Electric Current Chapter 7 – Section 2

Electric Current - The unit for current is amps (A) Flow of electrons through a wire. Current

Types of Current Ways in which electrons flow DC – direct current Electrons flow in same direction in a wire Ex. Battery AC – alternating current Electrons flow in both directions in a wire Ex. Power cords

Direct Current - battery http://www. energizer Batteries produce electrons through a chemical reaction.

Alternating Current - generator Electrons are produced at a power plant and sent through wires to homes & businesses.

Amount of current depends on : Voltage – the push behind current flow: measured in (V) volts. Resistance – the amount of friction between the electrons and the wire: measured in (Ω) ohm’s Low Resistance = good conductors High resistance = poor conductors or Resistors The amount of current in a circuit depends on the amount of voltage available to motivate the electrons, and also the amount of resistance in the circuit to oppose electron flow. Carpet vs. Hardwood

Calculating Current Current is calculated using Ohm’s Law Current = Voltage Resistance

Calculating Current Quantity Symbol Unit Abbreviation Current I Amps A Voltage V Volts Resistance R Ohms Ω

Calculating current

Practice How much current is in a circuit that includes a 9-volt battery and a bulb with a resistance of 3 ohms? How much current is in a circuit that includes a 9-volt battery and a bulb with a resistance of 12 ohms?

Practice What is the voltage of a circuit with 15 amps of current and toaster with 8 ohms of resistance? How many ohms of resistance must be present in a circuit that has 120 volts and a current of 10 amps?

Circuits - Path through which charges can be conducted Closed Circuit =complete path - bulb WILL light up Open Circuit = incomplete path - bulb will not light up

Parts of a Circuit Voltage source – battery Wire - conductor Resistor – bulb, buzzer, motor Switch – open/close circuit (optional)

Draw a circuit with … 1 battery & 1 light bulb & 1 resistor If the battery is 9 volts, the light bulb is 2 ohms, and the resistor is 1 ohm – what is the current of this circuit?

Types of circuits Two types of circuits: Series – ex: Christmas lights Parallel – ex: House

Series Circuits Series – single path for charges to flow If one element removed, circuit will not work Voltage across each device can be different – second bulb may have less voltage – light is dimmer

Parallel Circuits Parallel – multiple pathways Voltage across each device is the same Each resistor uses what it needs Can remove one element without interrupting flow

Is the following series or parallel? Series – all components are on the same path Parallel– all components are on their own path

Draw your own circuit Draw an open series circuit with 1 light bulb, 2 resistors, and a switch Draw a closed parallel circuit with 2 light bulbs and a switch

What is the total voltage of each circuit? A = B =

Magnets

Let’s see what you know about magnets!

1. Magnets not only pull things, but they can also push things away. A. true B. false

A. true

2. The Earth is one big giant magnet. A. true B. false

A. true

3. Two magnets with like poles will _________ each other. A. attract (pull together) B. repel (push apart)

B. repel (push apart)

4. Two magnets with unlike poles will _________ each other. A. attract (pull together) B. repel (push apart)

A. attract (pull together)

5. Compasses always point in which direction? A. North B. East C. South D. West

A. North

6. The space around a magnet is called its ____________. A. electrical field B. baseball field C. magnetic field

C. magnetic field

7. Magnets will only be attracted to objects that have this in them. A. silver B. copper C. iron D. gold

C. iron

8. Magnets can even work in water. A. true B. false

A. true

You are now an expert on magnets! Congratulations! You are now an expert on magnets!

What is a magnet? A magnet is an object that attracts certain materials - usually objects made of iron or steel.

A magnet has two ends called magnetic poles or just poles for short. A magnet’s pull is strongest at the poles.

magnetism Magnesia, province of Greece Unusual property of lodestone noted over 2000 years ago

12th C, Magnets first used in navigational compass, Chinese

16th C, William Gilbert “Every magnet has two poles, a north and a south.” “Like magnetic poles repel, unlike poles attract.”

Figure 5-5(a) A bar magnet and its magnetic dipole field.

Figure 5-3 A compass needle and the Earth. Any magnet will twist because of the forces between its poles and and those of the Earth. Every magnet has at least two poles.

Courtesy Andy Washnik Figure 5-5(b) Iron filings placed near a bar magnet align themselves along the field.

Figure 5-4 A magnetic field. Small magnets placed near a large one orient themselves along the lines of the magnetic field, as shown.

“Opposites attract. Likes repel.” The above describes both magnetic and electric force, but electric charges can be isolated, magnetic poles cannot.

Figure 5-6 Cut magnets. If you break a dipole magnet in two, you get two smaller dipole magnets, not an isolated north or south pole.

Magnets have Poles Just like the Earth has a North and South Pole, magnets also have two poles. This is because the Earth is actually a HUGE magnet itself! Similar magnetic poles repel, while opposite magnetic poles attract.

Magnets have a Magnetic Field Every magnet has a magnetic field around it. It can be thought of as a line of force running from the north end of the magnet to the south end of a magnet. Earth’s magnetic field is what causes the needle of a compass to point north and south.

1820, Hans Oersted …connected a battery to let electric current flow, and noticed a compass needle twitch and move.

Electricity & Magnetism: “ two sides of the same coin “ Every time an electric charge moves, a magnetic field is created. (electromagnet) Every time a magnetic field varies, an electric field is created. (motor)

Electric motors convert electricity into magnetic fields, for useful rotary motion

Figure 5-8 An electric motor. The simplest motors work by placing an electromagnet that can rotate between two permanent magnets. (a) When the current is turned on, the north and south poles of the electromagnet are attracted to the south and north poles of the permanent magnets. (b)–(d) As the electromagnet rotates, the current direction is switched, causing the electromagnet to continue rotating.

Electric motors convert electricity into magnetic fields, for useful rotary motion

Electrical Generators …are the exact opposite of electric motors: they convert rotary motion into electrical energy. link

Figure 5-10 An electric generator. As long as the loop of wire rotates, there is a changing magnetic field near the loop and a current flows in the wire.

Electric Motor M Electrical Energy Mechanical Energy DC Motor

Electromagnetic Devices Motors Converts electrical energy into mechanical energy Commutator – split ring, allows current to change direction every ½ revolution Brushes – connect wires to commutator 2/19/2019 Physical Science

Electric Generator G Mechanical Electrical Energy Energy Stationary magnets - rotating magnets - electromagnets

Generators Converts mechanical energy to electrical energy Loop turns within a magnetic field On each half rotation the current reverses direction – alternating current 2/19/2019 Physical Science