Data Sheet

-25-

19. Ohm’s Law

Using Ohm’s Law the resistance of each part could be

calculated.

Build the circuit to the left. In this circuit the motor (95),

the LED (69), and the lamp (76) are in series so they all see

the same current. If you had a voltmeter and measured the

voltage drop across each component, you would see that the

voltage drop across the LED (69) is much greater than the

voltage drop across the lamp (76) or motor (95).

According to Ohm’s Law, R = V÷I which means the internal

resistance of the LED (69) is much greater than the internal

resistance of the lamp (76) or motor (95). The high internal

resistance of the LED (69) is limiting the current in this circuit,

which is why the motor (95) spins very slowly and the lamp

(76) does not light. Each part is designed using Ohm’s Law to

perform best when they have full battery voltage.

20. Kirchhoff’s First Law

Kirchhoff’s rst law states: At any node (junction) in an

electrical circuit, the sum of currents owing into that node is

equal to the sum of currents owing out of that node.

Build the circuit on the left. The top block shown in red as

“2nd level” represents a node. If a positive current is coming

into a node and a negative current is leaving a node, then:

switch

+ I

motor

+ I

lamp&LED

= 0 or I

switch

= –I

motor

– I

lamp&LED

This shows that the current owing into node E1 from the

switch is equal to the current owing out of node E1 to the

motor and lamp&LED.

2nd level