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Ohm’s Law Physics Dr. Robert MacKay

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**Voltage (Volts) Electrical Pressure**

- + + - V

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**Current (Amps) Charge Flow**

I + + + + + + + + + + V + -

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**Resistance, R (Ohms, Ω) Impedence**

- - - - - - - - - - R Depends on Temperature Material of wire Area of wire Length of wire V + -

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**Resistance, R (Ohms, Ω) Impedence**

Area - - - - - - - - - - Length R Depends on: * Temperature * Material of wire * Cros sectional Area * Length

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**Resistance and Resistivity**

r= resistivity R=resistance

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**As a wire heats it up its resistance increases.**

Some thermometers work on this principle.

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

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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

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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

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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

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**Electric Power (Watts)**

Power = Current x Voltage 1 Watt = Amp x Volt

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

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

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**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

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

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**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

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

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**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

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

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

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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?

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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

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**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?

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**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

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

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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)

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

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

Req=10 W /10=1.0 W

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**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?

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**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

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**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?

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**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

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**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?

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**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

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