S.MORRIS 2006 ELECTRICAL CIRCUITS More free powerpoints at

Electricity and Voltage - Electricity is the flow of electrons (-) Charges (-) flow from HIGH voltage areas to LOW voltage areas –Voltage is like electrical pressure that pushes and pulls charges –Voltage Difference: the push/pull that causes charges to move and is measured in volts (V)

Voltage Voltage is created by –a chemical cell (battery) when it changes chemical energy to electrical energy – by a generator when it changes mechanical energy to electrical energy – by a solar cell when it changes light energy to electrical energy.

Voltage and Current When a wire connects the terminals of a battery or generators, then the voltage will push and pull electrons through a conductor. –One terminal has extra electrons thus a negative charge. The other terminal has a deficit of electrons and thus a positive charge. –Electrons in the wire are pushed by the negative terminal and pulled by the positive terminal through the wire Circuit: a closed, conducting path For changes to flow, the wire must always be connected in a circuit Electric Current: the flow of charges through a wire or any conductor. Measured in Amperes (A=Amps) Current is almost always the flow of electrons What happens if we break the circuit?

Check for Understanding What is voltage? How is voltage generated? (3 ways) What is current?

Resistance Resistance: the tendency for a material to oppose the flow of electrons Changes electrical energy into thermal energy and light Ex: lightbulb filament Resistance is measured in Ohms (Ω)

What Affects Resistance?

IV. Control the Flow A voltage difference causes the charges to flow Flow of charges= current (Amps or A) Electrical resistance restricts the movement of charges Resistance = current Pressure = current (Voltage Difference)

Ohm’s Law Current = voltage difference Resistance I = V/R or V=IR I= current (units = A, amps) V= voltage (units = V, volts) R = resistance (units = ohms Ω) Tutorial

Section 3 – Electrical Energy I. Series Circuit: the current has only one loop to flow through –things are wired one right after the other –If one thing (bulb) goes out every thing goes out –If the circuit is broken the entire flow of current stops

Series Circuit Current is the same at each point in the circuit When another resistor (light bulb) is added in series, the total resistance increases. When resistance increases, current will decrease. Decreased current means dimmer light.

II. Parallel Circuit: contains two or more branches for current to move through –current splits up to flow through the different branches –because all branches connect the same two points of the circuit – the voltage difference is the same in each branch –more current flows through the branches that have the lower resistance

Household Electrical Safety In a house, many appliances draw current from the same circuit –If more appliances are connected to a circuit, more current will flow through the wires –More current in wires = more heating in the wires –More heat causes insulation on wires to melt, which increases chances of fire To protect a house from this, all household circuits have a: 1.Fuse, or 2.Circuit breaker

Household Circuits: Fuse: a small piece of metal that melts if the current becomes too high Circuit Breaker: contains a small piece of metal that bends when it gets hot bending causes a switch to flip and opens the circuit

Let’s Compare Series and Parallel Circuits Series Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit ____________________ Adding resistance in series ____________________ Parallel Circuits _______ path(s) for current Current ________________ Voltage ________________ Break in circuit ____________________ Adding resistance in parallel ____________________

Let’s Compare Series and Parallel Circuits Series Circuits 1 path(s) for current Current is the same at every point Voltage drops at each resistor Break in circuit stops all current Adding resistance in series decreases total current (dimmer light bulbs) Parallel Circuits multiple path(s) for current Current can be different in each branch Voltage same across each resistance Break in circuit does not affect other bulbs Adding resistance in parallel increases total current

The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.

What is an electric current? An electric current is a flow of microscopic particles called electrons flowing through wires and components. + - In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.

simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. To make the circuit, these components are connected together with metal connecting wires. cell lamp switch wires

simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around. If there were any breaks in the circuit, the current could not flow.

circuit diagram cellswitchlampwires Scientists usually draw electric circuits using symbols;

circuit diagrams In circuit diagrams components are represented by the following symbols; cellbatteryswitchlamp motorammetervoltmeter buzzer resistorvariable resistor

types of circuit There are two types of electrical circuits; SERIES CIRCUITSPARALLEL CIRCUITS

The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. SERIES CIRCUITS If one bulb ‘ blows ’ it breaks the whole circuit and all the bulbs go out.

PARALLEL CIRCUITS The current has a choice of routes. The components are connected side by side. If one bulb ‘ blows ’ there is still be a complete circuit to the other bulb so it stays alight.

measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A

measuring current A A This is how we draw an ammeter in a circuit. SERIES CIRCUIT PARALLEL CIRCUIT

measuring current SERIES CIRCUIT PARALLEL CIRCUIT current is the same at all points in the circuit. 2A current is shared between the components 2A 1A

copy the following circuits and fill in the missing ammeter readings. ? ? 4A 3A? ? 1A ? 3A 1A

measuring voltage The ‘ electrical push ’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V

Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. measuring voltage Unlike an ammeter a voltmeter is connected across the components Scientist usually use the term Potential Difference (pd) when they talk about voltage.

measuring voltage V This is how we draw a voltmeter in a circuit. SERIES CIRCUITPARALLEL CIRCUIT V

V measuring voltage V V V

series circuit 1.5V voltage is shared between the components 1.5V 3V

voltage is the same in all parts of the circuit. 3V parallel circuit 3V

measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.

measuring current & voltage V V 6V 4A A A a)

measuring current & voltage V V 6V 4A A A A b)

answers 3V 6V 4A 6V 4A 2A 4A a)b)