Ohms Law

Energy & Environment

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Ohms Law

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High Resistance

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Low Resistance

EMF -

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Electricity through a circuit depends on:

EMF/voltage Current

Dependent on the Resistance of the circuit

Discovered by George Ohm

Current (amps)

Resistance (ohm)

Potential Difference (volts)

Ohms Laws Example 1A bicycle light has a resistance

of 3 ohms (Ω )

It uses a current of 0.6 Amps

What voltage does it work at?

Voltage= I x R = current x resistance = 0.6 amps x 3 ohms =1.8 volts

Ohms Law Example 2The power of a torch is 1.5 watts

The current is 0.5 amps

What is the resistance?

Power =EMF x CurrentWatts= Volts x Amps

Power =EMF x CurrentEMF = Power Current = 1.5 watts 0.5 amps =3 volts

R = V I = 3 0.5 = 6 Ω

VA PABAL

Do you think the resistance is more in a long or short bit of wire?

Laws of Resistance

Electrons are more likely to bump into other atoms

Ω-

ΩThe resistance of a wire increases with length

Ω Ω- -

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Short Long

Do you think the resistance is more in a thick or thin bit of wire?

The resistance of a wire decreases with increasing thickness

Laws of ResistanceDo you think the resistance varies with the type of material?

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High Resistance

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Low Resistance

Yes, metals have low resistance and non metals have high resistance

The more free electrons there are the lower the resistance

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Do you think the temperature effects the resistance?Yes, when materials get hot, the atoms get energy so move around making it more difficult for the electrons to move

Is Resistance Bad?

But sometimes we add resistors to a circuit

These reduce the current so that the appliance doesn’t not get overpowered and overheat.

Some resistance can be bad as it uses up power and can mean that we need a larger EMF e.g a 12V battery instead of a 9V

If you connect an LED diode to 9V it will burn out quickly.

If you connect an LED diode to 9V it will burn out quickly. But if you connect a proper resistor to the LED, then it will limit the current to the diode and it will not burn out

One Road Tolls (resistors)

Resistance: If electrons were cars….

Road (wire) with lots of cars (electrons)

Road Toll (resistor) reducing the flow of cars (electrons)

Road (wire) with fewer cars

Road Tolls (resistors) in parallel

Road Tolls (resistors) in series

Resistance: If electrons were cars….

Road (wire) with lots of cars (electrons)

Road (resistor) Tolls in series reducing the flow of cars (electrons)

Road (wire) with fewer cars

Road (wire) with lots of cars (electrons)

Road tolls (resistor) in parallel increasing the flow of cars (electrons)

Whilst this is a good example to show the difference of resistors in series & parallel it does not show

a true representation of the current. In a series circuit the

current is always the same.Itotal=I1=I2=I3

Resisters in Series Circuits

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R1= 2Ω

R2 = 2Ω

What do you think the total resistance of this circuit is?

Rtotal= R1 + R2

Sometimes you need a resistor with a value you don’t have. So you can use a combination to create the value you need

Rtotal= R1 + R2 = 2 + 2 = 4 Ω

Resisters in Parallel Circuits

1 = 1 + 1Rtotal R1 R2

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R2 = 2ΩR1 = 2Ω

The two resisters will not be added as the electrons will only flow through one or the other.

1 = 1 + 1Rtotal R1 R2 = 1 + 1 2 2 =1 Ω

What do you think the total resistance of this circuit is?

How do we get this?

Parallel Resistors proof

1 = 1 + 1Rtotal R1 R2

I= V R

V1 = Itotal –I2

R1

Itotal =I1 +I2

Vtotal =V1 =V2

V1 =V1 - V1

R1 Rtotal R2

1 =1 - 1R1 Rtotal R2

1 =1 + 1Rtotal R1 R2

V1 = R1 X I1

V1 = I1

R1

V1 = Vtotal – V2 R1 Rtotal R2

Series Circuits

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Current is the same throughout

EMF is the sum of the voltages in the circuit

Total resistance is the sum of the resistance in the circuit

Itotal=I1=I2=I3 Rtotal=R1+R2+R3

(EMF)vtotal=V1+V2+V3

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Parallel Circuits

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Itotal=I1+I2+I3

1 =1 +1 +1Rtotal R1 R2 R3

(EMF)vtotal=V1=V2=V3

Current is the addition over the branches

EMF is the same of the voltages across the branches

1/Total Resistance = 1/resistance 1+1/resistance 2 + 1/resistance 3

Series Circuits Exercise

1.What is the total resistance of this circuit?2.What is the current through the lamp?3.What is the EMF from the battery?

Rtotal= R1 + R2 + R3 = 3 + 2 +1 = 6 Ω

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R1= 3Ω

R2 = 2ΩR3 = 1Ω

I1= 2 amps

I2= 2 ampsI3= ?

Itotal= I1 = I2 = I3

= 2=2=? = 2 amps

V3= ? V2=?

V1= ?

Series Circuits Exercise

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R1= 3Ω

R2 = 2ΩR2 = 1Ω

I1= 2 amps

I2= 2 ampsI3= 2 amps

Vtotal= V1 + V2 + V3

= ?+?+?

V3= ? V2=?

V2= ?

3.What is the EMF from the battery?

V1= I1 x R1

= 2x 3 =6 volts

V2= I2 x R2

= 2x 2 =4 volts

V3= I3 x R3

= 2x 1 =2 volts

Vtotal= 6+4+2 = 12 volts

Parallel Circuits Exercise

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I2= 3 ampsI1= ? amps

V1= ? V2=?I3= 2 amps

V3= ?

R1 = 1Ω R2 = 2Ω R3 = 3Ω

1.What is the total resistance of this circuit?2.What is the current through the lamp?3.What is the EMF from the battery?

Parallel Circuits Exercise

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I2= 3 ampsI1= ? amps

V1= ? V2=?I3= 2 amps

V3= ?

R1 = 1Ω R2 = 2Ω R3 = 3Ω

1.What is the total resistance of this circuit?2.What is the current through the lamp?3.What is the EMF from the battery?

1 = 1 + 1 + 1Rtotal R1 R2 R3 = 1 + 1 + 1 1 2 3 =1.83

Rtotal = 1 1.83 = 0.54Ω

I1 = V1 = ? R 1 1

How can we find the voltage? What is special about the voltage across parallel circuits? (EMF)vtotal=V1=V2=V3

Parallel Circuits Exercise

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I2= 3 ampsI1= ? amps

V1= ? V2=?I3= 2 amps

V3= ?

R1 = 1Ω R2 = 2Ω R3 = 3Ω

2.What is the current through the lamp?3.What is the EMF from the battery?

(EMF)vtotal=V1=V2=V3

V2= I x R = 3 x 2 = 6 volts

So V1=V2=6 volts

I1 = V1 = 6 = 6 amps R 1 1

(EMF)vtotal=V1=V2=V3=6 volts

Series & Parallel Circuit Exercise

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I2= 3 amps

I3= ? ampsV3= ?

V2=?

I1= 2 amps

V1= ?

R3 = 1Ω

R2 = 2ΩR1 = 3Ω

1.What is the total resistance of this circuit?

Rparallel

1 = 1 + 1Rtotal R1 R2 = 1 + 1 3 2 = 0.83 Rtotal = 1 0.83 = 1.2Ω

Rtotal = Rparallel + R series

R series= 1 Ω

Rtotal = Rparallel + R series

= 1.2 + 1 =2.2 Ω

1. Draw a series and parallel circuit2. What happens to the current in a series and

a parallel circuit?3. What increases resistance?4. Why do we add resisters to circuits?5. Explain what happens to resistance in series

& parallel circuits.

Summary Questions