Download Ohm’s law - Gabrielse

Ohm’s law
V
I
R
J. Gabrielse
Voltage (V)
Definition: what pushes charges
Unit: volt (V)
Measured by putting voltmeter in parallel.
-
+
J. Gabrielse
Current (I)
Definition: moving electrical charge
Unit: ampere (A)  sometimes called Amps
Measured by putting ammeter in series.
Why?  So the current has to run through the
meter.
- +
J. Gabrielse
Resistance (R)
Definition: how hard it is for charges to move
Unit: ohm (Ω)
Measured by putting multimeter in parallel.
Disconnect all batteries.
The multimeter shows the
resistance between the two
points you touch.
J. Gabrielse
Ohm’s Law: V = I R
What is the equation for V?
V= ?
IR
V
I
I = ?
R
R= ?
J. Gabrielse
Ohm’s Law: V = I R
What is the equation for I?
V= ?
IR
V
I
R
V
I = ?
R
R= ?
J. Gabrielse
Ohm’s Law: V = I R
What is the equation for R?
V= ?
IR
V
I
I =
R
V
R
V
R= ?
I
J. Gabrielse
Resistors in Series
-
RA
What -is the
+
resistance of the
circuit?
+
RB
=
Requivalent
Requivalent =
We can replace RA
and RB with an
equivalent resistor,
RA + RB
Requivalent.
J. Gabrielse
Resistors in Series
Example
-
30 Ω
-
+
20 Ω
+
=
Requivalent
Requivalent
= 50 Ω
Requivalent = RA + RB = 30Ω + 20Ω = 50Ω
J. Gabrielse
Resistors in Parallel
RA
1
R equivalent
-
=
In parallel circuits we
can also replace RA
and RB with one
equivalent resistor,
Requivalent.
+
RB
-
+
Requivalent
1
1


 ... 
RA RB
R equivalent 
1
 1
1 



B 
 R A J.RGabrielse
Resistors in Parallel
Example
30 Ω
-
+
=
20 Ω
-
+
Requivalent
1
1
R equivalentWhat


 12
is
R
?
equivalent
 1
1   1  1 

 


 R A R B   30 20 
J. Gabrielse
Notes
5. Ohm’s Law: V = I R
a. Voltage (V): what pushes charges through a circuit
i. Unit: volt (V)
ii. Measured by putting meter in parallel.
iii. Gets used up in a circuit.
b. Current (I): moving electrical charge
i. Unit: ampere (A)
ii. Measured by putting meter in series.
c. Resistance (R): how hard it is for charges to move
i. Unit: ohm (Ω)
ii. Resistors in series: Requivalent = RA + RB
1
iii. Resistors in parallel: R
equivalent 
 1
1 



 RA RB 
J. Gabrielse
Your Turn:
Practice Problems
p. 515: 5, 6, 7
Questions?  Come see me.
Get your class work grade before you leave.
J. Gabrielse