Electromagnets and Transformers

CP11a Transformers

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Re-cap magnetic fields of wires and electromagnets

• Complete the multiple choice questions on the next slides

Multiple Choice

How can you test to see if there is a current running through a wire?

• listen carefully to see if you hear a buzzing sound

• place several compasses around the wire, they will point in a circle, showing a magnetic field

• place several compasses around the wire, they will point outwards, showing a magnetic field

Multiple Choice

An electromagnet is a magnet that

• can be turned on and off and whose strength can be varied

• always has a magnetic field around it

• has very little strength

Multiple Select

Permanent Magnets and Electromagnets are similar in which TWO ways?

• they can both be turned off and on

• they both have a north and south pole

• they have a similar magnetic field surrounding them

Multiple Select

What THREE ways are electromagnets different from permanent magnets?

• they can be turned off and on

• their strength can be increased by increasing the current in the wire

• they are smaller in size

• their magnetic poles (North and South) can be reversed

What are transformers used for and how can the output voltage be calculated?

• State uses of transformers

• Describe the structure of a transformer

• Recall the equation for electrical power

• Apply the law of conservation of energy to derive an equation for transformers

• Calculate the output voltage

Transformers

• Transformers are used to increase or decrease the potential difference (voltage) of an electricity supply.

• Mains electricity in the UK is supplied at 230 V

• The transformer in this shaving unit is 'stepping down' the voltage to 115 V to be suitable for shavers.

​Make notes from the following slides and from the video

​GCSE Physics - How Transformers Work #82 - YouTube

Transformers

• A transformer consists of three main parts

• A soft iron core

• A primary coil, wound around the core, connected to an alternating current supply.

• A secondary coil wrapped around the opposite side of the core

• The coils are not connected to each other

Transformers

• The magnetic field from the primary coil causes an electric voltage to be induced in the secondary coil.

• This is called electromagnetic induction.

• The size of the induced voltage depends on the number of coils on the primary and secondary coils

Multiple Choice

A transformer is a device that

• turns cars into robots

• changes electrical power

• changes electrical voltage

Multiple Choice

The core of a transformer is made of iron because...

• it is strong

• it conducts electricity

• it is a soft magnetic material, which can be controlled by electricity

• It is a hard magnetic material which remains permanently magnetised

Multiple Choice

A transformer that icreases voltage is called a

• step up transformer

• step down transformer

Multiple Choice

The primary and secondary coils are not connected so how is electricity passed from one coil to the other?

• the changing magnetic field of the first coil creates a current in the other coil

• electricity passes through the iron core to the second coil

Electrical power and conservation of energy

• Recap equation:

• electrical power = current x potential difference P = I x V

• Since energy can not be created or destroyed, the power supplied to the transformer in the primary coil must be equal to the power transferred in the secondary coil.

• Since the power is constant a step-up transformer steps up voltage but steps down current​

• So the equation in the picture can be applied:

Multiple Choice

The voltage supplied to a circuit is 17 V and the current running through is 10 A. What is the power generated?

• 170 W
• 170 A
• 1.7 V
• 1.7 W

Multiple Choice

A step down transformer...

• decreases the voltage but increases the current

• decreases the voltage and decreases the current

• increases the voltage and increases the current

• increases the voltage but decreases the current

Multiple Choice

Watts is the same as...

• Joules per second
• Voltage per second
• Current per second

Multiple Choice

Electrical power can be calculated using

• Resistance x Voltage
• Voltage x Current 
• Voltage ÷ Current

Multiple Choice

In a transformer, the input power is __________ the output power.

• less than

• equal to

• greater than

Fill in the Blank

A step-up transformer has an input power of 75 W. What is the output power?

Multiple Choice

What would happen to the power output of a transformer if it wasn't 100% efficient?

• Increase

• Decrease

• Stay the same

Multiple Choice

A transformer has a current of 10 A in the primary coil and 25 A in the secondary coil. When a potential difference of 120 V is applied to the primary coil, what is the voltage in the secondary coil?

• 48V

• 30V

• 20V

Need help with the last question?

• A transformer has a current of 10 A in the primary coil and 25 A in the secondary coil. When a potential difference of 120 V is applied to the primary coil, what is the voltage in the secondary coil?

• Vp x Ip = Vs x Is

• 120 x 10 = Vs x 25

• Vs = (120 x 10 ) / 25

Multiple Choice

What's the missing value? Vp = ?, Ip = 2 A, Vs=200 V, Is = 0.1 A

• Vp = 10 V

• Vp = 20 V

• Vp = 100 V

Multiple Choice

What's the missing value? Vp = 11,000 V, Ip = ?, Vs=230 V, Is = 11 A

• Ip = 0.23 A

• ip = 4.3 A

• Ip = 100 A

Multiple Choice

What's the missing value? Vp = 33 kV, Ip = 2 A, Vs = 230 V, Is = ?

• Is = 0.23 A

• Is = 33 A

• Is = 287 A

Multiple Choice

What happens to the output current when a transformer causes the potential difference of the supply to increase?

• current increases

• Current decreases

• current stays the same

Multiple Choice

What happens to the output current when a transformer causes the potential difference of the supply to decrease?

• current increases

• Current decreases

• current stays the same