1. Ohm’s Law
Physics
Dr. Robert MacKay

2. Voltage (Volts) Electrical Pressure- + + - V

3. Current (Amps) Charge FlowI + + + + + + + + + + V + -

4. Resistance, R (Ohms, Ω) Impedence- - - - - - - - - -
R Depends on
Temperature
Material of wire
Area of wire
Length of wire V + -

5. Resistance, R (Ohms, Ω) Impedence
Area - - - - - - - - - -
Length
R Depends on:
* Temperature
* Material of wire
* Cros sectional Area
* Length

6. Resistance and Resistivity
r= resistivity
R=resistance

8. As a wire heats it up its resistance increases.
Some thermometers work on this principle.

9. Ohm’s Law V= I R I - - - - - - - - - - V + -

10. Exercise
Find the electric voltage required to have 10 Amps of current flow through a 2 Ω resistor.
Given: I= 10 A and R=2 Ω
Wanted: V
Solution: V=I R
V= (10 A) (2 Ω) = 20 Volts

11. Exercise
Find the electric current flowing in a 10 Ω resistor (light bulb) when connected to a 20 Volt battery.
Given: V= 20 Vand R=10 Ω
Wanted: I
Solution: V=I R or I=V / R
I= (20 V) / (10 Ω) = 2 Amps

12. Exercise
When a 60 Volt battery is connected to a circuit 4.0 amps of current flow from the battery. What is the circuit’s resistance.
Given: V= 60 V and I=4.0 Amps
Wanted: R
Solution: V=I R or R=V / I
I= (60 V) / (4.0 Amps) = 15 Ω

13. Electric Power (Watts)
Power = Current x Voltage
1 Watt = Amp x Volt

14. Curcuits
R=10 Ω R
10 Ω I
P= ?
I =? A V
20 V P
V= 20 V

15. Curcuits V= I R or I = V / R R=10 Ω R 10 Ω I 2.0 A P= ? I =? A V 20 V

16. Curcuits V= I R or I = V / R R=10 Ω R 10 Ω I 2.0 Amps P= ? I =? A V
40.0 Watts P
V= 20 V
P= I V = 2A(20V) = 40W

17. Curcuits
R=? R I
P= 60 W
I =? A V
120 V P
60 W
V= 120 V

18. Curcuits V= I R R=? R I P= 60 W I =? A V 120 V P 60 W V= 120 V
P= I V or
I=P/V=60W/120V=0.5A

19. Curcuits
R=? R I
P= 60 W
0.50A
I =? A V
120 V P
60 W
V= 120 V

20. Curcuits R=? R I P= 60 W 0.50A I =? A V 120 V P 60 W V= 120 V
R=V/I=120V/0.5A=240W

21. Curcuits
R=? R
240W I
P= 60 W
0.50A
I =? A V
120 V P
60 W
V= 120 V

23. Curcuits R=10 Ω R I =2 A 10 Ω I P= 40 Watts 2 A A V 20 V 40 Watts P

26. Two light bulbs are connected in series to a battery the resistance of each is 20 W. What is the total effective resistance of this series combination?

27. Two light bulbs are connected in series to a battery the resistance of each is 20 W. What is the total effective resistance of this series combination?
Req=R1+R2= 20 W + 20 W= 40 W

28. Two light bulbs are connected in series to a 40 Volt battery
Two light bulbs are connected in series to a 40 Volt battery. The resistance of each is 20 W. What is the total Current leaving the battery?

29. Two light bulbs are connected in series to a 40 Volt battery
Two light bulbs are connected in series to a 40 Volt battery. The resistance of each is 20 W. What is the total Current leaving the battery?
V=I R or I=V/R (R=Req=40 W)
I=40V/40 W =1.0 Amp

34. Curcuits R=10 Ω R I =2 A 10 Ω I P= 40 Watts 2 A A V 20 V 40 Watts P

40. A bit of trivia
When 2 equal resistors are connected in parallel the effective resistance is half the value of each. (two 10 W resistor Req=5 W)
When 3 equal resistors are connected in parallel the effective resistance is one-third the value of each. (three10 W resistor Req=3.33 W)
When 4 equal resistors are connected in parallel the effective resistance is one-forth the value of each. (four10 W resistor Req=2.5 W)
When N equal resistors are connected in parallel the effective resistance is 1/N the value of each. (N 10 W resistor Req=10/N W)

41. What is the effective total resistance when 10 equal 10 W resistors are connected in parallel?

42. What is the effective total resistance when 10 equal 10 W resistors are connected in parallel?
Req=10 W /10=1.0 W

43. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?

44. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?
V=V1=V2=20 V

45. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the current flowing through each resistor?

46. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the voltage difference across each resistor?
I=V/R I1=V1/R1 and I2=V2/R2
I1=20V/ 20 W =1.0 A
I2=20V/ 20 W =1.0 A

47. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the total effective resistance of this circuit?

48. Two 20 W Resistors are connected in parallel to a 20 V battery
Two 20 W Resistors are connected in parallel to a 20 V battery. What is the current flowing through each resistor?
R=V/I=20V/2.0A=10 W