Units
Give the SI unit for force.
Newton (N)
A measure of the magnetic intensity.
Magnetic field strength
What does emf stand for?
a) Electron magnetic force
b) Electromotive force
B
Function of a step down transformer
Reduce voltage
V/d is used to calculate
Electric field strength
Give the SI unit for electric field strength.
V/m V/C
A change that occurs as a result of movement in a magnetic field.
Which formula represents back emf?
Emf = V/R
Emf = V – IR
Emf = V - IR
Function of a step up transformer.
Increase voltage
Give the torque formula for a simple loop in a magnetic field
Torque = n I A B L Sin Ꝋ
Give the SI unit for Magnetic Flux
Weber (Wb)
A measure of the magnetic field that passes an area.
Magnetic flux
A DC motor has a supply voltage of 10V. When the rotor is not turning the resistance is 2 ohm.
Calculate the current.
I = V/R
I = 10 / 2
I = 5A
In the formula
VP IP = VS IS
What does VP represent
VP = Voltage Primary
Give the torque formula for a motor in a magnetic field.
Torque = n I A B Cos Ꝋ
Give the SI unit for induced emf.
Volt (V)
Convert mechanical energy into electrical energy.
Generator
A DC motor has a supply voltage of 10V. When the rotar is not turning the resistance is 2 ohm. When connected to a supply voltage the motor draws 3A.
What is the back emf when the motor is turning freely?
V – emf = IR
Emf = V – IR
Emf = 10 – (3 x 2)
Emf = 4V
In the formula
VP IP = VS IS
What does IS and IP represent
IS = Current Secondary
IP = Current Primary
A transformer is connected to a primary voltage of 220V and produces output (secondary) voltage of 1000V.
The transformer draws secondary current of 40A.
Analyse the structure and operation of the transformer.
Find the primary current.
VP IP = VS IS
IP = VS x IS /VP
IP = 1000 x 40 / 220
IP = 181A
Give the SI units for
- Magnetic Field strength
- Current
- Potential difference
Magnetic field strength = T (Tesla)
Current = A (Amperes)
Potential Difference = V (Volts)
Changing current and magnetic fields produce a torque in the rotor.
AC Motor
A DC motor has a supply voltage of 20V. When the rotar is not turning the resistance is 5 ohm. When connected to a supply voltage the motor draws 3A.
What is the back emf when the motor is turning freely?
V – emf = IR
Emf = V – IR
Emf = 20 – (3 x 5)
Emf = 5V
A transformer is connected to a primary voltage of 240V and produces output (secondary) voltage of 20V.
The transformer draws primary current of 5A.
Analyse the structure and operation of the transformer.
Find the secondary current.
VP IP = VS IS
IS = VP x IP /VS
IS = 240 x 5 / 20
IS =
A transformer is connected to a primary voltage of 100V and produces output (secondary) voltage of 240V.
The transformer draws secondary current of 60A.
Analyse the structure and operation of the transformer.
Find the primary current.
VP IP = VS IS
IP = VS x IS /VP
IP = 240 x 60 / 100
IP = 144A