3. Ohm’s Law
Ohm’s Law - The resistance does not
depend on the voltage or the current flowing
through the circuit.
∆V = I R
∆V = Potential difference or Voltage drop = V, volts
I = Current = A, amps
R = Resistance = Ω, ohms
4. Charge in circuits
q = It
Coulomb =C=As or A = C/s
Elementary charge 1.6 X 10-19 C
5. Power
Power is the rate of doing work. Power is the
rate at which energy is transferred. It is
measured in Watts.
P= ∆E P = Power = W = J/s
t
P = I ∆V
P = I2 R P = (∆V)2
R
6. Measurements
Ohmeter measures resistance in ohms (Ω)
– Hook up in parallel outside circuit
Voltmeter measures voltage in volts (V)
– Hook up in parallel with circuit running
Ammeter measures current in amps (A)
– Hook up in series. Remove a wire in the circuit
and place ammeter in its place.
8. Series Circuit – one current path
The Equivalent Resistance is greater than the
largest resistor
RT = R1 + R2 + ...
The current is the same in each element since
there is only one current path
IT = I1 = I2 = ...
The total voltage is the sum of the individual
voltages and should equal the voltage of the
battery
∆VT = ∆V1+ ∆V2+...
9. Parallel Circuit –
more than one path
The Equivalent Resistance is less than the
smallest resistor
1/RT = 1/R1 + 1/R2 + …
The total current is the sum of the
individual currents
IT = I1 + I2 + ...
The voltage drop is the same across each
resistor because they each have a path
from the battery
∆VT = ∆V1 = ∆V2 = ...
10. Adding a Resistor to a Circuit
Add a resistor in series
1) total resistance increases
2) therefore, current must go down
Add a resistor in parallel
1) total resistance decreases
2) therefore, current must go up
11. Solving Series and Parallel Circuits
Calculate the total resistance. Calculate
each parallel branch resistance then add the
remaining series branches. Make simplified
equivalent circuit drawings in order to solve
the circuits.
Calculate the total current
Use the total current as the current to the
series part of the circuit to calculate its
voltage drop.
Calculate the parallel voltage drop by
subtracting the series drop from the total
voltage drop.
Use a chart to track what is
known/unknown.
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