1. Worksheet: Electric current, battery and bulb

Activity 1-1

The torch is avery simple electric circuit. Try to find out how it works, build up your own electric circuit and try to design a simple electric device.

  1. Dismantle the torch and examine its components. What are they?
  1. Draw acircuit diagram for the torch. Mark the current direction.
  1. Label each torch component and describe its function.
  1. Check the material of the torch case. What is it made of? Is it a part of the circuit?
  1. Put the torch back to its initial shape.
  1. Build your own simple electric circuit that makes the bulb light up. Check the bulb parameters first. Sketch the circuit diagram.

Activity 1-2

Now you know how to construct a simple electric circuit that lights up a bulb. Now try to design a simple electric device. You can use extra switches, wires and bulbs. You can use these materials:

  • Three bulbs (e.g. 4,5V/0,3A), typ? battery (4,5V), leads, one-way (single-pole-single-throw) switch, two-way (single-pole-double-throw) switch, double-pole-double-throw switch

Invent and construct the electric circuits according to the description. In order to understand how the more complicated switches work, look up the information at

  1. Christmas tree lights: You want to light up your Christmas tree with three bulbs. What happens if one of the bulbs fails? Connect them the way that if one of the bulbs fails, the other two are still lit. Sketch the circuit diagram.
  1. Lighting a tunnel: A person walking through the tunnel turns a lamp in the first half of it and then he turns a second lamp for the second half of the tunnel and the first one is turned off. Connect the two bulbs the way it works according to the description. Sketch the circuit diagram.
  1. Entry and exit light switches: A room has two doors. Light switches are at both doors. Either switch turns the light in the room and off. Connect the bulbs the way it works according to the description. Sketch the circuit diagram.

  1. Worksheet: What material conducts electric current?

In this activity you have to design and carry out an experiment to examine different materials (wires of different materials, pencil lead, match, piece of plastic, distilled water, tap (salty, sweet) water, glass, porcelain, china plate with metal strip, etc.) and their conductivity. Use a bulb as an indicator of current.

  1. Draw a circuit diagram in order to investigate the ability of different materials to conduct electric current.
  1. Fill in the table according to your observation. Tick into the appropriate box.

Material / Bulb brightness
dim bright
  1. Which material is the best conductor?
  1. Which material is the worst conductor?

  1. Worksheet: Measuring current and voltage

In this activity you are going to learn how to measure current and voltage in simple electric circuit. Firstly, set up the simple electric circuit and connect the current and the voltage sensor as in the figure.

Picture - Simple electric circuit – battery, bulb, leads, switch, current sensor
  1. Open the file “Measuring current and voltage”. The amount of current is displayed digitally. Write down the current flowing through the circuit.
    I1 =
  1. Now connect the current sensor from the other side of the bulb. Read its value and compare it with the previous reading.
    I2 =
  1. What happens with the current value when you exchange the current sensor leads?
  1. Connect the current sensor so that it displays positive values. Display the current vs. time diagram. Start measuring and try closing the switch for a few seconds and then opening it for a several seconds. Sketch your result.

Figure - Simple electric circuit – battery, bulb, leads, switch, voltage sensor across the battery
  1. Now disconnect the current sensor and set the circuit as in the figure but do not connect the voltage sensor yet.
  1. Firstly, connect both clips of one voltage sensor together. Observe the reading. Next connect both clips to the same point in the circuit. Close the switch. Then connect both clips at the two ends of the same wire. Close the switch. Finally connect the voltage sensor clips to the battery as in the figure. Close the switch. Test your predictions.

Prediction / Result
U (V) / U (V)
Clips together
Clips at the same point
Clips at the two ends of the same wire
Clips at the battery
  1. In the same circuit, how would you expect the voltage across the battery to compare to the voltage across the bulb with the switch open and closed? Test your predictions.

Prediction / Result
U battery (V) / U bulb (V) / U battery (V) / U bulb (V)
Switch open / U = / U = / U = / U =
Switch closed / U = / U = / U = / U =
  1. Explain the results. What is going on as the switch is closed and opened?
  1. Now connect a voltage and a current sensor as in the figure so that you are measuring the voltage across the battery and current through the battery at the same time. Display current vs. time diagram and voltage vs. time diagram.

Figure - Simple electric circuit – battery, bulb, leads, switch, voltage sensor across the battery, current sensor next to the battery
  1. Start measuring and open and close the switch several times. Sketch your graphs and write down the results.

U battery (V) / I (A)
Switch open
Switch closed
  1. Explain your results. What happens to the current through the battery and the voltage across it when the switch is closed and open?
  1. Now suppose you connect a second bulb in the circuit as shown in the figure. How do the readings change? Predict.

Figure - Simple electric circuit – battery, two bulbs, leads, switch, voltage sensor across the battery, current sensor next to the battery
  1. Connect the circuit with two bulbs and test your prediction.

Prediction / Result
U battery (V) / I (A) / U battery (V) / I (A)
Single bulb in a circuit
Two bulbs in series
  1. Explain the results. Does the battery appear to be a source of constant current, constant voltage, or neither when different elements are added to a circuit?

  1. Worksheet: Electric element in a dc circuit

Activity 4-1 Resistor and Ohm’s law

In this activity you are going to use common electric component called resistor that is usually connected to a circuit to make current more difficult to flow. This property to resist the current is described by the physical quantity of resistance, marked R. Now you are going to investigate how the voltage across a resistor influences the current flowing through itand what role is played by its resistance.

  1. Open the file “Current-voltage relationship”. Set up a simple electric circuit with a resistor and connect the current and the voltage sensor as in the figure. In the experiment you will use a variable power supply in order to change the voltage across the resistor while watching the corresponding current through it.

Simple electric circuit – power supply variable, resistor, current and voltage sensor
Fig.
  1. Do not start measuring yet. Imagine you turn the dial on the power supply and hence increase the voltage across the resistor. What happens with the current? Draw your prediction into the graph below.

Prediction / Result
  1. Start measuring current and voltage. Turn the dial on the power supply slowly from 0V up to 10V within 10 seconds. Do not exceed the recommended maximum voltage. Compare your result with the prediction. Does it agree?
  1. Fill in the table below for at least three voltage values. For each reading check the ratio between the voltage across the resistor and current.

U (V) / I(A) / (V/A)
1.
2.
3.
4.

What is this ratio between the voltage and the current in each case?

  1. Describe the result of your measurement. What is the mathematical relationship between the current flowing through the resistor and the voltage across it?
  1. Try to fit the graph with the appropriate function. Use the fit routine in the software. Write down the function type and the value of its parameters.

f(x)=a=

  1. Identify the physical meaning of the variables x, y in the function y=f(x).

x =y =

  1. The relationship that you have observed is known as Ohm’s law. In order to put the law into its normal form we have to define another physical quantity known as conductivity marked G. The unit of the conductivity is the Siemens, marked S. Conductivity is defined as the slope of the graph (parameter a). Its inverted value is known as resistance marked R. The unit of the resistance is the ohm, marked , Try to define conductivity and resistance in terms of UandI.

G =R =

  1. State the mathematical relationship between the current flowing through the resistor and the voltage across it using the quantities of U, I and R (G, eventually).

I =

This formula is known as Ohm´s law. Circuit elements that obey Ohm´s law are said to be ohmic.

  1. Based on your measurement, is the value of resistance constant or does it change as the current through the resistor changes?
  1. What is the value of the resistance of your resistor? Use the appropriate parameter of the function used to fit your measurement. How does it agree with the value written on the label?

Rmeasured = Rwritten =

  1. Note that resistors are manufactured such that their actual value is within a tolerance. For most resistors, the tolerance is 5% or 10%. Determine the tolerance of the measured resistor and calculate the range of values for it. Is the measured value within the tolerance?

Tolerance in % =Range of values: Rmeasured =

  1. Repeat the measuring procedure for a resistor with higher resistance. Draw your prediction of current-voltage diagram first.

First resistor
Result from the previous measurement / Second resistor with higher resistance Prediction
  1. Describe the difference between I-Udiagrams of two resistors with different resistance.

Activity 4-2 Light bulb and Ohm’s law

In the activity 4-1 you have discovered that for a resistor the relationship between the current through the resistor and the voltage across it is proportional. In the following activity you are going to explore the same relationship for a bulb.

  1. Open the file “Current-voltage relationship”. Replace the resistor by the bulb as in figure.

Simple electric circuit – power supply variable, bulb, current and voltage sensor
Fig.
  1. Do not start measuring yet. Imagine you turn the dial on the power supply and hence increase the voltage across the bulb. What happens to the brightness of the bulb?
  1. What happens with the current? Draw your prediction of the I-U relationshipinto the graph below.

Prediction / Result
  1. Start measuring current and voltage. Check your bulb parameters. Turn the dial on the power supply slowly from 0V up to the maximum voltage within 10 seconds. Do not exceed the recommended maximum voltage since it can burn out the bulb. Compare your result with the prediction. Does it agree?
  2. Compare the result for the bulb to that for the resistor. Describe the differences.
  1. Based on your measurement, is the value of resistance constant or does it change as the current through the bulb changes?
  1. Find out the resistance of the bulb for at least three values of current flowing through it (I1 < I2 < I3).

I(A) / U (V) / R=()
1.
2.
3.
4.
  1. How does the resistance change with increasing current?
  1. Does your bulb follow Ohm’s law? Is a bulb an ohmic circuit element? Explain.

Activity 4-3 Other electric elements in a dc circuit

You have already investigated the behaviour of a resistor and a bulb in a dc circuit. There are many other electric elements that can be parts of an electric circuit. Now you can extent your investigation exploring the behaviour of devices such as diodes, LEDs and Zener diodes.

  1. Open the file “Current-voltage relationship”. Replace the resistor by the diode (e.g. LED – light-emitting diode) as in figure.

Simple electric circuit – power supply variable, diode, current and voltage sensor
  1. Do not start measuring yet. Imagine you turn the dial on the power supply and hence increase the voltage across the diode. What happens with the current? Draw your prediction of the I-U relationshipinto the graph below.

Prediction / Result
  1. Start measuring current and voltage. Check your diode parameters first. Turn the dial on the power supply slowly from 0V up to the maximum voltage within 10 seconds. Do not exceed the recommended maximum voltage since it can burn the diode. Now turn the dial back, reverse the leads of the diode and try again. What do you observe?
  1. You have observed that diode behaves differently in response to different direction of current having a preferred current direction. Does a resistor or a bulb behave the same way?
  1. Now place the diode into the position the current flows through the circuit. Start measuring current and voltage again. Compare your result with your prediction. Does it agree?
  1. Compare the result for the diode to that for the resistor. Describe the differences.
  1. Based on your measurement, is the value of resistance constant or does it change as the current through the diode changes?
  1. Find out the resistance of the diode for at least three values of current flowing through it (I1 < I2 < I3).

I(A) / U (V) / R=()
1.
2.
3.
4.
  1. What can you conclude about the resistance of the diode?
  1. Does your diode follow Ohm’s law? Is a diode an ohmic circuit element? Explain.

Activity 4-4 What electric element is inside the black box?

You have already investigated the behaviour of three elements – resistor, bulb and diode often used in electric circuits. Now you have five boxes available, each with one of the investigated elements. You are going to reveal the black box content using variable power supply, leads, switch and voltage and current sensor.

Design your experiment, plan the measuring procedure and draw conclusions.

Figure of black box with the question mark.
  1. Worksheet: Human body and Ohm´s Law

In simple terms the human body can be considered as a circuit through which an applied potential difference will drive a current. The body acts as a resistor with resistance R depending on the path the current flows in the body. As we know from Ohm's Law, the current flowing will depend on the voltage applied as well as on the resistance of the current path. For example, if you touch a high voltage wire in a cable, current passing from the wire through you to the ground can cause an electric shock. Your body is controlled by electrical nerve impulses, so electric currents can disrupt normal bodily functions. It is the current, not the voltage that determines the severity of the electric shock. The current travelling through a body can damage your organs, with the heart, brain and spinal cord being particularly susceptible.

Resistance of the human body

The human body composed largely of waterhas very low resistance. That means that your blood and fluids with high amount of conductive chemicals are good conductors with a resistance of about 200 . However, current must first pass through the skin that has very high resistance, the value depending on its nature, on the possible presence of water, and on whether it has become burned. Thus, most of the resistance to the passage of current through the human body is at the points of entry and exit through the skin. A person with naturally hard and dry skin can have a resistance of 500 000while soft and sweaty palms may have resistance 10 to 50 times lower. The skin resistance becomes very lowif it has been burnedbecause of the presence of conducting particles of carbon or if it has been wounded because of the presence of blood or much thinner skin. A person standing in saltwater has a skin resistance of only 500.

When current travels through your body it must pass basically through three series resistances: your skin (your fingers), internal part of your body, and your skin again (your toes). Adding up the resistance of your moist fingers (e.g. 20000 ), your body fluids (e.g. 200) and your toes (e.g. 30000 ) under the voltage of 230V you get a current of approx. 0,005A that passes through your body.

How much current is harmful?

Individual body chemistry has a significant impact on how electric current affects on individual. Also the alternating current is more dangerous than the direct one. There are few reliable figures for shock current effects because they differ from person to person and for a particular person with time and also depend on the current path. For example, the shock current of 500mA may have no lasting ill effects if its duration is less than 20ms, but 50mA for 10s could well prove to be fatal. The most dangerous results are ventricular fibrillation (where heart beat sequence is disrupted) and compression of the chest, resulting in a failure to breathe.

Rough limits say:

  • Currentgreater than1mA causes discomfort.
  • Above 16mA you loose control of your muscles and they undergo contractions.
  • Between 25mA and 100mA, you have difficulty breathing and eventually respiration stops.
  • Between 100mA and 200mA, your heart stops pumping and undergoes contractions called ventricular fibrillation.
  • Above 200mA irreversible heart damage occurs.

How to increase safety?