Bring a number of small electrical devices into the classroom. They can either work on batteries or need a socket to function, but the common element must be that they need electricity. Such devices can include electric toothbrushes, nightlights, portable gaming machines and cellphones. Ask students to tell you what these devices have in common. Guide them with questions such as "How do they function?" or "What do they need in order to work?" to reach the conclusion that electricity runs these devices.
Describe electrical conductors as materials allowing electricity to "go through them." Make a drawing of a rectangular conductor on the board, containing small circles for the electrical charges. Use the "+" and "-" symbols to depict positive and negative charges and show how they are realigned when they're near electrically charged matter (polarization). Erase the design, divide the board in half and ask children to tell you materials which block or don't block the flow of electricity. You can help them by asking questions such as "Are light bulbs made of wood?" or "Why do wires have plastic around them?"
The concept of the electrical circuit is quite easy for third graders to visualize with a few simple drawings. Draw an electrical device on top of the board and two lines, starting from the left and right side of the device and ending at either a socket's or a battery's sides (the voltage source). Explain that the electric current moves in circles within the closed route called "electrical circuit." On the left line connecting the device with the voltage source, draw a switch. This way you can show how people can stop the flow of electricity by "opening the circuit."
Bring a few 1.5-volt bulbs in the classroom, along with copper wires (with a plastic cover) and as many D batteries as bulbs. Ask children to form groups so that each team gets a bulb, two wires and a battery. Ask them to reproduce the circuit drawn on the board by connecting the wires' ends with the two sides of the battery and the bulb's electrical contact (its bottom). Instruct children to hold each wire from the plastic part -- the danger is minimal anyway -- and allow them to light up the bulb.