulb relates to the brightness of the bulb.  The higher the power, the brighter the bulb.  Power is measured in Watts [W]  For example, think of the bulbs you use at home. The 100W bulbs are brighter than the 50W bulbs.

Teknologi dan Rekayasa Bulbs in series experiment  One bulb connected to the batteries. Add another bulb to the circuit in series.  Q: When the second bulb is added, will the bulbs become brighter, dimmer, or not change?  We can use Ohm’s Law to approximate what will happen in the circuit in theory:

Teknologi dan Rekayasa Bulbs in series experiment continued…

Teknologi dan Rekayasa Bulbs in parallel experiment One bulb connected to the batteries. Add a second bulb to the circuit in parallel. Q: What happens when the second bulb is added?  We can use Ohm’s Law to approximate what will happen in the circuit:

Teknologi dan Rekayasa Light bulbs are not linear  The resistance of light bulbs increases with temperature The filaments of light bulbs are made of Tungsten, which is a very good conductor. It heats up easily.

Teknologi dan Rekayasa As light bulbs warm up, their resistance increases. If the current through them remains constant: They glow slightly dimmer when first plugged in. Why? The bulbs are cooler when first plugged in so their resistance is lower. As they heat up their resistance increases but I remains constant  P increases Most ohmic resistors will behave non-linearly outside of a given range of temperature, pressure, etc.

Teknologi dan Rekayasa Voltage versus Current for Constant Resistance The light bulb does not have a linear relationship. The resistance of the bulb increases as the temperature of the bulb increases.

Teknologi dan Rekayasa “Memory Bulbs” Experiment  Touch each bulb in succession with the wire, each time completing the series circuit  Q:What is going to happen?  Pay close attention to what happens to each of the bulbs as I close each circuit.

Teknologi dan Rekayasa “Memory Bulbs” Continued…  How did THAT happen??  Temperature of bulbs increases  resistance increases  power dissipation (brightness) of bulbs increases  Filaments stay hot after having been turned off  In series, current through each resistor is constant  smallest resistor (coolest bulb) has least power dissipation, therefore it is the dimmest bulb

Teknologi dan Rekayasa Conclusion  Ohmic resistors obey Ohm’s Law linearly  Resistance is affected by temperature. The resistance of a conductor increases as its temperature increases.  Light bulbs do not obey Ohm’s Law linearly  As their temperature increases, the power dissipated by the bulb increases i.e. They are brighter when they are hotter

Teknologi dan Rekayasa You’re turn to do some experiments!  Now you get to try some experiments of your own, but first, a quick tutorial on the equipment you will be using

Teknologi dan Rekayasa The equipment you’ll be using: - Voltmeter - Breadboard - Resistors - 9V battery Let’s do a quick review…

Teknologi dan Rekayasa Voltmeter

Teknologi dan Rekayasa Measuring Voltage

Teknologi dan Rekayasa Breadboards  You encountered breadboards early in the year. Let’s review them: The breadboard How the holes on the top of the board are connected:

Teknologi dan Rekayasa Series Resistors are connected such that the current can only take one path

Teknologi dan Rekayasa Parallel Resistors are connected such that the current can take multiple paths

Teknologi dan Rekayasa Real data In reality, the data we get is not the same as what we get in theory. Why? Because when we calculate numbers in theory, we are dealing with an ideal system. In reality there are sources of error in every aspect, which make our numbers imperfect.

Teknologi dan Rekayasa