Magnetic fields & Electromagnetism Flashcards
Magnetic field
A magnetic field exists wherever there is force on a magnetic pole. As with electric and gravitational fields, a magnetic field is a field of force. Magnetic field can be made in two ways: using a permanent magnet, or using the movement of electric charges, usually by having an electric current.
Solenoid
A long current-carrying coil used to generate a uniform magnetic field within its core. Using a ferrous core such as iron, increases the strength of the magnetic field.
How do magnetic fields arise?
All magnetic fields are created by moving charges. (In the case of a wire, the moving charges are free electrons.) This is even true for a permanent bar magnet. In a permanent magnet, the magnetic field is produced by the movement of electrons within the atoms of the magnet. Each electron represents a tiny current as it circulates around within its atom, and this current sets up a magnetic field. In a ferrous material, such as iron, the weak fields due to all the electrons combine together to make a strong field, which spreads out into the space beyond the magnet. In non-magnetic materials, the fields produced by the electrons cancel each other out.
Right-hand grip rule
A rule for finding the direction of the magnetic field inside a solenoid. If the right hand grips the solenoid with the fingers following the direction of the conventional current around the solenoid, then the thumb points in the direction of the magnetic field.
Right-hand rule
A rule for finding the direction of the magnetic field around a straight, current-carrying wire. If the right hand grips the wire, with the thumb pointing in the direction of the current, the fingers will curl around in the direction of the magnetic field.
Alternative way to identify the poles of an electromagnet
Another way to identify the poles of an electromagnet is to look at it end on, and decide which way round the current is flowing. You can remember that clockwise is a south pole, anticlockwise is a north pole.
Fleming’s left-hand (motor) rule
This rule is used to predict the force experienced by a current-carrying conductor placed in an external magnetic field: thumb → motion, first finger → magnetic field and second finger → conventional current.
Motor effect
The term used when a current-carrying wire in the presence of a magnetic field experience is a force.
Define Magnetic flux density
The magnetic flux density at a point in space is the force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point.
Explain Magnetic flux density + Equation
The strength of a magnetic field is known as its magnetic flux density, with symbol B. Sometimes it is known as the magnetic field strength. (You can imagine this quantity to represent the number of magnetic field lines passing through a region per unit area.) The magnetic flux density is greater close to the pole of a bar magnet, and gets smaller as you move away from it.
We define gravitational field strength g at a point as the force per unit mass:
g = F / m
Electric field strength E is defined as the force per unit positive charge:
F = F / Q
In a similar way, magnetic flux density is defined in terms of the magnetic force experienced by a current-carrying conductor placed at right angles to an (external) magnetic field. For a uniform magnetic field, the flux density B is defined by the equation:
B = F / IL
where F is the force experienced by a current-carrying conductor, I is the current in the conductor and L is the length of the conductor in the uniform magnetic field of flux density B. The direction of the force F is given by Fleming’s left-hand rule.
The unit for magnetic flux density is the tesla, T. It follows from the equation for B that 1 T = 1 N A^−1 m^−1.
The force on the conductor is given by the equation:
F = BIL
Note that you can only use this equation when the field is at right angles to the current.
Measuring Magnetic flux density with a hall probe (Practical Activity 24.2)
The simplest device for measuring magnetic flux density B is a Hall probe When the probe is held so that the field lines are passing at right angles through the flat face of the probe, the meter gives a reading of the value of B. If the probe is not held in the correct orientation, the reading on the meter will be reduced. A Hall probe is sensitive enough to measure the Earth’s magnetic flux density. The probe is first held so that the Earth’s field lines are passing directly through it, as shown in Figure 24.13. In this orientation, the reading on the voltmeter will be a maximum (positive). The probe is then rotated through 180° so that the magnetic field lines are passing through it in the opposite direction. The change in the reading of the meter is twice the Earth’s magnetic flux density (maximum negative reading —> so change b/w max positive and max negative reading is twice the magnetic flux density).
Measuring Magnetic flux density with a current balance version 1 (Practical Activity 24.2)
Figure 24.14 shows a simple arrangement that can be used to determine the flux density between two magnets. The magnetic field between these magnets is (roughly) uniform. The length L of the current-carrying wire in the uniform magnetic field can be measured using a ruler. When there is no current in the wire, the magnet arrangement is placed on the top pan and the balance is zeroed. Now, when a current I flows in the wire, its value is shown by the ammeter. The wire experiences an upward force and, according to Newton’s third law of motion, there is an equal and opposite force on the magnets. The magnets are pushed downwards and a reading appears on the scale of the balance. The force F is given by mg, where m is the mass indicated on the balance in kilograms and g is the acceleration of free fall (9.81 ms−2). Knowing F, I and L, the magnetic flux density B between the magnets can be determined using the equation:
B = F / IL
You can also use the arrangement in Figure 24.14 to show that the force is directly proportional to the current.
A system like this in effect ‘weighs’ the force on the current-carrying conductor, and is an example of a current balance.
Measuring Magnetic flux density with a current balance version 2 (Practical Activity 24.2)
Another version of a current balance is shown in Figure 24.15. This consists of a wire frame that is balanced on two pivots. When a current flows through the frame, the magnetic field pushes the frame downwards. By adding small weights to the other side of the frame, you can restore it to a balanced position.
Worked Example 1 - An electric motor has a rectangular loop of wire with the dimensions shown in Figure 24.19. The loop is in a magnetic field of flux density 0.10 T. The current in the loop is 2.0 A. Calculate the torque (moment) that acts on the loop in the position shown.
Step 1 - The quantities we know are:
B = 0.10 T, I = 2.0 A and L = 0.05 m
Step 2 - Now we can calculate the force on one side of the loop using the equation F = BIL:
F = 0.10 × 2.0 × 0.05 = 0.01N
Step 3 - The two forces on opposite sides of the loop are equal and anti-parallel. In other words, they form a couple. From Chapter 4, you should recall that the torque (moment) of a couple is equal to the magnitude of one of the forces times the perpendicular distance between them. The two forces are separated by 0.08 m, so:
torque = force × separation = 0.01 × 0.08
= 8 × 10 ^−4 Nm
Force on a current-carrying conductor at an angle other than 90°
To calculate the force, we need to find the component of the magnetic flux density B at right angles to the current. This component is B sin θ, where θ is the angle between the magnetic field and the current or the conductor. Substituting this into the equation F = BIL gives:
F = BILsinθ
