PHY400 – Lab6 – Electrical Resistance & Ohm’s Law

Pre-Lab Questions6

Topic:Electrical Resistance and Ohm’s Law

Objective: To find the resistance of a given resistor.

Apparatus: Power Supply, Ammeter, Voltmeter, Rheostat, Resistor, and

Connecting Wires

Learning Outcomes: After doing this activity the students should be able to

(i)state the quantity supplied by a battery or power supply.

(ii)describe the nature of electrical current that depends on the external resistances of the circuit.

(iii)state the speed of electrical current flowing in a circuit.

(iv)stateOhm’s law.

(v)state the electrical resistance of a human body and the effects of electrical current on the body.

(vi)draw a circuit diagram for an experiment to determine the resistance of a resistor.

(vii)state the relevant data to be taken in the experiment an organize them in an appropriate table.

PRE-LAB QUESTIONS 6

  1. What is the definition of electrical resistance?
  2. What is the physical quantity supplied by a battery or power supply?
  3. Is the current derived from a battery always constant?
  4. How fast does an electric current flow in a circuit?
  5. What is meant by an ohmic conductor? State two examples of ohmic conductors.
  6. Draw a circuit diagram with all the necessary components that will enable you to perform an experiment to determine the resistance of a given resistor.

ii) Electric Shock

Table 1: Effects of electric currents on the body

Current (A) / Effect
0.001 / Can be felt
0.005 / Is painful
0.010 / Causes involuntary muscle control
0.015 / Causes loss of muscle control
0.070 / If through the heart, serious disruption; probably fatal if current lasts for more than 1 s

An electric shock requires difference in electric potential between one part of your body and another part.The electric current passing through the human body is usually determined by the skin resistance, which ranges from about 1000 ohms for wet skin to about 500,000 ohms for dry skin. The internal resistance of the body is small, being between 100 and 500 ohms. (Singapore Science Centre)

  1. Find the current that flows through your body when you simultaneously touch the positive and negative terminal of a 1.5 V battery while your skin is (i) wet (ii) dry.
  2. What is the electrical voltage that can be fatal if applied to our dry body?

References

http: Singapore Science Centre. Question No. 4870 : What is the electrical

resistance of the human body?

Experiment 6

Topic:Electrical Resistance and Ohm’s Law

Objective: To determine the resistance of a given resistor.

Learning Outcomes: After doing the experiment the student would be able to

(i)state the meaning of a battery, resistor, resistance, current, Ohm’s law and an ideal circuit.

(ii)connect the circuit and measure the current passing through and voltage across a given resistor.

(iii)determine the resistance of a given resistor and its uncertainty.

(iv)compare the resistance of an unknown resistor determined from the graph of V versus I, to the resistance determined by direct measurement by using an ohm meter.

(v)apply V = IR to solve related problems.

Apparatus: Power Supply, Ammeter, Voltmeter, Rheostat, Resistor,Multimetre and

Connecting Wires

Introduction

Battery

A battery is a devise that uses chemical energy to separate positive and negative charge, producing potential difference between its terminals. The potential difference causes the current to flow in the wire … (Walker 2004)

A battery uses chemical reactions to produce a difference in electric potential between its two ends, or terminals. The symbol for battery is

When the battery is connected to a circuit, electrons move in a closed path from the negative terminal of the battery to the positive terminal.

In batteries, … a charge separation is maintained between two points, thus creating electric fields that can then do work. It is clear that chemical reactions supplies the energy… (the battery) is the source of energy to the circuit.(Dana 1981)

Battery does not store charge. Battery stores energy. The energy in the battery is transformed to mechanical energy, light energy and thermal energy as it pushes charges through wires and (other electrical components (loads) in the circuit) . Thus, … a battery acts as a pump that absorbs electrons at the negative terminals and releases higher potential energy electrons from the positive terminals. (Cummings, Laws et al. 2004)

Current can pass from one terminal of a battery through conductors back to the other terminal of the battery, if all the components form a complete loop or circuit.

Observations indicate that both batteries and any current-carrying wires connected to them are electrically neutral.

Resistor and Resistance

A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm’s law: . [R = resistance of a resistor; V = potential difference or voltage across the resistor; I = electrical current flowing through the resistor]

In other words, a resistor is a conductor whose function in a circuit is to obey Ohm’s law so that it provides a specified resistance to the flow of charge independent of the potential difference impressed across it.

The resistance of a given circuit elements is defined as the ratio of the potential difference across the element to the current through the elements. (Cummings, Laws et al. 2004)

The 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. For most conductors, increased temperature means increased resistance. The resistance of some materials, called superconductor, reaches zero at very low temperatures.

Electrical resistance is measured in units called ohms (symbol: ) This unit is named after Georg Simon Ohm, a German physicist who in 1826 discovered a simple and very important relationship among voltage, current, and resistance.

Current

The electric current is defined as the rate of flow of charges, (steady current).

The direction of the current in an electrical circuit is the direction in which a positive test charge would move.

In typical circuits the charges that flow are actually the negatively charged electrons. As a result, the flow of electrons and the current arrow point in opposite direction.

Electrons move rather slowly through a typical wire. They suffer numerous collisions with the atoms in the wire, and hence their path is rather tortuous and round about – its average speed is about 104 m/s or 0.01 cm/s. Thus if you switch on the headlights of a car, for example, an electron leaving the battery will take about an hour to reach the light bulb, yet the lights seem to shine from the instant the switch is turned on. How is this possible?

The answer is that as an electron begins to move away from the battery it exerts a force on its neighbors, causing them to move in the same general direction, and in turn, to exert a force on their neighbors, and so on. This process generates a propagating influence that travels through the wire at nearly the speed of light. (Walker 2004)

The amount of current that flows in a circuit depends not only on the voltage but also on the electrical resistance.

Ohm’s Law

A devise is said to obey Ohm’s law whenever the current, I through it is always directly proportional to the potential difference applied V. (V  I). The resistance R of the devise is constant. V = RI.

R is the resistance of the material. A material that obeys Ohm’s law is called Ohmic (linear) conductor which those that do not are called non-Ohmic (non-linear) conductor.

Ammeter, Voltmeter and Rheostat

An ammeter measures the current through a resistor in a circuit and it is connected in series to the resistor while a voltmeter measures the potential difference across the resistor.

A rheostat is a device whose resistance can be varied to provide different potential difference across a resistor in a circuit.

Ideal Circuits

The ideal circuits have three characteristics:

  1. They are powered by ideal batteries. An ideal battery “maintains a constant potential difference (voltage) across its terminals”.
  2. All circuit elements, other than the battery and the connecting wires, are ohmic devices having a significant resistance. An ohmic device has a constant value of R that is not a function of current passing through it.
  3. Ideal connecting wires connect the battery to circuit elements. We assume there is no potential drop across connecting wires. i.e. the resistance of the connecting wire is negligible compared to the whole resistance of the circuit. (Cummings, Laws et al. 2004)

Electric Shock

Table 1: Effects of electric currents on the body(Hewitt 2002)

Current (A) / Effect
0.001 / Can be felt
0.005 / Is painful
0.010 / Causes involuntary muscle control
0.015 / Causes loss of muscle control
0.070 / If through the heart, serious disruption; probably fatal if current lasts
for more than 1 s.

An electric shock requires difference in electric potential – a voltage difference –

between one part of your body and another part.

Instructions

  1. Discuss the formula, the quantities to be measured and the arrangement of circuit in order to achieve the objective of this experiment.
  1. Make the relevant measurements and record them.
  1. Plot the relevant graph and determine the resistance and its uncertainty from this graph.
  1. Compare the value of the resistance determined from the graph to the value determined by direct measurement using an ohm metre.
  1. Write your report and hand it in by the end of the lab period.
  1. Reminders:a) Have the instructor check the circuit before closing the switch.

b) To prevent unnecessary heating in the circuit, the switch should be closed

long enough to obtain the necessary readings.

What should be in your report :

1. Objective :

2. Apparatus : Equipments that you use in the experiment

3. Theory : The concepts and formula that were used in the experiment

including the circuit diagram.

4. Procedures : Steps that you take to perform the experiments

5. Data and Results:

Organize your data in an appropriate table. Be consistent and correct in the use of significant figures. Give the quantity its units and uncertainties.The calculation of the results and its uncertainty should be shown here.

  1. Conclusions : State the results based on the objectives. Include the uncertainties and the possible sources of uncertainties.

Note: Include the answer to the Post-Laboratory Questions in your report.

Post-Laboratory Questions

  1. Does the resistor obey Ohm’s law? State your reason.
  1. Describe the energy transformation that took place in the circuit once all the connections are complete and the current is flowing in the circuit.
  1. Calculate the voltage that is needed across a 1000- resistor so that the current reaches a fatal level? Refer Table 1.
  2. What is the common effect on resistance when the temperature of a resistor is increased?
  1. Why does the value of R in an electric circuit changes when the circuit is left on for a long time?
  1. Why does an ammeter have a low resistance while a voltmeter has high resistance?

References

Cummings, K., P. W. Laws, et al. (2004). Understanding Physics, John Wiley & Sons, Inc.

Dana, R. (1981). "How Batteries Work: A Gravitational Analog." American Journal of Physics 51(9): 829-831.

Hewitt, P. G. (2002). Conceptual Physics, Addison Wesley.

Walker, J. S. (2004). Physics. New Jersey, Pearson Education, Inc.

1